WBBSE 10th Class Science Solutions Physical Science & Environment Chapter 6 Current Electricity

09/18/2023 Ajit kumar 89 Views 0 Comments

West Bengal Board 10th Class Science Solutions Physical Science & Environment Chapter 6 Current Electricity

WBBSE 10th Class Physical Science & Environment Solutions

Synopsis

Coulomb’s Law, Potential Difference, EMF

Electric charge: Electric charge is the physical property of a matter due to which it exerts force on a charged body or on an uncharged body.
There are two types of charges-positive charge and negative charge. Similar charges repel each other while opposite charges attract each other.
Coulomb’s law: The mutual attractive or repulsive force between two point charges at rest is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
This law was given by the French scientist is the distance Charles Coulomb. If r and between two point charges q₁ from q₂, Coulomb’s law mutual force,

where k is the constant of proportionality. The value of k depends on the surrounding medium and the system of unit used.
Units of charge in CGS system and SI are esu of charge and C (coulomb) respectively. 1C = 3 × 10⁹ esu of charge
1 esu charge: If two point charges of the same magnitude and of the same nature are placed in vacuum 1 cm apart and exert a repulsive force of 1 dyn on each other, then each charge is called a unit charge or 1 esu of charge or 1 statcoulomb in CGS system.
1 coulomb charge: If two point charges of the same amount and of the same nature are placed in vacuum at a distance of 1 m and exert a repulsive force of 9 × 10⁹ N on each other, then each charge is called a unit charge or 1 C (coulomb) in Sl.
Electric field: When a charge is kept at any place, it creates a field around it and if any other charge is kept there, it experiences an electric force. This area or field is called electric field.
Electric potential: Potential at a point in an electric field is defined as the amount of work done in bringing a unit positive charge from infinity to that point.
Units of electric potential in CGS system and in Sl are respectively esu potential or statvolt and volt.
1 volt of potential difference: If 1 joule of work has to be done to move 1 coulomb of positive charge from one point to another point in an electric field, then the potential difference between these two points is called 1 volt of potential difference.

TOPIC – A

Coulomb’s Law, Potential Difference, EMF

SHORT AND LONG ANSWER TYPE QUESTIONS

Q.1 Write the mathematical form of Coulomb’s law.

Ans. Let two point charges q₁ and q₂ be placed at the points A and B respectively. The distance between these two charges is r. Now if F is the mutual force between the two charges, then according to Coulomb’s law,

F ∝ q₁q₂ [when r is constant] and

where k is a constant whose value depends on the surrounding medium and the system of units used.

Q.2 Give the definition of unit charge in CGS system with the help of Coulomb’s law.

Ans. Two point charges q₁ and q₂ are placed at a distance of r. Now if the mutual force between them is F, then according to Coulomb’s law,

So, we get from equation (1), q² = 1 or, q = ±1 .

Therefore, if two point charges of the same magnitude and of the same nature are placed in vacuum 1 cm apart and exert a repulsive force of 1 dyn on each other, then each charge is called a unit charge or 1 esu of charge or 1 statcoulomb in CGS system.

Q.3 Give the definition of unit charge in Sl with the help of Coulomb’s law.

Ans. Two point charges q₁ and q₂ are placed at a distance of r. Now if the mutual force between them is F, then according to coulomb’s law,

Therefore, if two point charges of the same amount and of the same nature are placed in vacuum at a distance of 1 m and exert a repulsive force of 9 × 10⁹ N on each other, then each charge is called a unit charge or 1 C (coulomb) in Sl.

Q. 4 Establish a relation between the units of charge in CGS system and SI.

Ans. The unit of charge in SI is C and the unit of charge in CGS system is esu of charge.

Suppose, 1 C = x esu of charge

Now if two point charges of q₁ = q₂ = 1 C = x esu are kept at a distance of r = 1m = 100 cm, then the mutual repulsive force is given by

Q.5 Can we term Coulomb’s constant of the mutual force between two point charges as universal?

Ans. The mathematical form of the Coulomb’s law regarding electrical force between two point charges q₁ and q₂ at a distance of r from each other in a medium is F = k · q₁q₂/r² where k is called Coulomb’s constant.

The constant k depends on the nature of the medium. Therefore, mutual electrical force between two point charges depends on the nature of the medium and the value of this force is different in different media. Hence, Coulomb’s constant cannot be termed as universal.

Q.6 What is electric charge? How many types of charges are there? Name them.

Ans. Electric charge is that physical property of a matter due to which it exerts force on an electrified body or on an uncharged body.

It is of two types, namely positive charge and negative charge.

Q.7 When a material is electrically charged by rubbing, then what is the charge on the two materials?

Ans. When a material is rubbed with another material, then some electrons get transferred from one material to the other. The material which has deficit of electrons is positively charged and the material which has excess of electrons is negatively charged.

Q.8 When does an electrically charged material attract another material? When does it repel another material?

Ans. An electrically charged material attracts an uncharged material and also a material having a charge of opposite nature.

Ans. It repels a material having charge of similar nature.

Q.9 Write Coulomb’s law.

Ans. The mutual attractive or repulsive force between two point charges at rest is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.

Q.10 when the distance between two point charges increases, how does the value of the mutual attractive or repulsive force between them change?

Ans. When the distance between two point charges increases, the value of the mutual attractive or repulsive force gets reduced in proportion to the square of the distance between the two.

Q.11 Define electric field.

Ans. When a charge is kept at any place, it creates a field around it and if any other charge is placed in that area or field, it experiences an electric force. This area or field is known as an electric field.

Q.12 Define electric potential.

Ans. Electric potential at any point in an electric field is the total work done in moving a unit positive charge from infinity to that point under the influence of an external electric field.

In other words, we may say the total amount of work that has to be done in bringing one unit positive charge from infinity to a point near another positive charge is called the electric potential of that point with reference to the second charge.

Q.13 What are the units of electric potential 1+2 in CGS system and SI? Define them.

Ans. The units of electric potential in CGS system and SI are esu of potential or statvolt and volt (V) respectively.

Esu of potential or statvolt: The potential at a point is said to be one esu of potential or one statvolt if 1 erg of work is done in bringing a positive charge of 1 esu from infinity to that point in an electric field.

Volt (V): The potential at a point is said to be one volt if 1 J (joule) of work is done in bringing a positive charge of 1 coulomb from infinity to that point in an electric field.

Q.14 What is electromotive force?

Or, In reality electromotive force (emf) is not any force-explain the fact.

Ans. The electromotive force of an electrical source is defined as the amount of electrical energy that is produced in the source due to the transfer of a positive charge from a lower potential to a higher potential.

In reality, electromotive force is not any force at all. It is the energy that is required to move per unit charge from lower potential to higher potential.

Q.15 Define the unit of emf in Sl.

Ans. Unit of emf in SI is J/C or V (volt).

1 volt of electromotive force: If 1 J of work has to be done to move a positive charge of 1 coulomb from a point of low potential to a point of high potential in an electrical cell, then the magnitude of the electromotive force of that electrical cell is 1 J/C or 1. V.

Q.16 ‘The emf of an electrical cell is 10 V’ – what do you mean by this statement?

Ans. ‘The emf of an electrical cell is 10 V’ means that 10 J of work has to be done in order to move an electric charge of 1 C from a lower potential point to a higher potential point in the electrical cell.

Q.17 Define electromotive force for an open fine electromotive force for circuit.

Ans. The electromotive force of a cell for an open circuit is defined as the amount of work done by an external agency to move a unit positive electric charge from the positive pole of the cell to its negative pole.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. Coulomb force is

A. purely attractive force

B. purely repulsive force

C. either attractive or repulsive

D. neither attractive nor repulsive

Ans. C

2. Electric charges are of

A. a single type

B. two types

C. three types

D. four types

Ans. B

3. The charge of an electron is

A. –3.2 × 10^–19C

B. –1.6 × 10^–19C

C. 1.6 × 10^–19C

D. 3.2 × 10^–19C

Ans. B

4. 450 V = how many statvolt?

A. 0.5

B. 1

C. 2

D. 1.5

Ans. D

5. 0.1 C = how many esu of charge?

A. 3 × 10¹⁰

B. 3 × 10⁹

C. 3 × 10⁸

D. 3 × 10⁷

Ans. C

6. 1 J/C is equal to

A. 1 W

B. 1 Ω

C. 1 V

D. 1 A

Ans. C

7. 2 C charge is equivalent to the charge of how many electrons?

A. 6.25 × 10¹⁸

B. 6.25 × 10¹⁹

C. 1.25 × 10¹⁹

D. 12.5 × 10¹⁹

Ans. C

8. If the charge of an electron be 1.6 × 10¹⁹ C, which one of the following cannot be the charge of a body?

A. 1.6 × 10^–19 C

B. 0.8 × 10^–19 C

C. 3.2 × 10^–19 C

D. 4.8 × 10^–19 C

Ans. B

9. Unit of electromotive force is

A. volt

B. coulomb

C. ohm

D. ampere

Ans. A

10. Electromotive force is measured by

A. ammeter

B. voltmeter

C. galvanometer

D. potentiometer

Ans. B

Answer in brief

1. What is the unit of electric charge?

Ans. The unit of electric charge in CGS system is esu of charge or statcoulomb.

2. What is the unit of electric charge in SI?

Ans. The unit of electric charge in SI is coulomb (C).

3. On which factors, does the value of the constant k of Coulomb’s law depend?

Ans. The value of the constant k depends on the nature of the surrounding medium and the system of unit.

4. What type of quantity is electric potential?

Ans. Electric potential is a scalar quantity.

5. What is the unit of electric potential in CGS unit?

Ans. The unit of electric potential in CGS unit is esu of potential or statvolt.

6. What is the unit of electric potential in SI?

Ans. The unit of electric potential in Sl is volt (V).

7. V · C is the unit of which physical quantity?

Ans. V · C is equal to J (joule) which is the unit of work and energy.

8. What is the unit of electromotive force (emf) in SI?

Ans. The unit of electromotive force (emf) in Sl is volt (V).

9. Is Coulomb’s law a universal law?

Ans. No, Coulomb’s law is not a universal law.

10. Does the mass of an uncharged body increase or decrease when it is charged with a positive charge?

Ans. When an uncharged body is charged with a positive charge, its mass decreases.

11. Does the mass of an uncharged body increase or decrease when it is charged with a negative charge?

Ans. When an uncharged body is charged with a negative charge, its mass increases.

12. Unit of which physical quantity is obtained when the unit of electric charge is divided by the unit of electric current?

Ans. Unit of time is obtained when the unit of electric charge is divided by the unit of electric current.

13. Unit of which physical quantity is obtained when the unit of potential difference is multiplied by the unit of charge?

Ans. Unit of work is obtained when the unit of potential difference is multiplied by the unit of electric charge.

