MCQ Current Electricity Class 12 Physics Chapter 3

1. Path followed by free electrons in conductor, when it is placed in an external electric field is:

a) Straight line

b) Circular

c) Curved path

d) Elliptical

Ans. Option C Curved path

2. Average thermal speed of electrons in a conductor is:

a) 3 x 108 m/s

b) 10-4 m/s

c) 0 m/s

d) 10-6 m/s
Ans. Option C : 0 m/s as the electrons have random motion in the absence of electric field inside the conductor. Their thermal velocity averaged to be zero.

3. A cell of emf 8V with small finite internal resistance is charged with the help of an external battery. Terminal Potential drop across the cell while charging would be:

a) Greater than 8 V

b) Less than 8 V

c) Zero

d) Equal to 8 V

Ans: Option a : Greater than 8V while charging terminal drop of a cell is V = E + Ir as r being non zero V will always be more than E(i.e. 8V).

4. When three cells each of emf 3V and internal resistance 1 ohm is connected in series across a resistor. Then, a graph of terminal drop of the combination vs current drawn from it is found to be a straight line. Then Its Y-intercept and slope represents:

a) 9 V and effective internal resistance

b) 3 V and negative of effective internal resistance

c) 3 V and effective internal resistance

d) 9 V and negative of effective internal resistance

Ans: Option d: graph of terminal drop vs current is a straight line with negative slope, so Y, intercept represents effective emf of the combination (in series Eeff = E1+ E2+ E3 = 9V) and negative slope gives us effective internal resistance.

5. Name the material that offers less resistance with increase in temperature

a) Au

b) Ag

c) Si

d) Hg at 40C

Ans: Option c: remaining all are metals whose resistivity increase with increase in temperature. Whereas Si is a semiconductor, whose resistivity decreases with increase in temperature.

6. When a resistor x is kept in the left gap and y in the right gap of a meter bridge, balancing length is found to be l cm. what resistance is to be placed in the left gap to have the same balancing length l cm on placing x in the right gap?

a) X2/y

b) X2y

c) Y2x

d) Y2/x

Ans: Option A: in the first case x/y = l/(100-l) Second case z/x = l/(100-l), on comparing both z = x2/y

7. The best instrument for accurate measurement of EMF of a cell is-

a) Potentiometer

b) metre bridge

c) Voltmeter

d) ammeter and voltmeter
Ans: Option a: Potentio meter, as it draws no current from the cell

8. Name the physical quantity that is conserved in Kirchhoff’s loop rule:

a) Charge

b) mass

c) Energy

d) Momentum

Ans: Option C. Energy is conserved in Kirchhoff’s loop rule. Where as charge is conserved in Kirchhoff’s junction rule

9. How does the balancing length of a potentiometer change on increasing the resistance offered by the rheostat that is connected in series with the primary cell.

a) Increases

b) Decreases

c) Remains same

d) Turns zero

Ans: Option b: on increasing the resistance offered by primary resistance, total current in the primary decreases which will decrease the potential drop across the potentiometer wire. So to balance the given potential balancing length will increase.

10. a cell of emf 5V and internal resistance 1 ohm is connected across a heating element
of resistance 9 ohm, find the amount of heat lost in one sec is:

a) 2.25 Watt

b) 2.25 Joule

c) 2.5 Watt

d) 2.5 Joule

Ans: Option b: H = i2Rt here t= 1 sec, R=9 ohm and I = E/(R+r) = 5/(9+1)=0.5 A H= 0.25 x 9 x 1 = 2.25 joule

Q.No. 11 to 16 are assertion and reason questions, which consists of two statements, typed as Assertion and Reason. While answering these questions you are required to choose any one of the following four responses.
a) If both Assertion and Reason are True and the Reason is a correct explanation of Assertion.
b) If both Assertion and the Reason are True but the Reason is not a correct explanation of the Assertion.
c) If Assertion is true but the Reason is False
d) If both Assertion and the Reason are false.

11. Assertion: A domestic electric appliance working on a three pin, will continue working if the thick pin is removed.
Reason: The thick pin is used only as a safety device.

a) If both Assertion and Reason are True and the Reason is a correct explanation of Assertion.
b) If both Assertion and the Reason are True but the Reason is not a correct explanation of the Assertion.
c) If Assertion is true but the Reason is False
d) If both Assertion and the Reason are false.

Ans. Option A

12. Assertion: Conductivity of a metal is much higher than that of an electrolyte at room temperature.
Reason: Free electron density in metals is much lesser than the density of ions in
electrolytes and also free electrons have smaller mobility than ions.