14. What is the carrier of electricity in a metal conductor?

Ans. Free electron is the carrier of electricity in a metal conductor.

Fill in the blanks

1. A body is positively charged if there is ………….. of electrons.

Ans. deficit

2. A body is negatively charged if there is ………. of electrons in it due to friction.

Ans. excess

3. The active mutual force between two point charges ………….. on the nature of the medium.

Ans. depends

4. The potential at a particular point is ………..volt if 1/300 erg of work is done in bringing one esu of positive charge from infinity to that point in an electric field.

Ans. one

5. Some other form of energy is transformed into ……….. in an electric cell.

Ans. electrical energy

6. Force and electromotive force (emf) are …………. quantities.

Ans. different

7. Scientist ……….. named two opposite type of charges as positive and negative.

Ans. Benjamin Franklin

8. Electric charge is a ………… physical quantity.

Ans. scalar

State whether true or false

1. Charge of an electron is -1.6 × 10^-19 C.

Ans. True

2. Like charges attract each other and unlike charges repels each other.

Ans. False

3. 1C = 3 × 10¹⁰ esu of charge.

Ans. False

4. According to the Coulomb’s law, mutual force of attraction or repulsion is directly proportional to the square of the distance between the two given point charges.

Ans. False

5. When a body is earthed, electrons from the earth flow into the body. This means that the body gets positively charged.

Ans. False

6. In electric cell heat energy is converted into electric energy.

Ans. False

TOPIC – B

Ohm’s Law

SHORT AND LONG ANSWER TYPE QUESTIONS

Q.1 What are the directions of electric current and the flow of electrons in a metal conductor present in an electric circuit?

Ans. In a metallic conductor, free electrons are the carriers of electricity. If a potential difference is applied across the two sides of a conductor, free electrons move from lower potential to higher potential. As a result, it is said that a current is passing through the conductor.

In a metallic conductor, the direction of electric current is opposite to the motion of the electrons. In other words, it has been assumed that the electrons flow from higher potential to lower potential.

Q.2 Write down Ohm’s law.

Or, Explain the formula by which the relationship between the potential difference across the two ends of a conductor and the value of current through it can be known.

Ans. According to Ohm’s law, if the temperature and other physical conditions remain constant, the amount of current flowing through a conductor is directly proportional to the potential difference across the ends of the conductor.

Let us assume the potential across the two ends of a conductor are V_A and V_B respectively. If V_A > V_B, then the current flows from A to B.

Now, if I is the current, then according to Ohm’s law,

Q.3 What do you mean by the resistance of a conductor? Define resistance from Ohm’s law.

Ans. Resistance is that property of a conductor due to which it obstructs the flow of current through it. If a current / flows through a conductor when V is the potential difference across its two ends, then according to Ohm’s law,

This constant is called the resistance of the conductor. In other words, the ratio between the potential difference across the conductor and the amount of current flowing through the conductor is called the resistance.

Q.4 What are ohmic conductor and nonnd nonohmic conductor? Give examples.

Ans. Ohmic conductor: Those conductors which follow Ohm’s law or those conductors for which I-V graph is a straight line passing through the origin, are called ohmic conductors.

Example: any metallic conductor

Non-ohmic conductor: Those conductors which do not follow Ohm’s law or those conductors for which I-V graph is not a straight line passing through the origin, are called non-ohmic conductors.

Example: electrolytic solution

Q.5 What are the factors on which the resistance of a conductor depends?

Ans. Factors on which the resistance of a conductor depends are: (1)length of the conductor, (2) cross sectional area of the conductor, (3) nature of the material of the conductor, (4) temperature.

In addition, for some conductors, resistance depends on: (1) intensity of light, (2) strength of magnetic field, (3) pressure.

Q.6 Two terminals of an electrical cell are connected by a wire. After that, the wire is folded into two and connected again with the same two terminals. In this condition, does the amount of current increase or decrease as compared to the first case?

Ans. As the wire is folded into two and is connected with the cell, the resistance of the folded wire will be half of the resistance of the original wire due to reduction of its length into half. Again, due to doubling of the area of cross section, resistance of the folded wire will be half of the original wire. Therefore, due to change of length and cross section of the wire, the resistance of the folded wire will be (1/2×1/2) or 1/4th of its original resistance. Since the potential difference of the cell is constant, current through the folded wire is 4 times the current through the original wire as resistance and current are inversely proportional to each other.

Q.7 A thin and a thick wire of the same length are connected with the same battery. In which wire is the value of current more and why? Explain.

Ans. When a thin and a thick wire of the same length are connected with the same battery, value of current in the thick wire is more.

The resistance of a conductor is inversely proportional to the cross section, so the resistance of a thick wire will be less. Again, if the potential difference remains the same, value of current is inversely proportional to the resistance. Therefore, as a result of decrease of resistance of the thick wire, value of current in the conductor will be more. Similarly, if the wire is thin, i.e., the cross section is reduced, the resistance of the wire will increase. Hence, as a result of increase of resistance, the value of current through the thin I wire will be less.

Q.8 A copper wire is heated keeping its length and radius constant. Find the change in its resistance. If we take a carbon thread Instead of a copper wire, will there be any change in the resistance?

Ans. If a copper wire is heated by keeping its length and resistance constant, its resistance increases.

If a carbon thread is used instead of a copper wire and heated, the resistance of the thread will decrease. This is because the resistance of all pure metals and most metallic alloys increase with increase of temperature. But the resistance of carbon, electrolytes, rubber, silicon etc. decreases with increase of temperature.

Q.9 Write down the use of insulators in te down of the daily life.

Ans. If electric current passes through a conductor and if we touch it during that time, an electric shock can be felt. This is due to the fact that our body is a good conductor of electricity. Insulators protect us from this. That is why an insulating material like coating of polyethylene is always applied on a conducting wire. For the same reason, a ceramic insulator is used in the overhead wire of trains. Also, handles of electrical instruments like pliers, tester etc. are made up of insulating materials.

Q.10 What do you mean by superconductivity? Draw a resistivity vs temperature graph to explain this.

Ans. In general, if the temperature of a metal is reduced, its resistivity decreases. There is a type of metal or compound whose resistivity becomes zero if their temperature becomes less than a particular temperature. This particular temperature is called critical temperature. This type of phenomenon is called superconductivity and those materials are called superconductors.

In the given figure change of resistivity with temperature of this type of material is shown. In the graph, point C is the critical temperature.

For example, mercury at 4.2 K temperature, lead at 7 K temperature and Niobium Nitride (NbN) at 16 K temperature become superconductors.

Q.11 Explain series combination and parallel combination of resistance.

Ans. Series combination: When some resistances are connected such that the extreme end of one is connected with the beginning of the next resistance and so on, then the resistance are said to be connected in series. When this combination is joined in an electrical circuit, the same current flows through each resistance after applying a potential difference across the combination.

Parallel combination: When some resistances are connected such that one of the ends of all the resistances is connected at a fixed point while the other end is connected to another point, then the resistances are said to be in parallel combination. When this combination is joined in an electrical circuit, potential difference across each of the resistance remains the same.

Q.12 Write the characteristics of the series combination of resistances.

Ans. The characteristics of the series combination of resistances are:

Same amount of electric current passes through each resistance.
Equivalent resistance of the combination is the algebraic sum of each individual resistance.
When a potential difference is applied between the two sides of the combination, potential difference between the two sides of each resistance is directly proportional to the respective resistance.
Equivalent resistance of the combination is more than the value of each resistance.

Q.13 Write the characteristics of the parallel combination of resistances.

Ans. The characteristics of the parallel combination of resistance are:

The potential difference across the two terminals of each resistance is the same.
The reciprocal of the equivalent resistance of the combination is equal to the sum of the reciprocals of all the resistances.
The equivalent resistance of the combination is smaller than the smallest resistance of the combination.
As the terminal potential difference is constant, hence the value of current passing through a particular resistance is inversely proportional to that resistance.

Q.14 What are the constituents of a simple electrical circuit? What is the direction of electric current in the entire circuit?

Ans. There are two parts of a simple electrical circuit- (1) external circuit (entire portion except the cell) and (2) internal circuit (internal portion of the cell). At least one resistance must be present in the external circuit.

Once the key of the circuit is closed, an electric current starts flowing through the circuit and also through the resistance from the end of higher potential to the end of lower potential. In the internal circuit, this flow is from the negative terminal of the cell to the positive terminal.

Q.15 An electrical circuit has been formed by connecting a resistor, a voltmeter, an ammeter, an electric cell and a few pieces of conducting wires.

(1) Out of these, which electrical parts have their own resistances and which parts have no resistance?

(2) If we consider the parts to be ideal, then which ones will be without any resistance?

Ans. (1) According to the question, all the parts, namely, resistor, voltmeter, ammeter, electric cell, conducting wires possess some resistance. Out of these, the resistance of the conducting wire is considered to be negligible compared to the other parts.

(2) If all the constituents had been ideal, then except the resistor and voltmeter, then all the other constituents namely ammeter, electric cell and conducting wires would have been without resistance.

Q.16 What is internal resistance of an electric cell? What is the reason for the existence of this resistance?

Ans. Resistance of the electric cell connected in an electrical circuit is the internal resistance of cell.

When an electric cell sends current through a closed circuit, then current flows also through the cell. The active liquid or electrolytic material placed between the two electrodes of this cell offers resistance to this current. This resistance inside the cell is the internal resistance,

Q.17 What is lost volt? What is the cause of its origin?

Ans. During the flow of electric current through the resistances of the external circuit and the internal circuit, some amount of work has to be done by each electric charge to those resistances. Therefore, some amount of electrical energy is converted to some other form of energy while passing through each resistance. The energy converted in the case of internal circuit is heat energy which cannot be used for any practical purpose. This wasted energy inside an electrical cell is known as lost volt.

The presence of the internal resistance of the electrical cell is the cause of origin of lost volt.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. Which metal is the best conductor of electricity?

A. gold

B. silver

C. aluminium

D. copper

Ans. B

2. What is the equivalent resistance of a parallel combination of resistances 3 Ω and 6 Ω ?

A. 2 Ω

B. 4 Ω

C. 9 Ω

D. 3 Ω

Ans. A

3. When the potential difference is 10 V and the amount of current is 5 A, then the resistance is

A. 5 Ω

B. 4 Ω

C. 3 Ω

D. 2 Ω

Ans. D

4. If R is the equivalent resistance of a parallel combination of three resistances R_₁, R_₂ and R₃ (R_₁ > R₂ > R₃), then

A. R > R_₁

B. R > R₂

C. R > R₃

D. R < R₃ < R₂ < R₁

Ans. D

5. The resistance of a superconductor at critical temperature is

A. infinite

B. zero

C. 10⁶ Ω

D. 10⁹ Ω

Ans. B

6. When temperature increases, the resistance of a semiconductor

A. decreases

B. increases

C. first increases then decreases

D. first decreases then increases

Ans. A

7. What is the value of R at the time of a short circuit?

A. ∞

B. 0

C. 10⁶ Ω

D. 10¹⁰ Ω

Ans. B

8. What is the value of R in an open circuit?

A. ∞

B. 0

C. 10 Ω

D. 1000 Ω

Ans. A

9. 1 A is equal to

A. 1 C²/s

B. 1 C/s²

C. 1 C/s

D. 1 s/C

Ans. C

10. The carrier of electricity in a metal conductor is

A. orbital electrons

B. electrons and ions

C. free electrons

D. ions

Ans. C

11. When a charge of 120 C passes through a conductor for 2 minutes, what is the amount of electric current?

A. 1 A

B. 2 A

C. 0.5 A

D. 0.25 A

Ans. A

12. 1 A = how many mA?

A. 10

B. 100

C. 1000

D. 10⁶

Ans. C

13. When 10¹⁹ electrons flow through any cross section of a conductor in 2 seconds, the amount of electric current

A. 0.8 A

B. 1.6 A

C. 2.4 A

D. 3.2 A

Ans. A

14. 1 V/A is equal to

A. 1 J

B. 1 C

C. 1 Ω

D. 1 W

Ans. C

15. If temperature increases, resistance of a metal

A. increases at first, then decreases

B. decreases at first, then increases

C. increases

D. decreases

Ans. C

16. The unit of resistivity is

A. Ω · m²

B. Ω · m

C. Ω/m

D. Ω/m²

Ans. B

17. The value of resistivity is highest in the case of

A. metal

B. insulator

C. semiconductor

D. superconductor

Ans. B

18. The value of resistivity is lowest in the case of

A. metal

B. insulator

C. semiconductor

D. superconductor

Ans. D

19. In a series combination of several resistances, which of the following remains unchanged for each resistance?

A. potential difference

B. consumed power

C. current

D. none of these

Ans. C

20. In a parallel combination of several resistances, which of the following remains unchanged for each resistance?

A. potential difference

B. consumed power

C. current

D. none of these

Ans. A

Answer in brief

1. What is the unit of resistance in SI?

Ans. Unit of resistance in SI is ohm (Ω).

2. Give example of an ohmic conductor.

Ans. All metal conductors are ohmic conductors.

3. Give examples of three non-ohmic devices.

Ans. Three non-ohmic devices are transistor, LED and filament lamp.

4. What is the term used to describe the fall of potential inside an electric cell?

Ans. The fall of potential inside on electric cell is called lost volts.

5. When an electric cell is connected with an external resistance, what is the direction of current flow in the external resistance?

Ans. The direction of current flow in the external resistance is from the positive pole towards the negative pole.

6. When an electrical cell is connected with an external resistance, what is the direction of current flow inside the cell?

Ans. The direction of current flow inside the cell is from the negative pole towards the positive pole.

7. What is the unit of resistivity in SI?

Ans. The unit of resistivity in SI is Ω · m.

8. What is the unit of conductivity in SI?

Ans. The unit of conductivity in Sl is S · m−¹.

9. What is the unit of conductance in SI?

Ans. The unit of conductance in SI is siemens (S).

10. What is the value of resistivity of a conductor at a temperature less than the critical temperature?

Ans. The value of resistivity of a conductor at a temperature less than the critical temperature is zero.

11. How does the resistivity of a metal change if its temperature is increased?

Ans. The resistivity of a metal increases if its temperature is increased.

12 How does the resistivity of a semiconductor change if its temperature is increased?

Ans. The resistivity of a semiconductor decreases if its temperature is increased.

13. How would you connect some resistances, in series combination in parallel combination, to get a resistance higher than the resistance of the individual constituents?

Ans. To get a resistance higher than the resistance of the individual constituents, the resistances have to be connected in a series combination.

14. How would you connect some resistances, in series combination or in parallel combination, to get a resistance lower than the resistance of the individual constituents?

Ans. To get a resistance lower than the resistance of the individual constituents, the resistances have to be connected in a parallel combination.

15. The household electrical instruments are connected in which combination?

Ans. The household electrical instruments are always connected in parallel combination.

16. Which physical quantity, electric current or potential difference, remains constant for all the resistances connected in a series combination?

Ans. Electric current remains the same through each resistance connected in a series combination of resistances.

17. Which physical quantity, electric current or potential difference, remains constant for all the resistances connected in a parallel combination?

Ans. Potential difference across the two ends of each resistance remains constant in a parallel combination of resistances.

18. What are the values of two resistances such that the amount of current passing through them remains the same, when connected in series and in parallel combination?

Ans. The values of the two resistances should be equal.

19. Which instrument is used to measure electric current?

Ans. Ammeter is used to measure electric current.

20. Which instrument is used to measure potential difference?

Ans. Voltmeter is used to measure potential difference.

21. What is the condition for steady electric current through a conductor?

Ans. Steady electric current is obtained when the potential difference across the two ends of the conductor remains constant.

22. How does the resistance of the metal selenium change when light rays fall on it?

Ans. When light rays fall on the metal selenium, its resistance decreases.

23. How does the resistance of bismuth change when it is kept in a megnetic field?

Ans. When bismuth is kept in a magnetic field, its resistance increases.

24. How does the resistance of carbon change when pressure on it is increased?

Ans. When pressure on carbon is increased, its resistance decreases.

25. Write one use of superconductivity.

Ans. Superconductive magnet is a very powerful electromagnet. This type of magnet is used in a particle accelerator.

Fill in the blanks

1. Electric current is a ………. quantity.

Ans. scalar

2. Ω · cm is the practical unit of ……….

Ans. resistivity

3. The resistivity of a metal is …………. than that of a semiconductor.

Ans. less

4. If temperature increases, the resistance of germanium (Ge) …………

Ans. decreases

5. When a suitable impurity is doped inside a semiconductor, its resistivity …………..

Ans. decreases

6 .Electric current does not follow the law of ………… addition.

Ans. vector

7. When the potential difference across the two ends of a conductor is changed, the …………. passing through the conductor also changes.

Ans. amount of current

8. The nature of the I-V curve of a metallic conductor is a …………

Ans. straight line

9. If r is the internal resistance of a cell and I is the value of current, then rI is the ………..

Ans. lost volt

10. The …………. of a conductor depends on its length, cross sectional area and its constituent material.

Ans. resistance

11. When temperature decrease, the resistivity of a metal also ………….

Ans. decreases

12. The handle of an electrical instrument is made of an …………. material in order to avoid electric shock.

Ans. insulating

TOPIC – C

Heating Effect of Electric Current, Electrical Power

SHORT AND LONG ANSWER TYPE QUESTIONS

Q.1 Write down Joule’s laws of heating effect of electric current.

Ans. In the year 1841, British physicist James Prescott Joule published three laws with respect to heating effect of electric current. These are:

Law of electric current: The amount of heat produced in a conductor (H) is directly proportional to the square of the current passed (I), when the resistance (R) of the conductor and the time of flow of current (t) remain constant, i.e.,
H ∝ I²[when R and t remain constant].
Law of resistance: The amount of heat produced in a conductor (H) is directly proportional to the resistance of the conductor (R), when the amount of current (I) and the time of flow of current (t) remain constant, i.e.,
H ∝ R [when I and t are constants].
Law of time: The amount of heat produced in a conductor (H) is directly proportional to the time for which the current passes (t), when the amount of current (I) and resistance of the conductor (R) remain constant, i.e.,
H ∝ t [when R and I are constants).

Q.2 Electric current is passing through a metallic wire. How does the amount of heat that is produced according to Joule’s law, change for the following cases?

(1) Value of electric current is tripled, keeping resistance and time of flow of current unchanged.

(2) Potential difference across the two ends of the wire is doubled, keeping resistance and time of flow of current unchanged.

Ans. (1) According to Joule’s law, the amount of heat produced in a metallic wire is directly proportional to the square of the current passed, when the resistance of the conductor and time of flow of current remain constant. In this case, as the value of current is tripled, heat produced becomes 3² or 9 times.

(2) Amount of heat produced is given by

So, amount of heat produced in a metallic wire is directly proportional to the square of the potential difference across the two ends of the wire, when the resistance of the conductor and time of flow of current remain constant. In this case, as the value of potential difference is doubled, heat produced becomes 2² or 4 times.

Q.3 What is the change in the heat produced in a conductor due to flow of current in the following two cases? (1) Current is allowed to flow in the opposite direction (2) ac is applied instead of dc.

Ans. (1) Heat produced in a conductor due to flow of current does not depend on the direction of the current. If the magnitude of current and time of its flow through the conductor remain constant for a fixed resistance, then amount of heat produced does not change even if the direction of current is changed.

(2) When ac is sent through a conductor instead of dc, then also heat is produced. If the magnitude of current and time of its flow through the conductor remain unchanged, then amount of heat produced in the conductor remains unchanged even if ac is applied instead of dc.

Q.4 How is the rating of an electrical lamp done?

Or, What do you mean by voltage rating and power rating of an electric lamp?

Ans. Any electrical appliance is manufactured in such a way that at a particular potential difference, it would be most effective without getting damaged. This value of potential difference is mentioned on the body of the appliance. This is called voltage rating.

If 220 V is written on the body of an electrical lamp, it indicates that the lamp, if connected to a 220 V supply line, would be most effective without getting damaged. If the potential difference is less than 220 V, it glows with less brightness and if the potential difference is more than 220 V, this lamp may get damaged or burnt. In addition to voltage rating, there is another rating which is called power rating. This is also known as watt rating.

Power rating or watt rating is the electrical energy that is consumed per unit time by the electric lamp when a potential difference mentioned in the voltage rating is applied to it.

Q.5 Two filament bulbs of ratings 3 V-1 W and 3 V-2 W are connected in series and in parallel combination. In each case, which bulb does glow brighter?

Ans. Resistance of 3V-1W bulb is given by

Case 1: When the bulbs are connected in series, current passing through them is the same. Here, the bulb which has greater resistance produces more heat and thus glows brighter. So, 3 V-1 W bulb glows brighter.

Case 2: When the bulbs are connected in parallel, potential difference across the two ends is the same. Here, the bulb which has less resistance produces more heat and thus glows brighter. So, 3 V-2 W bulb glows brighter.

Q.6 What happens if a 220V-100 W lamp is connected to a 440V line? Further, what happens if it is connected to a 160 V line?

Ans. When a 220 V – 100 W lamp is connected to a 440 V line, current greater than the highest admissible amount of current passes through it and as a result, huge amount of heat is produced and the coil of the lamp is burnt or damaged. So, the lamp glows brightly for a very short interval of time and then gets fused.