Ans. Option C

13. Assertion: When Wheatstone bridge is balanced, the current through the cell depends on the resistance of the galvanometer.
Reason: In balanced condition, current through the galvanometer is very high.

Ans. Option D

14. Assertion: Potentiometer is an ideal instrument to measure the potential difference.
Reason: Potential gradient along the potentiometer wire can be made very small.

Ans. Option B

15. Assertion: The value of temperature of coefficient of resistance is positive for metals.
Reason: The value of temperature or coefficient of resistance is negative for insulators.

Ans. Option B

16. Assertion: When identical cells are connected in parallel to an external load, the effective emf increases.
Reason: All the cells will be sending unequal currents to the external load in the same direction.

Ans. Option D

Question 17-20 are case study based and source based questions, in which relevant data will be given in the form of a diagram or paragraph or the principle behind the topic. Based on which 4 MCQs will be asked.

17. Potentiometer: A potentiometer is an instrument that measures the terminal potential difference with high accuracy without drawing any current from the unknown source. It is based on the principle that if constant current is passed through a wire of uniform cross-section, then potential difference across any segment of the wire is proportional to its length. Sensitivity of potentiometer inversely depends on its potential gradient. In the below circuit length of AB is 1.00m emf of Ep is 5.0V, resistance offered by AB is 0.08 ohm per cm and Voltmeter reads 2 V with a balancing length of 50 cm.

i) What is the resistance offered by the rheostat in ohm in the above condition is:

a) 1

b) 2

c) 0.5

d) 0.25

Ans: Option b. As 50 cm is balancing 2.0V entire wire AB can balance 4.0 V as emf of primary is 5.0V, drop across Rheostat be 1.0V.
Resistance offered by AB is 8ohm so for a drop of 4.0V current through it will be 0.5A.
Now for a current of 0.5A and drop of 1.0V rheostat has to offer 2 ohm of resistance.

ii) On slightly increasing the emf of the primary cell how would the balancing length change?

a) Increases

b) decreases

c) remains same

d) turns zero
Ans. Option b. On increasing primary emf, potential gradient will increase which will decrease the balancing length.

iii) If voltmeter is replaced with series combination of two cells of emf E1 and E2 (E1>E2), balancing length is found to be 80cm, when the same cells are connected in opposite new balancing length is found to be 40 cm. the ratio of emf of the two cells(E1/E2) is:

a) 1:2

b) 2:1

c) 1:3

d) 3:1
Ans: Option d. E1+E2 = k 80, E1-E2=k40, on adding both E1=k60, and E2=k20 => E1/E2 = 3:1

iv) Sensitivity of a potentiometer can be increased by :

a) Decreasing potential gradient along the wire

b) Increasing potential gradient along the wire

c) Decreasing current through the wire

d) Increasing current through the wire
Ans: Option a

18. Electric energy: Whenever an electric current is passed through a conductor, it becomes hot after
some time. The phenomenon of the production of heat in a resistor by the flow of an electric current through it is called heating effect of current or Joule heating. Thus, the electrical energy supplied by the source of emf is converted into heat. In a purely resistive circuit, the energy expended by the source entirely appears as heat. But if the circuit has an active element like a motor, then a part of the energy supplied by the source goes to do useful work and the rest appears as heat.

i) Which of the following statements is not correct?

a) Heat produced in a conductor depends only on its resistance

b) Heat produced in a conductor depends only on current passing through it

c) With increase in time heat produced in a conductor decreases.

d) All of the above
Ans: Option d, Aloof the given statements are false

ii) If the coil of a heater is cut to one third, what would happen to heat produced?

a) Tripled

b) Becomes one third

c) Remains same

d) Becomes nine times
Ans: Option A. as length becomes one third resistance will also become one third. Heat produced is H = (v2/R)t, as R’ = R/3 => H’ = 3H

iii) 60W and 100W are joined in series and connected to the mains. Which bulbs will glow brighter?

a) 60W

b) 100W

c) both bulbs glow brighter

d) none will glow

Ans: Option A, H is directly proportional to R where R is inversely proportional to P. So more the power rating less the heat produced in series.

iv) Distance transmission of electrical energy is to be done at

a) High voltages to increase the power dissipated

b) High voltages to decrease the power dissipated

c) Low voltages to decrease the power dissipated

d) Low voltages to increase the power dissipated

Ans: Option b, power dissipated in transmission line is Pc = P2R/V2, here P is the power to be delivered, which remains constant, Pc α 1/V2, more the voltage less the power dissipated.