If the lamp is connected to a 160 V line, current lower than the highest admissible amount of current passes through it and as a result, the lamp glows less brightly.

Q.7 Describe an electric lamp and its working principle.

Ans. The practical application of the heating effect of electric current is best demonstrated in the

case of an electric lamp. Two thick conducting wires are inserted in a sealed glass bulb. A long filament made of tungsten is attached to the two ends of the wires. Now if current is passed through it, the filament gets extremely heated and emits light due to incandescence. As the melting point of tungsten is high (3380°C), it does not melt at high temperature. Again, to prevent it from corrosion, the bulb is filled with some inert gas like argon and in some cases, also with nitrogen gas. Nowadays, the filament is made up of wolframite (an alloy of iron, manganese and tungsten) instead of tungsten.

Q.8 Why is an electric bulb filled with an inert gas or nitrogen instead of making it free from air or filling it up with air?

Ans. When an electric bulb is made free from air, its filament vaporises at high temperature. After a few days, the filament decays, disintegrates and then finally vaporises to form a coating inside the bulb. As a result, the transparency of the glass is reduced and brightness of the bulb is decreased. On the other hand, if the bulb is filled with air, the filament will be corroded by oxygen of air to produce a metallic oxide at high temperature and the working power of the bulb is slowly reduced. For all these reasons, an electric bulb is filled with an inert gas or nitrogen. Only then there is no possibility of corrosion and as vaporisation is very low, the bulb works for a longer period of time.

Q.9 Describe an electric heater and its working principle.

Ans. In an electric heater, there is a notched roundshaped disc made of a bad conductor of electricity, like mica or fire clay, inside a metallic container. A long wire made of nichrome (an alloy of Ni, Cr and Fe) is set inside by twisting it like a spring. If the two ends of the nichrome wire are connected to an electric line with the help of a

plug, the coil becomes hot and red. As the resistivity of nichrome is high, it can be heated more and it is not corroded easily. Further, melting point of nichrome is high. For these reasons a nichrome wire is used in a heater. There is a base of porcelain on the mica or fire clay disc on which the cooking utensils are kept so that there is no possibility of connection between the cooking utensils and electricity.

Q.10 Though heat is generated continuously in an electric heater, its temperature becomes stable after sometime. What is the reason behind this? Sometime.

Ans. When electrical current is passed through a heater, heat is produced in the coil of the heater according to Joule’s law and the temperature of the coil increases. As a result, difference of temperature between the coil and the surrounding goes on increasing. More the difference of temperature, more is the rate of radiation of heat by the coil. In this way, the temperature of the coil increases and at a particular temperature, rate of production of heat in the coil is equal to the rate of radiation of heat by the coil. In this state, the temperature of the coil stops increasing further and becomes stable.

Q.11 Describe an electric iron and its working principle.

Ans. A nichrome wire coil is fitted within two trianguler shaped mica sheets in an electric iron. This is kept securely in an iron covering. When current is passed through the coil, it gets heated and as a result, the iron covering also gets heated. Clothes can be ironed with the help of this heated iron covering.

The bottom surface of the iron covering is made smooth. As the coil made of nichrome wire is I covered with mica sheets, there is no chance of getting any electric shock. But when there is a hole in the mica sheet, there arises some possibility of getting a shock from that iron. The handle of the iron is made of a material which is bad conductor of heat, so that at the time of ironing, no heat is felt by our hands.

Q.12 What is a fuse? Write down the working principle of an electric fuse. Why is it used?

Ans. Electric fuse is an arrangement that protects a circuit from excess flow of electric current. This is a type of wire with low melting point and is made of an alloy of tin and lead (tin 75%, lead 25% ). The wire is joined with a holder made of porcelain and then connected to an electric line.

Electric fuse is connected in a series combination with the live wire of an electric line. The melting point of a fuse wire is low and it cannot carry current of a value more than that of a fixed value. Due to any reason, when value of current in that electric line increases suddenly such that its crosses that fixed value, the wire melts to disconnect the circuit. Thus, electric fuse saves electrical appliances from getting burnt or damaged.

Due to joining of a fuse wire in a line, costly electrical appliances in the house (for example, refrigerator, washing machine, TV etc.) are protected against damage in case of excess flow of electric current. Above all, possibility of a fire due to burning of a line by short circuit in the house is also considerably reduced.

Q.13 Make a comparative discussion on Incandescent lamp, CFL and LED with respect to saving of energy.

Ans. In an incandescent lamp, nearly 98% of consumed electrical energy is converted to heat energy and only 2% produces visible light.

CFL (Compact Fluorescent Lamp) converts 7% to 9% of consumed energy into visible light whereas LED (Light Emilting Diode) converts nearly 4% to 18% of consumed energy into visible light. So, incandescent lamp does not save energy. CFL and LED save more energy than the incandescent lamp. Though CFL and LED are costlier than incandescent lamps, these reduce electric bill by saving a good deal of electrical energy in the long run.

Q.14 What are the main advantages of LED bulb in comparison to CFL?

Ans. LED bulb is an energy-saver compared to CFL in many ways. Some of the main advantages of LED bulb are as follows:

LED bulb produces very low green house gas.
Consumption of energy is comparatively lower in LED bulb.
[For example, whereas a CFL of 12-15 W consumes 55 kW-h of energy in a particular time interval, an LED bulb consumes only 28 kW.h of energy in the same time interval.]
The light of an LED bulb goes out immediately when it is switched off but the same does not happen in case of a CFL.
The life of an LED bulb is many times more than that of a CFL.
[For example, whereas one CFL can glow easily for 10000 hours, an LED bulb gives light for nearly 50000 hours.]
Light energy received from an LED bulb is much more than the same from one CFL of the same wattage.

Q.15 The coil of a heater does not radiate light whereas the filament of an electric lamp radiates light. Why?

Ans. The filament of an electric lamp is made of tungsten and is very thin. Again, the coil of a heater is made of nichrome whose area of cross section is more than the filament. As a result, the resistance of the filament of lamp is comparatively greater than the coil. Due to its high resistance, the filament is heated more and after becoming white hot, it starts radiating light. But as the resistance of the coil of the heater is less, it is not heated to that extent and does not become white hot to radiate light.

Q.16 How is the energy rating mark of an electrical machine done? There are two washing machines manufactured by two different companies. Four stars are marked on one machine and five stars are marked on another. Which one is more economical in terms of consumption of electrical energy?

Ans. The same machine of different models consume different amounts of electrical energy. To show this difference in consumption of electrical energy, some stars are marked on the machines. Amount of energy consumed by the machine can be compared by the number of stars. More the number of these stars on the body of a machine, less is the amount of energy spent by that machine as compared to other models. Therefore more stars signify more savings. This is how energy rating mark of an electrical machine is done.

Five stars marked washing machines is more economical in terms of consumption of electrical energy.

Q17. When we received the electric bill, we found that 24 units were consumed last month. What is the meaning of this unit? What is its name?

Ans. The meaning of consumption of 24 units in the last month is that 24 BOT units of electrical energy were spent in that house last month. That is the unit of consumed electrical power.

The name of the unit is Board of Trade Unit (BOT unit) or kilowatt-hour (kW.h).

Q.18 What do you mean by a short circult?

Ans. When two electrical lines of opposite nature or two opposite poles of an electrical cell are connected through very low resistance, excessive amount of current flows in the circuit. This is called short circuit. As excessive amount of heat is produced in a short circuit, sometimes leading to a fire in the circuit or a damage to the circuit components.

Q.19 220 V-100 W is written on the body of an electric lamp. What information does it convey?

Ans. If 200 V100 W is written on the body of an electric lamp, we can infer that if a potential difference of 220 V is applied across the two ends of the lamp, it will glow with maximum brightness and 100J of electrical energy is consumed by the lamp every second.

Q.20 Why is the filament of an electric lamp nt of an electric la made of tungsten?

Ans. Tungsten does not melt even at high temperatures as its melting point is 3380°C. In addition to this, as the resistivity of tungsten is high, the resistance of the wire is also high and thus it becomes very hot due to current flow to radiate light. Also, thin and long wire can be drawn from tungsten. For all these reasons, the filament of an electric lamp is made of tungsten.

Q.21 There are two thick conducting wires in an electric bulb at the ends of which the filament is attached. Why is light not emitted from them, even though the same amount of current passes through these wires?

Ans. As these two wires are short and their cross sectional area is large, the resistance is low. As the resistance is low, the heat produced is less in spite of passing the same amount of current. That is why no light is emitted.

Q.22 Why is a nichrome wire used in an wire used in an electric heater?

Ans. Nichrome is an alloy of nickel, chromium and iron. As the resistivity of nichrome is high, so it can be heated more and as its melting point is also high (1400°C approx.), it will not melt so easily. Further, it does not corrode easily at high temperature. For all these reasons, a nichrome wire is used in a heater.

Q.23 What do you mean by a 10A fuse?

Ans. A 10A fuse means that if an electric current of more than 10A passes through that fuse wire, the wire will become extremely hot and will melt immediately to disconnect the circuit. As a result, electric current through the circuit will stop flowing.

Q.24 The same amount of current is passed through one thin and another thick wire of the same material and of the same length for the same period of time. Heating of which wire will be more?

Ans. The resistance of a thin wire more than that of a thick wire of the same material and of the same length. Now we know that, H=I²Rt. Therefore, the thin wire will become hotter than the thick wire when the same current is passed through it for the same period of time.

Q.25 Two wires of the same material and of the same cross sectional area are taken. The length of the first one is greater than that of the second. When the same amount of current is passed through both the wires for the same period of time, heating of which wire will be more?

Ans. Out of the two wires made of the same material and having the same cross sectional area, the one with greater length has higher resistance. Therefore the resistance of the first wire is more than that of the second wire. So, when the same amount of current passed through both the wires for the same period of time, the first wire gets more heated.

Q.26 If same amount of electric current is passed for the same time duration through two copper wires-one thick and one thinheating of which wire will be more? Explain with reasons.

Ans. Between the two copper wire, the resistance of the thin wire is more. We know that if electric current and time of flow of current remain unchanged, then heat produced in the conductor is directly proportional to the resistance of the wire. Therefore, heat produced will be more in the case of the thin copper wire.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. Keeping the resistance and time constant, the amount of current is doubled, heat produced becomes

A. two times

B. four times

C. six times

D. eight times

Ans. B

2. A fuse wire is made of

A. tin

B. lead

C. an alloy of tin and lead

D. an alloy of aluminium and copper

Ans. C

3. The resistance of a 240 V – 60 W lamp is

A. 480 Ω

B. 960 Ω

C. 240 Ω

D. 720 Ω

Ans. B

4. What is the emf of the cell, if 10 J. of work is done in moving 2 C of charge once around an electric circuit?

A. 10 V

B. 5 V

C. 2.5 V

D. 1 V

Ans. B

5. Star sign on an electric machine indicates

A voltage rating

B. watt rating

C. energy rating

D. ampere rating

Ans. C

6. Power consumed to send a current of 2 A through a potential difference of 5 V is

A. 20 W

B. 5 W

C. 10 W

D. 15 W

Ans. C

7. Power consumed to send a current of 4 A through a resistance of 2 Ω is

A. 64 W

B. 16 W

C. 24 W

D. 32 W

Ans. D

8. Power consumed to maintain a potential difference of 10 V at the two ends of a resistance of 10 Ω is

A. 10 W

B. 100 W

C. 1 W

D. 1000 W

Ans. A

9. 0.2 W. h = how many joules?

A. 360

B. 720

C. 1080

D. 1440

Ans. B

10. Which of the following bulbs has the highest resistance?

A. 220 V-25 W

B. 220 W-60 W

C. . 220 V-100 W

D. 220 V-40 W

Ans. A

11. Which of the following bulbs has the lowest resistance?

A. 220 V-25 W

B. 220 V-60 W

C. 220 V-100 W

D. 220 V-40 W

Ans. C

12. When the following four bulbs are connected in series, which one will glow brightest?

A. 220 V-25 W

B. 220 V-60 W

C. 220 V-100 W

D. 20 V-40 W

Ans. A

13. Among the following light emitters of equal watts, which one saves maximum power?

A. CFL

B. LED bulb

C. incandescent bulb

D. tube light

Ans. B

14. Electric iron works on the principle of

A. action of magnet on electric current

B. action of electric current on magnet

C. action of electromagnetic induction

D. production of heat due to flow of current

Ans. D

15. A constant potential difference is applied at the two sides of a uniform wire. Heat produced is doubled if

A. radius of the wire is doubled

B. both length and radius are doubled

C. both length and radius are halved

D. length is doubled but radius is halved

Ans. C

16. The resistance of a 220 V-100 W lamp is

A. 968 Ω

B. 1936 Ω

C. 484 Ω

D. 242 Ω

Ans. C

17. A current of 1 A is passed through a resistance of 10 Ω for 4.2 minutes. What is the amount of heat produced?

A. 500 cal

B. 600 cal

C. 700 cal

D. 800 cal

Ans. B

18. 1 kW = how many W?

A. 100

B. 1000

C. 500

D. 5000

Ans. B

19. 1 MW = how many W?

A. 10²

B. 10⁶

C. 10³

D. 10⁴

Ans. B

20. 1 W · h = how many J?

A. 36

B. 360

C. 3600

D. 36000

Ans. C

21. The amount of heat produced in a resistor when a current is passed through it can be found using

A. Faraday’s law

B. Ampere’s law

C. Joule’s law

D. Ohm’s law

Ans. C

Answer in brief

1. What is the step taken to prevent the corrosion of the filament of an electric bulb?

Ans. To protect the filament from oxygen in the air, the air in the bulb is either removed or replaced with an inert gas like neon or argon.

2. What is a gas-filled bulb?

Ans. A bulb filled with an inert gas or nitrogen (N₂) is called a gas-filled bulb.

3. Between the filament and the fuse wire of an electric bulb, which one has greater cross section?

Ans. The cross section of a fuse wire is more than that of a filament of an electric bulb.

4. What is the material used for making the heating coil of a heater?

Ans. The heating coil of a heater is made of an alloy of Ni, Cr and Fe known as nichrome.

5. What happens when there is a hole in the mica sheet of an electric iron?

Ans. There is possibility of getting an electric shock if there is a hole in the mica sheet of an electric iron.

6. Which material is used for the manufacturing of an electric fuse?

Ans. An alloy of tin and lead (tin 25%, lead 75%) is used for the manufacturing of an electric fuse.

7. What is the nature of the resistivity and the melting point of a fuse wire?

Ans. Both the resistivity and the melting point of a fuse wire are very low.

8.What is the unit of electric power in SI?

Ans. The unit of electric power in SI is watt (W).

9. What is the voltage at which a 220 V-100 w lamp glows with maximum brightness?

Ans. A lamp of 220 V-100 W glows with maximum brightness when the potential difference across it is 220 V.

10. While purchasing an electrical appliance, which of the following factors should be given maximum importance?

Voltage rating, watt rating, energy rating.

Ans. While purchasing an electrical appliance, the utmost importance should be given to energy rating.

11. What is the commercial unit of electrical energy?

Ans. BOT unit is the commercial unit of electrical energy.

12. There are two bulbs of 220 V-100 W and 220 V-50 W respectively. Which one has more resistance?

Ans. The 220 V-50 W bulb has more resistance than the 220 V-100 W bulb.

13. Write full form of CFL.

Ans. CFL stands for Compact Fluorescent Lamp.

14. Which harmful element is used in CFL?

Ans. Mercury is used in CFL.

15. What is the full form of LED?

Ans. The full form of LED is Light Emitting Diode.

16. Between CFL and LED which have long lifespan?

Ans. Between CFL and LED, LED have long lifespan.

Fill in the blanks

1. The unit of electrical energy in SI is …………..

Ans. joule

2. The value of the mechanical equivalent of heat is …………

Ans. 4.2 J/cal

3. An electric fuse saves us from accidents and short-circuits by ………….. when an excess current flows through it.

Ans. melting

4. The filament of an electric bulb is made of ………..

Ans. tungsten

5. The coil of the nichrome wire in an electric iron is covered with a ……….. foil.

Ans. mica

6. 1 MW = …………… kW

Ans. 1000

7. The unit of electrical power is multiplied by the unit of ………… to get an unit of electrical energy.

Ans. time

8. W · h is the unit of …………..

Ans. electrical energy

9. In a plant producing thermal electricity, plenty of ………….. gas is emitted which pollutes the environment.

Ans. CO₂

10. 1BOT unit = …………. W · h

Ans. 1000

State whether true or false

1. In an electric cell, electrical energy gets converted into chemical energy.

Ans. False

2. If the potential difference across the two ends of a wire is halved, keeping resistance and time of flow of current same, then the amount of heat produced will become 4 times.

Ans. False

3. The filament of an electric bulb is made of tungsten.

Ans. True

4. Lamps, electric fans and refrigerators used in our house are connected in series combination.

Ans. False

5. CFL, LED lamps are more efficient than incandescent lamps.

Ans. True

6. A fuse is used to avoid short circuit damage.

Ans. True

7. If ac is passed through a conductor istead of dc, heat is not produced.

Ans. False

8. 1 watt = 1 V × 1 A

Ans. True

10. Resistance of a 220 V-100 W lamp is 484Ω.

Ans. True

11. Poisonous mercury vapour used in CFL.

Ans. True

TOPIC – D

Electromagnetism

SHORT AND LONG ANSWER TYPE QUESTIONS

Q.1 Describe Oersted’s experiment to demonstrate the magnetic effect of electric current.

Or, Show the magnetic effect of electric current with the help of a simple experiment.

Ans. Scientist Hans Christian Oersted performed an experiment in 1820 to demonstrate the magnetic effect of electric current. The following experiment is done on the basis of the same.

Required instruments: Two batteries of 1.5 V each, a thick copper wire, connecting wires, battery holder, switch, magnetic needle, resistor.

Experiment: Two batteries are put in the battery holder and then connection is made as shown in Fig 30 with thick copper wire, resistor (R) and switch through connecting wires. The entire setup is kept by the side of a table. AB is the thick copper wire. Now wire AB is placed outside the table so that wire AB does not fall off or slide. Further, wire AB is kept such that A remains in the south and B in the north. The magnetic needle is kept below the wire AB with the switch remaining off. In this state, magnetic needle remains along the direction of north-south. Now, when the switch is flipped on, it can be seen that the north pole of the magnetic needle is deflected towards the west.

Now, the poles of the batteries are interchanged. As a result, flow of current is from B to A in the wire AB or from north to south direction. As done previously, if the magnetic needle is kept below the wire AB, it is observed that the deflection of the north pole of the magnetic needle is towards east.

Conclusion: From this experiment of Oersted, we can conclude that a magnetic field is produced around a current carrying wire and if the direction of current is reversed, the direction of magnetic field is also reversed.

Q.2 In which direction does the deflection of the north pole of a magnetic needle take place when it has been kept below a current carrying wire in Oersted’s experiment. when the direction of current flow is from: (1) south to north (2) north to south?

Ans. (1) When the direction of current flow through the wire is from south to north, then the north pole of the magnetic needle gets deflected towards west.

(2) When the direction of current flow through the wire is from north to south, then the north pole of the magnetic needle gets deflected towards east.

Q. 3 Explain the similarity between the magnetic fields created by a bar magnet and by the flow of current in a circular conductor.

Ans. The lines of force obtained in case of a bar magnet [Fig. 32] and in case of a circular current carrying conductor [Fig. 33] show that a circular current carrying conductor is equivalent to a bar magnet since their lines of force are similar.

In Fig. 33, if the current flows in a clockwise direction in the circular conductor, the front face

of the circular conductor acts like the south pole of a bar magnet.

If seen from the opposite side, the direction of flow is anticlockwise and that side acts like the north pole.

Q.4 If a current carrying circular coil is kept in a hanging position, what is the final alignment of the coil? Why?

Ans. When a current carrying circular coil is kept hanging, it settles such that its axis is along north-south direction. If we look at the coil from the south, flow of current is clockwise and if it is seen from the north, flow of current is anticlockwise. This is because a current carrying circular coil acts like a bar magnet.

The side from which the flow of current is found to be clockwise acts as the south pole and the side from which the flow of current is found to be anticlockwise acts as the north pole.

Q.5 Write down Fleming’s left hand rule.

Ans. According to Fleming’s left hand rule, if the thumb, the forefinger and the middle finger of the left hand is held mutually perpendicular to each other in such a way that the forefinger points to the direction of the magnetic field and the middle finger points to the direction of the electric current, then the thumb indicates the direction of motion of the conductor or the direction of the force acting on the conductor.

Q.6 Explain the working principle, construction and working of a Barlow’s wheel with a diagram.

Ans. Working principle: A force acts on an electrical conductor due to the effect of magnetic field. In Barlow’s wheel, a star-shaped metal wheel with several spokes rotates continuously due to this force. Here, electrical energy is converted into mechanical energy.

Construction: A star-shaped copper wheel with several sharp teeth is mounted on a metal rod (B) in such a way that the wheel rotates along a horizontal axis in a perpendicular plane. The metal rod is attached with the wooden base by the help of a stand (7). Just below the wheel and above the wooden base, a container filled with mercury (M) is placed in such a way that each tip of the tooth just dips into the container while rotating. The container (M) is placed in between the two poles of a horse-shoe magnet (NS). Now the axis of the wheel and mercury are connected to a battery and switch with the help of two binding screws (S₁ and S₂) and conducting wires. Working: If the switch is flipped on, electric current passes along the path shown by arrows. Now, as the direction of the magnetic field and the lirection of current through the wheel are mutually perpendicular to each other, so, according to Fleming’s left hand rule, a force is applied on the tooth of the wheel which touches mercury.