19. Kirchhoff’s laws(Wheatstone bridge):

These are two basic rules that will help us to solve the circuits that can not be resolved as simple series or parallel combinations. These laws deal with currents and voltage drops in a circuit.

i) What is the effective resistance between A and B in the above circuit

a) 4 ohm

b) 2 ohm

c) 1 ohm

d) 2.4 ohm

Ans: Option a. above circuit is a balanced wheatstone bridge so the effective resistance turns out to be 4 ohm

ii) What is the total current in the circuit

a) 0 A

b) 2 A

c) 1 A

d) 0.5 A

Ans: Option b, as total resistance is 4 ohm and total voltage applied is 8 V from ohm’s law total current will be 2 A

iii) Current through 8 ohm resistor is:

a) 0 A

b) 2 A

c) 1 A

d) 0.5 A

Ans: Option A, as bridge is balanced no current will flow through 8 Ohm resistor

iv) What will be the drop across 3 Ohm resistor

a) 1 V

b) 2V

c) 3 V

d) 0V

Ans: Option C, as current through circuit is 2 A, from symmetry it’ll get divided equally about upper and lower branch i.e. 1 A each. As 1 A current in passing through 3 Ohm resistor, drop across it will be 3V

20. Current in a conductor :

Metal’s have a large number of free electrons, nearly 1028 per cubic metre. In the absence of an electric field, the average terminal speed of the electrons in random motion at room temperature is of the order of 105m/s. When a potential difference V is applied across the two ends of a given conductor, the free electrons in the conductor
experience a force and are accelerated towards the positive end of the conductor. On heir way, they suffer frequent collisions with the ions/atoms of the conductor and lose their gained kinetic energy. After each collision, the free electrons are again accelerated due to electric field, towards the positive end of the conductor and lose
their gained kinetic energy in the next collision with the ions/atoms of the conductor. The average speed of the free electrons with which they drift towards the positive end of the conductor under the effect of applied electric fields is called drift speed it can also be expressed in terms of current as vd=i/neA.

i) Magnitude of drift velocity per unit electric field is;

a) Current density

b) current

c) resistivity

d) mobility
Ans: Option d, mobility

ii) The drift speed of the electrons depends on:

a) Dimensions of the conductor

b) Number density of free electrons in the conductor

c) Both a and b

d) Neither a nor b
Ans: Option C, vd = I/neA

iii) We are able to obtain fairly large currents in a conductor because

a) The electron drift speed is usually very large

b) The number density of free electrons is very high and this can compensate for the low values of the electron drift speed and the very small magnitude of the electron charge

c) The number density of free electrons as well as the electron drift speeds are very large and these compensate for very small magnitude of the electron charge

d) The very small magnitude of the electron charge has to be divided by the still smaller product of the number density and drift speed to get the electric current.
Ans: Option b.

iv) The number density of free electrons in copper conductor is 8.5 x 1028m-3, how long does an electron take to drift from one end of a wire 3.0m long to its other end? The area of cross-section of the wire is 2.0 x 10-6 m2 and it is carrying a current of 3.0A.

a) 8.1 x 104 s

b) 2.7 x 104s

c) 9 x 103 s

d) 3 x 103 s

Ans: Option b, I = neAvd and vd=length/t => t=neA(length)/I

21. An electric current is passed through a circuit containing two wires of same material, connected in parallel. If the lengths and radii of the wires are in the ratio of 3:2 and 2:3, then the ratio of the current passing through the wire will be

a) 2:3

b) 3:2

c) 8:27

d) 27:8

Ans: optin 3, 8:27 as I inversely depends on R in parallel and R proportional to L/r2

22. From the graph between current I and voltage V shown below, identify the portion corresponding to negative resistance.

a) AB

b) BC

c) CD

d) DE

Ans: Option C, CD this is the region where current is falling with raise in voltage

23. Two wires of same material have length L and 2L and cross-sectional areas 4A and A respectively. The ratio their specific resistance would be:

a) 1 : 2

b) 8 : 1

c) 1 : 8

d) 1 : 1
Ans: Option d. as both materials being same, specific resistance remains same

24. Two cells of emf’s approximately 5V and 10V are to be accurately compared using a potentiometer of length 400m.

a) The battery that run the potentiometer should have voltage of 8 V

b) The battery of potentiometer can have a voltage of 15V and R adjusted so that the potential drop across the wire slightly exceeds 10V

c) The first portion of 50cm of wire itself should have a potential drop of 10V

d) Potentiometer is usually used for comparing resistance and not voltages.
Ans: Option B: driving battery emf should always more than the emf of secondary cells

25. A 220V-100W bulb is connected to a source of 180V. the power consumed by it will be nearly :

a) 32 w

b) 67 W

c) 100 W

d) 75 W
Ans: Option b: 67