Due to this force, the wheel rotates towards right. Now as the wheel starts rotating, the tip of the tooth which is in contact with mercury rotates and the circuit gets disconnected. But due to the inertia of motion, the next tooth comes in contact with mercury and again the whole process continues and the wheel of Barlow’s wheel keeps rotating continuously.

Q.7 What happens to the rotation of Barlow’s wheel if:

(1) the current flow is in opposite direction?

(2) the two poles of the magnet are reversed?

(3) the current flow is in opposite direction and the poles of the magnet are reversed simultaneously?

(4) ac is applied Instead of dc?

Ans. (1) If the current flows in the opposite direction in a Barlow’s wheel, then by keeping the direction of magnetic field the same, the wheel starts rotating in the opposite direction.

(2) If the current flows in the same direction but the two poles of the magnet are reversed, the Barlow’s wheel starts rotating in the opposite direction.

(3) If the current flows in the Barlow’s wheel in the opposite direction and the two poles are reversed simultaneously, the Barlow’s wheel keeps rotating in the same direction.

(4) If ac is applied instead of dc, the rotation of the Barlow’s wheel stops.

Q.8 Describe the construction and working instruction and workin of an electric motor.

Ans. Construction: The main parts of an electric motor are : (1) field magnet, (2) armature, (3) commutator and (4) brush.

(1) Field magnet: This is a powerful horseshoe electromagnet. The strength of the magnet may be increased as required by increasing the amount of current or the number of wounds per unit length.

(2) Armature: This is a rectangular coil ABCD made of insulated copper wire. The coil is wound on a soft iron bar. The coil is kept in such a way that the two arms of the coil, AB and CD are at right angles to the magnetic field.

(3) Commutator: The two ends of the armature are joined with two half-rings E and F. (This is also called split-ring device.) This is called a commutator. When the armature rotates, these two half-rings also rotate.

(4) Brush: The outer surfaces of the two half-rings touch two carbon brushes B₁ and B₂. A battery and a switch are connected to these two brushes through conducting wires.

Working: When current is passed, it goes from A to B in the arm AB and from C to D in the arm CD. According to Fleming’s left hand rule, it is observed that the force acting on AB acts downwards and the force acting on CD acts upwards. Hence, the coil rotates anticlockwise. When the coil ABCD is vertical, then two brushes B₁ and B₂ come in the gap between the two half-rings E and F and connection is broken. But due to inertia of motion, half-ring F touches brush B₁ and half-ring E touches brush B₂. As a result, direction of current in the arm AB is from B to A and in the arm CD, it is from D to C. In this state, according to Fleming’s left hand rule, the arm AB is deflected upwards and the arm CD is deflected downwards. In this way, current continues to flow in the circuit and the armature rotates in the same direction.

Q.9 What is the direction of magnetic field? How would you determine the direction of a magnetic field?

Ans. The direction towards which a magnetic field exerts force on a small isolated north pole placed in it gives the direction of the magnetic field.

As it is not possible to get an isolated magnetic pole, so the deflection of the north pole of a magnetic needle is used to determine the direction of a magnetic field.

Q.10 When an electric current is passed through a conducting wire, does the wire get magnetised?

Ans. No, the wire does not get magnetised in this case. If we keep some iron filings on a piece of paper which is in contact with a conducting wire, we would find that the iron filings are not attracted by the wire.

Q.11 Write down Ampere’s swimming rule.

Ans. According to Ampere’s swimming rule, if a man swims along the current carrying wire in the direction of the flow of current with his face always towards the compass needle, then the north pole of the compass needle gets deflected towards his left hand.

Q.12 Write down the right hand grip rule.

Ans. According to right hand grip rule, if a current carrying wire is held in right hand such that the thumb points towards the direction of flow of current, then the other fingers holding the wire indicate the direction of magnetic field lines.

Q.13 Can Barlow’s wheel be called a motor?

Ans. In an electric motor, electrical energy is transformed into kinetic energy. In Barlow’s wheel, electrical energy is transformed into rotational kinetic energy. Hence, Barlow’s wheel can be called an electric motor.

Q.14 How would you increase the speed of the speed of rotation of a Barlow’s wheel?

Ans. When the pole strength of the magnet or the amount of electric current increases, then the speed of rotation of Barlow’s wheel also increases.

Q.15 When current is passed through a conducting circular coll, which magnetic which magneti pole is formed in which face?

Ans. South pole is formed on that face of the coil through which current flows in a clockwise direction, whereas north pole is formed on that surface of the coil through which current flows in an anticlockwise direction.

Q.16 What is an electric motor?

Ans. Electric motor is an electrical appliance where a current carrying coil rotates around a fixed axis under the influence of a uniform magnetic field and transforms electrical energy into mechanical energy.

Q.17 A dc motor is rotating clockwise. How is it possible to change this direction?

Ans. When the pole of the magnet is reversed or the direction of current is reversed, then the direction of rotation of a motor is also reversed, i.e., it rotaties in an anticlockwise direction.

Q.18 Write the uses of an electric motor. How would you increase the speed of rotation of the armature of an electric motor?

Ans. Electric motor is used in electric fans, MP3 players, pumps, trains, rolling mills etc.

The speed of rotation of the armature can be increased by increasing the number of windings or increasing the current or increasing the strength of the magnetic field.

Q.19 The north pole and the south pole of a magnetic needle are not indicated. How would you identify the poles of a magnet with the help of a conducting wire and a battery?

Ans. The conducting wire is connected to the battery. Now, the wire is kept in such a way that the electric current in the wire flows from south to north. If the magnetic needle is then kept below the wire, the pole that gets deflected towards the west is the north pole. The remaining one is the south pole.

Q.20 The positive terminal and the negative terminal of a battery are not indicated. How would you identify the terminals of the battery with the help of a conducting wire and a magnetic needle?

Ans. Generally the magnetic needle aligns itself in the north-south direction. The conducting wire is connected with the battery and is placed along the north-south direction. Now, the magnetic needle is kept below the wire. If the north pole is deflected towards west, then the wire at the south end has been connected with the positive terminal and if the north pole is deflected towards east, then the wire at the south end has been connected with the negative terminal.

Q.21 What is the function of carbon brushes in dc motor?

Ans. Carbon brushes make contact with the commutator to supply current to the armature in dc motor.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. Electrical energy is produced from mechanical energy in

A. Barlow’s wheel

B. electric motor

C. electric generator

D. transformer

Ans. C

2. Barlow’s wheel rotates when

A. dc is passed

B. ac is passed

C. ac or dc is passed

D. no current is passed

Ans. A

3. Barlow’s wheel is a

A. simple dc motor

B. simple ac motor

C. dc generator

D. ac generator

Ans. A

4. The liquid used in the arrangement of Barlow’s wheel is

A. glycerin

B. alcohol

C. mercury

D. kerosene

Ans. C

5. Which of the following converts electrical energy into mechanical energy?

A. dynamo

B. transformer

C. electric motor

D. inductor

Ans. C

6. Rotation speed of the armature of a motor can be increased by

A. increasing current intensity

B. increasing number of turns of the coil

C. increasing the strength of the magnetic field

D. all of these

Ans. D

7. According to Fleming’s left hand rule, the middle finger indicates the direction of

A. magnetic field

B. current

C. motion

D. force

Ans. B

8. Rotation of Barlow’s wheel occurs according to

A. Fleming’s left hand rule

B. Ampere’s swimming rule

C. right hand grip rule

D. Fleming’s right hand rule

Ans. A

Answer in brief

1. What can be determined with the help of Ampere’s swimming rule?

Ans. The direction of the magnetic field around a long wire carrying electric current can be determined with the help of Ampere’s swimming rule.

2. When a man swims towards east, what is the direction of deflection of the north pole of the magnetic needle according to the Ampere’s swimming rule? [Suppose the current flows from west to east.]

Ans. There is no deflection of the north pole of the magnetic needle.

3. According to Ampere’s swimming rule, in which direction a man should swim for the deflection of the north pole of a magnetic needle to be towards west. [Suppose the current flows from south to north.]

Ans. The man should swim from south towards north.

4. In a magnetic field, when a force acts on the south pole of a magnet towards the east direction, then what is the direction of the magnetic field?

Ans. The direction of the magnetic field is towards the west.

5. Would two magnetic lines of force intersect each other?

Ans. No, two magnetic lines of force would never intersect each other.

6. Can ac (alternating current) rotate Barlow’s wheel?

Ans. No, an ac cannot rotate a Barlow’s wheel.

7. What is magnetic flux?

Ans. The number of magnetic lines of force passing perpendicularly through a plane placed in a magnetic field is called magnetic flux.

8. What type of electric current can rotate a Barlow’s wheel?

Ans. dc (direct current) can rotate a Barlow’s wheel.

9. What type of energy transformation does take place in an electric motor?

Ans. Electrical energy is transformed into mechanical energy in an electric motor.

10. What type of energy transformation does take place in a dynamo?

Ans. Mechanical energy is transformed into electrical energy in a dynamo.

11. Between thermal energy and hydroelectric energy which one is renewable?

Ans. Hydroelectric energy is renewable energy.

12. How do you determine the direction of rotation of Barlow’s wheel?

Ans. The direction of rotation of Barlow’s wheel is determined by Fleming’s left hand rule.

13. When the south pole of a bar magnet is brought towards a closed coil along its axis, what is the direction of current in the front face of the coil?

Ans. The direction of current in the front face of the coil is clockwise.

14. Name the device which is used to reverse the direction of current in the coil of a motor after every half rotation.

Ans. Commutator.

15. State the function of a split ring in a de motor.

Ans. Function of split ring in a dc motor is to reverse the direction of current in the coil of a motor after every half rotation.

Fill in the blanks

1. In Fleming’s left hand rule, the forefinger indicates the direction of ………..

Ans. magnetic field

2. Magnetic lines of force emerge from the ………… pole of a magnet.

Ans. north

3. When an electric current is passed through a conducting wire, a …………. field is produced around it.

Ans. magnetic

4. When current is flowing through a conductor, the conductor is not ……….

Ans. magnetised

5. For a circular current carrying loop, the face of the coil in which current appears clockwise develops ………. pole.

Ans. north

6. If ac is passed through a dc motor, its armature does not …………

Ans. rotate

7. Magnetic lines of forces may be …………. curve.

Ans. closed

State whether true or false

1. Direction of rotation of Barlow’s wheel is determined by right hand grip rule.

Ans. False

2. A static charge may produce magnetic field.

Ans. False

3. The direction of magnetic field at any point on a magnetic line of force is along the tangent drawn on that point.

Ans. True

4. In right hand thumb rule the thumb indicates the direction of flow of current.

Ans. True

5. A circular wire carrying current acts as a bar magnet.

Ans. True

6. Magnetic field intensity inside a spiral current carrying conductor increases when an iron rod is placed inside the coil.

Ans. True

7. Strength of an electromagnet can be increased as much as desired.

Ans. False

8. Galvanometer is an electrical device which is based on the principle of magnetic effect of electric current.

Ans. True

9. The magnitude of force applied on a long straight current carrying wire is maximum when it is placed perpendicular to the direction of magnetic field.

Ans. True

10. Direction of the magnetic field around a long wire carrying current is determined by right hand thumb rule.

Ans. True

TOPIC – E

Electromagnetic Induction and Domestic Electrical Circuit

SHORT AND LONG ANSWER TYPE QUESTIONS

Q.1 Write down Faraday’s laws of electromagnetic induction.

Or, Write down the two laws of Faraday regarding electromagnetic induction.

Ans. First law: Whenever there is a change in the magnetic flux linked with a closed coil, an electromotive force is induced in the coil. The induced emf lasts as long as the magnetic flux continues to change.

Second law: In case of electromagnetic induction, the magnitude of the induced emf (electromotive force) is directly proportional to the rate of change of magnetic flux linked with the coil.

Q.2 State Lenz’s law. Explain Lenz’s law with the help of the law of conservation of energy.

Ans. According to Lenz’s law, the direction of any magnetic induction effect is such as to oppose the cause of the effect.

Let us assume that the N pole of a bar magnet is brought near a closed coil along its axis and due to this, the direction of induced current in the coil is clockwise. That means, S pole is formed in the front face of the coil. This S pole attracts the N pole of the bar magnet. As a result, the bar magnet accelerates towards the coil. In this case, it is found that without the supply of any external energy, kinetic energy of the magnet and also electrical energy are obtained which go against the principles of law of conservation of energy. Therefore, when the N pole of the bar magnet proceeds along the axis of the coil, then flow of current in the coil is anticlockwise. As a result, N pole is created in the front face of the coil instead of S pole and it repulses the N pole of the bar magnet. To move the bar magnet towards the coil along its axis, some mechanical work has to be done. This mechanical work is converted into electrical energy. Therefore, the direction of the induced emf is such that it opposes the very cause responsible for its production-this is Lenz’s law.

Q.3 Show that Lenz’s law supports the law of conservation of energy.

Ans. It is known from the phenomenon of electromagnetic induction that if there is a relative motion between a magnet and a coil, a current is induced in the coil. It has been found by experiment that if the magnet is brought near or taken away from the coil, some resistance acts always. As a result, in order to maintain a relative motion between the magnet and the coil, positive work has to be done against this resistance. This work is manifested as electrical energy in the coil. In other words, the law of conservation of energy is valid in this case. Lenz’s law states that the direction of induced emf is such that it opposes the cause responsible for its production. Hence, Lenz’s law supports the law of conservation of energy.

Q.4 By keeping a copper ring horizontal, a bar magnet is dropped freely from a steep height through the centre of the ring. Is the acceleration of the falling ring equal to or more than the acceleration due to gravity?

Ans. When the bar magnet is dropped from a steep height towards the centre of the copper ring, then magnetic flux linked to the copper ring increases, that is, there is a change of magnetic flux linked to the copper ring. As a result, an electromotive force is induced in the ring and electric current starts to flow. The direction of induced current is such that it opposes the very cause which produces it. In this case, the fall of the bar magnet is opposed. Therefore, a repulsive force acts upward on the bar magnet so that it falls downwards with lesser acceleration than the acceleration due to gravity.

Q.5 What do you mean by direct current and alternating current?

Ans. Direct current (dc): When the direction of an electric current is always the same, it is called dc.

Alternating current (ac): When the direction of an electric current reverses at regular intervals, it is called ac.

In Fig. 44(a) and Fig. 44(b), change of electric current (I) is shown with change of time (t) with the help of a graph. In Fig. 44(a), the direction of current does not change but in Fig. 44(b), the direction of current reverses at regular intervals. So in Fig. 44(a), current is dc and in Fig. 44(b), it is ac.

Q.6 What are the advantages of ac over dc?

Ans. The advantages of ac over dc are as follows:

With the help of a transformer, ac may be transformed from low voltage to high voltage (step-up transformer) and from high voltage to low voltage (step-down transformer). This advantage is not available in case of dc.
The cost of generation of ac by a generator is less than that of dc.
Loss of energy while transmission is comparatively less in ac.

Q.7 Discuss the working principle of an dynamo in brief.

Ans. In Fig. 45, the main parts of an ac dynamo are shown. The main parts are field magnet, armature, slip ring and brush. N and S are the two poles of a horse-shoe magnet, which is called a field magnet. But, generally, an electromagnet is used in place of a field magnet as a permanent magnet. This field magnet creates a nearly constant magnetic field between the two poles, directed from north pole to south pole. ABCD is a rectangular coil made by winding insulated copper wires on a rectangular bar made of soft iron. This is called an armature. The open ends of the armature coil are connected with two perfectly smooth round-shaped rings made of brass. These two rings R₁ and R₂ rotate with the rotation of the coil. B₁ and B₂ are two carbon brushes. When the coil ABCD rotates with the rings, the brushes touch the two rings lightly. A resistance R is connected to brushes B₁ and B₂ in the external circuit. Now, when the coil ABCD is rotated, magnetic flux linked with the coil changes. As a result, electromotive force is induced in the and current flows in the circuit.

When the arm AB of the coil ABCD comes down, then the direction of current in the resistance R becomes opposite to the direction of the current when the arm AB goes up. Hence, during one full rotation of the coil ABCD, the direction of current in the resistance R changes twice. In this way, as the coil ABCD rotates with a uniform angular velocity, the direction of current in the resistance changes direction periodically at a fixed interval of time and the generated current is ac in nature.

Q.8 Discuss the working principle of a dc dynamo in brief.

Ans. In Fig. 46, the main parts of a dc dynamo are shown. The main parts are field magnet, armature, commutator and brush. N and S are two poles of a horseshoe magnet, which is called a field magnet. But, in general, an electromagnet is used in place of a field magnet as a permanent magnet. This field magnet creates a nearly constant magnetic field between the two poles, directed from north pole to south pole. ABCD is a rectangular coil made by winding insulated copper wires on a soft iron cylindrical bar. This is called an armature. The open ends of the armature coil are connected with two semi-circular sheets R₁ and R₂ made of brass. R₁ and R₂ together form a commutator. When the coil rotates, the commutator also rotates. B₁ and B₂ are two carbon brushes. When the coil AB rotates with the commutator, then the brushes touch the two sheets of the commutator lightly. A resistance R is connected with the brushes B₁ and B₂ in the external circuit. Now when the coil rotates, magnetic flux linked with the coil changes. As a result, electromotive force is induced in the coil and current flows in the circuit. When current is passed through the coil in the direction of ABCD, then applying Fleming’s left hand rule we can say that the arm AB experiences a downward force while arm CD experiences an upward force. After crossing the vertical position, the arm AB tries to move upwards and the arm CD tries to come downwards due to inertia of motion. At that time, sheet R₁ comes in contact with brush B₂ and sheet R₂ comes in contact with brush B₁. As a result, the current is unidirectional instead of rotating in the opposite direction. Thus, it flows through the resistance R in the same direction in the external circuit and dc is generated.

Q.9 Write down the difference between dynamo and an electric motor.

Ans. The difference between a dynamo and an electric motor are as follows:

Q.10 Describe, in brief, the principle of generation of thermal electricity.

Ans. In a simple turbine, some blades are attached to the end of a rod which is connected to the coil of a dynamo. If the turbine is rotated, the coil also rotates and electrical energy is generated.

In a thermal power station, water is boiled by burning coal or any other fuel and transformed into steam. This steam rotates the blades of turbine and consequently, the armature coil of the dynamo rotates. As a result, there is a change in the magnetic flux linked with the coil and an electromotive force is induced. Thermal electricity is generated in this way.

This steam is again condensed in a condenser and is sent back by Rankine cycle to the place where it is heated.

Q.11 Briefly describe the principle of generation of hydroelectricity.

Ans. In a hydroelectric power plant, water is stored in reservoirs. Now this water stored at a great height is allowed to fall downwards. As the mass of water falls down, its potential energy decreases and kinetic energy increases. If a blade of a turbine is kept in this falling water, it rotates and the armature coil of the dynamo also rotates with it. As a result, there is a change in the magnetic flux linked with the coil causing an electromotive force to be induced and thus hydroelectricity is generated.

Q. 12 Explain the arrangement of electric line in a house with diagram.

Ans. For domestic supply of electricity, two wires are drawn from the overhead or underground cable of the electric supply company. One is live wire and the other is neutral wire. In general, live wire is covered with red plastic and neutral wire is covered with black plastic. According to international guidelines, live wire is now covered with brown plastic and neutral wire is covered with light blue (sky coloured) plastic.

Live wire and neutral wire are first connected with the meter. Now, the live wire is passed through the main fuse and neutral wire is taken directly from the meter and connected with the main switch. The line of the house can be switched ‘on’ and ‘off’ as per requirement with the help of the main switch. As the potential difference between the earth and the neutral line is not always zero, an iron rod is inserted inside the ground and that rod is connected with the conducting wire. This arrangement is called earthing and the wire used for this is called the earth wire. The earth wire is green or yellow in colour. The earth wire is taken to the switch board meant for electrical appliance (iron, refrigerator, table fan etc.) through the main switch. Live wire goes to different switch boards from the main switch through several branch lines. Points are made, according to requirement, in the switch board for use of fan, tv, lamp etc. Live wire is connected to each switch board through a fuse. Next, the wiring of the house is done from switch board through live wire and neutral wire. Lamp, refrigerator, electric fan etc., in the house remain in a parallel combination.

Q.13 What is earthing? Why is it done? How earthing is done?

Ans. Connecting an electrical circuit or an electrical appliance to the earth through a conducting wire is known as earthing.

Due to any defect in the electrical connection, the metal coating of the appliance may get electrified. There is a possibility of danger due to that. If anybody standing on the ground touches that appliance, the person may get an electric shock. Earthing is done to avoid that possibility.

For earthing, a conducting rod made of metal is driven into the ground. In the household circuit, there is an extra wire (earthing wire) connected to this rod. In case of a three-pin socket, the comparatively thicker hole is connected to this wire. The resistance of this wire is very low. There is a connection between the metal coating of the electrical appliance and the earthing wire. The potential of the earth is zero. So, if anyhow the coating of the instrument is electrified, that electricity goes to the earth directly. Therefore the possibility of getting a shock is minimised.

Q.14 Explain the functioning of a switch and tioning of a s a main switch.

Ans. Switch is an arrangement in an electrical circuit by which the flow of electric current can be made ‘off’ and ‘on’. Switch is generally made of ebonite and is fixed on the respective board attached on the surface of the wall. Generally, a 5A switch is used in the circuit of electric fan, bulb, tubelight and a 10A or 15A switch is used in the circuit of pump, refrigerator, heater.

Main switch is an arrangement which may be made ‘off’ and ‘on’ according to necessity to disconnect or connect the electric line of the house from the main electric supply line. As a result, the flow of electric current in homes can be stopped or started according to one’s will.

Q.15 What are electromagnetic induction and induction and induced current?

Ans. If there is change of magnetic flux associated with a closed coil, an electric current originates in the coil. This phenomenon is called electromagnetic induction and the current flowing in the coil is called induced current.

Q.16 What is induced electromotive force?

Ans. If there is a relative motion between a magnetic field and a conductor, then an electromotive force originates in the conductor which is called induced electromotive force.

Q.17 The north poles of two identical bar magnets are kept at the same height from the centres and at right angles to the planes of two identical closed circular conducting coils. Now both the bar magnets are brought near the coils to the same height, first one rapidly and the second one slowly. In which case is the induced current more?

Ans. Induction of current is more in the first coil because the first magnet was brought rapidly and thus, the rate of change of magnetic flux linked with the first coil is more than the second one. As a result, the amount of induced electromotive force is also more. Hence, induction of current is more in the first case.

Q.18 Along the axis of a circular coil made of wire, a cylindrical bar magnet is kept. The magnet rotates about the axis. Is there any induction of current in that coil?

Ans. When the magnet rotates about the common axis of the magnet and the circular coil, there is no change in the flux linked to the coil. So, no electromotive force is induced in the coil. As a result, no current is induced in the coil.

Q.19 A bar magnet is kept along the axis of a circular conducting coil. Now both the coil and the bar magnet are moved in the same direction with the same velocity. Is there any induction of current in the coil?

Ans. When both the coil and the bar magnet are moved in the same direction with the same velocity, there is no relative velocity between them. As a result, there is no change in the magnetic flux linked to the coil. Therefore, no electromotive force is induced and thus, no current is induced in the coil.

Q.20 What is a dynamo? How many types of dynamo are there? Name them.

Ans. Dynamo is a device in which mechanical energy converted into electrical energy by utilising the principle of electromagnetic induction.

It is of two types: (1) ac dynamo and (2) dc dynamo.

Q.21 What is the combination in which electric lamp, electric fan, refrigerator etc. are connected with the electric line of house? Why?

Ans. Electric lamp, electric fan, refrigerator etc. are connected in a parallel combination with the electric line of a house because any one of them can be switched on or off as per requirement and the potential difference across the two ends of every electrical appliance remains constant.

Q. 22 Write down the formation of a three-pin plug in brief.

Ans. In a three-pin plug, there are three metal pins. The top one is longer and thicker than the other two which are of equal size. The two lower pins are connected with the live wire and the neutral wire respectively. Earthing is done with the thicker and longer one.

Q.23 Why do electrical appliances like electric bulb, heater, iron etc. work both with ac and dc?

Ans. The electrical appliances like electric bulb, heater, iron etc. work on the principle that heat is produced when electric current passes through a conductor. Heat produced in a conductor due to the flow of electric current depends on the resistance, amount of current and time of flow of current, which are independent of direction. Hence, produced heat does not depend on ac or dc.

Q.24 why is the earth pin of a three-pin plug of a three-pin plug made longer and thicker?

Ans. The earth pin of a three-pin plug is made thicker so that this pin can not be inserted in the other two holes by mistake. Also, the earth pin is made longer so that the end of this pin gets connected with the socket before the other two pins and thereby reducing the chance of the user getting electric shock.

Q.25 Write down the construction of a socket in brief.

Ans. A socket is an important component of an electric circuit in which a plug is inserted. There are three holes in a socket. The lower two holes are connected with live wire and neutral wire whereas the upper, bigger hole is connected with earth wire. This socket is connected with the respective board.

Q.26 Why is the front portion of every pin of three-pin plug split?

Ans. The front portion of every pin of a three-pin plug is split length-wise so that proper connection can be made with the socket. Without proper connection between the pin and the socket, sparks may fly due to loose connection which can be very dangerous.

VERY SHORT ANSWER TYPE QUESTIONS

Choose the correct answer

1. The colour of a live wire is

A. brown

B. black

C. green

D. sky blue

Ans. A

2. The colour of a neutral wire is

A. brown

B. black

C. green

D. sky blue

Ans. B

3. Which of the following helps in the conversion of low voltage ac into high voltage ac?

A. converter

B. rectifier

C. step-up transformer

D. step-down transformer

Ans. C

4. Which of the following helps in the conversion of high voltage ac into low voltage ac?

A. converter

B. rectifier

C. step-up transformer

D. step-down transformer

Ans. D

5. Electromotive force (emf) is induced in a closed coil, when magnetic flux linked with the coil

A. purely increases

B. remains constant

C. purely decreases

D. either increases or decreases

Ans. D

6. The direction of the induced emf in a circuit is determined by which law?

A. Lenz’s law

B. Faraday’s first law of electromagnetic induction

C. Faraday’s second law

D. Ampere’s swimming rule

Ans. A

7. No emf is induced in a closed coil when magnetic flux linked with the coil

A. decreases

B. is constant

C. increases

D. changes

Ans. B

8. In electromagnetic induction, the induced emf in a coil is independent of

A. change in the flux

B. time

C. resistance of the coil

D. all of these

Ans. C

9. Lenz’s law is a consequence of the law of conservation of

A. charge

B. momentum

C. energy

D. mass

Ans. C

10. The instrument which converts mechanical energy into electrical energy is

A. dynamo

B. motor

C. galvanometer

D. Barlow’s wheel

Ans. A

11. The basic principle in which dc generator works is

A. magnetic effect of electricity

B. electromagnetic induction

C. chemical effect of electricity

D. heating effect of electricity

Ans. B

12. In domestic circuit appliances are connected

A. in series

B. in parallel

C. either in series or in parallel

D. both in series and parallel

Ans. B

13. In domestic circuit switch is connected in the

A. live wire

B. neutral wire

C. earth wire

D. fuse wire

Ans. A

14. The thicker and longer pin of the three-pin plug is connected to

A. live wire

B. neutral wire

C. live or neutral wire

D. earth wire

Ans. D

15. The main fuse is connected in

A. live wire

B. neutral wire

C. both the live and earth wire

D. earth wire

Ans. A

Answer in brief

1. When the north pole of a bar magnet is brought towards a closed coil along its axis, what is the direction of current in the front face of the coil?

Ans. The direction of current in the front face of the coil is anticlockwise.

2. When the north pole of a bar magnet is taken away from a closed coil along its axis, what is the direction of current in the front face of the coil?

Ans. The direction of current in the front face of the coil is clockwise.

3. When the south pole of a bar magnet is taken away from a closed coil along its axis, what is the direction of current in the front face of the coil?

Ans. The direction of current in the front face of the coil is anticlockwise.

4. With the help of which instrument, alternating current (ac) of high voltage can be transformed into ac of low voltage and vice versa?

Ans. With the help of a transformer, alternating current (ac) of high voltage can be transformed into ac of low voltage and vice versa.

5. In which direction does current pass through a live wire?

Ans. Current through a live wire passes from the direction of electricity supply station to the direction of electrical appliances.

6. In which direction does current pass through a neutral wire?

Ans. Current through a neutral wire passes from the direction of electrical appliances to the direction of electricity supply station.

7. What is the colour of live wire used in electric line in our homes?

Ans. The colour of live wire used in electric line in our homes is brown.

8. What is the colour of neutral wire used in electric line in our homes?

Ans. The colour of neutral wire used in electric line in our homes is sky blue.

9. By means of which gadget, the electric line in our homes can be switched on and off?

Ans. With the help of the main switch, the electric line in our homes can be switched on and off.

10. What is the colour of earthing wire?

Ans. The colour of earthing wire is either green or green with yellow stripe.

11. Which wire is connected with the top big hole of a three pin plug?

Ans. Earthing wire is connected with the top big hole of a three pin plug.

12. Which type of wire is generally used in house wiring?

Ans. Copper wire is generally used in house wiring.

13. Which metal wire is generally used for the transmission of electricity to a distant place?

Ans. Aluminium wire is generally used for the transmission of electricity to a distant place.

14. What is electromagnetic induction?

Ans. Electromagnetic induction is the phenomenon in which an emf is induced in a coil if there is a change in the magnetic flux linked with the coil.

Fill in the blanks

1. If there is change of ……….. linked with a closed coil, an emf is induced in the coil.

Ans. magnetic flux

2. The cost of production of dc with a generator is ……….. than that of ac.

Ans. more

3. When ac with high voltage is sent from a power station, loss of energy due to transmission of electricity is ……….

Ans. less

4. When magnetic flux in a closed coil remains ………….. , no emf is induced in the coil.

Ans. stationary

5. In …………… mechanical energy converted into electrical energy.

Ans. generator

6. Frequency of dc current is ………….

Ans. zero

7. ac can be converted into dc by ………….

Ans. rectifier

8. Lenz’s law actually follows the principle of conservation of ………….

Ans. energy

9. In our domestic supply line frequency of ac is …………… Hz.

Ans. 50

10. According to international convention, colour of the insulation of the neutral wire is ………..

Ans. blue

State whether true or false

1. Lenz’s law supports the law of conservation of energy.

Ans. True

2. Induced emf is produced in a closed coil when it is placed in a magnetic field.

Ans. False

3. If the north pole of a bar magnet moves towards a closed coil, the direction of induced current is anticlockwise in the front side of the coil.

Ans. False

4. Magnitude of induced emf can be calculated from Faraday’s first law.

Ans. False

5. In electroplating ac is used.

Ans. False

6. Production cost of ac is less than that of dc.

Ans. True

7. dc can be converted to ac by using converter.

Ans. True

8. EMF produced by a generator is directly proportional to its number of turns.

Ans. True

9. Switches are connected in neutral wire.

Ans. False

10. Fuse is always connected in the beginning of the circuit in live wire.

Ans. True

11. According to international convention, colours of insulation are brown for live, light blue of neutral and green for earth.

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WBBSE 10th Class Science Solutions Physical Science & Environment Chapter 6 Current Electricity

West Bengal Board 10th Class Science Solutions Physical Science & Environment Chapter 6 Current Electricity

WBBSE 10th Class Physical Science & Environment Solutions

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