Question Set C

CBSE 12th Physics Current Electricity Important Questions And Answers Study Material - QB365

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Questions:

Multiple Choice Question

51 x 1 = 51

If n,e, ፒ and m have their usual meanings, then the resistance of a wire of length and cross-sectional area A is given by

(a)

\(\frac{n e^2 A}{2 m \tau l}\)

(b)

\(\frac{m l}{n e^2 \tau A}\)

(c)

\(\frac{m \tau A}{n e^2 l}\)

(d)

\(\frac{n e^2 \tau A}{2 m l}\)

A car battery is charged by a 12 V supply and energy stored in it is 720 x 10⁵ J. The charge passed through the battery is

(a)

6.0 × 10⁴ C

(b)

5.8 × 10³ J

(c)

8.64 × 10⁶ J

(d)

1.6 × 10⁵ C

In a DCcircuit,the direction of current inside the battery and outside the battery, respectively are

(a)

positive to negative terminal and negative to positive terminal

(b)

positive to negative terminal and positive to negative terminal

(c)

negative to positive terminal and positive to negative terminal

(d)

negative to positive terminal and negative to positive terminal

A cell of emf (E) and internal resistance is connected across a variable external resistance R. The graph of terminal potential difference V as a function of R is

(a)

(b)

(c)

(d)

In a DC circuit, the direction of current inside the battery and outside the battery, respectively are

(a)

positive to negative terminal and negative to positive terminal

(b)

positive to negative terminal and positive to negative terminal

(c)

negative to positive terminal and positive to negative terminal

(d)

negative to positive terminal and negative to positive terminal

A constant voltage is applied between the two ends of a uniform metallic wire, heat His developed in it. If another wire of the same material, double the radius and twice the length as compared to original wire is used, then the heat developed in it will be

(a)

\(\frac{\mathrm{H}}{2}\)

(b)

(c)

(d)

If the potential difference V applied across a conductor is increased to 2V with its temperature kept constant, then the drift velocity of the free electrons in a conductor will

(a)

remain the same

(b)

become half of its previous value

(c)

be double of its initial value

(d)

become zero

Which of the following has negative temperature coefficient of resistivity?

(a)

Metal

(b)

Metal and semiconductor

(c)

Semiconductor

(d)

Metal and alloy

Kirchhoff's first rule, EI = 0 and second rule, ΣΙΑ = ΣΕ (where the symbols have their usual meanings) are respectively, based on

(a)

conservation of momentum and conservation of charge

(b)

conservation of energy and conservation of charge

(c)

conservation of charge and conservation of momentum

(d)

conservation of charge and conservation of energy

10.

If an ammeter is to be used in place of a voltmeter, then we must connect with the ammeter a

(a)

low resistance in parallel

(b)

low resistance in series

(c)

high resistance in parallel

(d)

high resistance in series

11.

The electric power consumed by a 220 V-100 W bulb, when operated at 110 V is

(a)

25 W

(b)

30 W

(c)

35 W

(d)

45 W

12.

A current of 0.8 A flows in a conductor of 40\(\Omega\) for 1 min. The heat produced in the conductor will be

(a)

1445 J

(b)

1536 J

(c)

1569 J

(d)

1640 J

13.

The current density due to drift of electrons in a conductor is given by (symbols have their usual meanings)

(a)

\(n e A v_d\)

(b)

\(\frac{n A v_d}{e}\)

(c)

\(\frac{n v_d}{e A}\)

(d)

\(n e v_d\)

14.

A conductor of 10\(\Omega\) is connected across a 6V ideal source. The power supplied by the source to the conductor is

(a)

1.8W

(b)

2.4W

(c)

3.6W

(d)

7.2W

15.

Pieces of copper and silicon are initially at room temperature. Both are heated to temperature T.
The conduction of

(a)

both increases

(b)

both decreases

(c)

copper increases and silicon decreases

(d)

copper decreases and silicon increases

16.

The current in a device varies with time t as I=6t, whereI is in mÁ and t is in s. The amount of charge that passes through thedevice during t=0s to t= 3s is

(a)

10 mC

(b)

18 mC

(c)

27 mC

(d)

54 mC

17.

A steady current of 8 mA flows through a wire.The number of electrons passing through a cross-section of the wire in 10 s is

(a)

\(4.0 \times 10^{16}\)

(b)

\(5.0 \times 10^{17}\)

(c)

\(1.6\times 10^{16}\)

(d)

\(1.0 \times 10^{17}\)

18.

If the length of potentiometer wire is increased, then the length of the previously obtained balance point will

(a)

increase.

(b)

decrease.

(c)

remain unchanged.

(d)

become two times.

19.

The terminal potential difference of a cell is greater than its e.m.f. when it is

(a)

being discharged.

(b)

in open circuit

(c)

being charged.

(d)

being either charged or discharged.

20.

The magnitude and direction of the current in the circuit shown will be

(a)

7/3A from a to b through e

(b)

7/3A from b to a through e

(c)

1 A from b to a through e

(d)

1 A from a to b through e

21.

Kirchhoff's junction rule is a reflection of

(a)

conservation of current density vector.

(b)

conservation of potential

(c)

the fact that the momentum with which a charged particle approaches a junction is unchanged (as a vector) as the charged particle leaves the junction

(d)

the fact that there is no accumulation of charges at a junction.

22.

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

(a)

(b)

(c)

(d)

23.

Three resistors each of 2 ohm are connected together in a triangular shape. The resistance between any two vertices will be

(a)

4/3 ohm

(b)

3/4 ohm

(c)

3 ohm

(d)

6 ohm

24.

For a cell, the graph between the potential difference (V) across the terminals of the cell and the current (I) drawn from the cell is shown in the figure.

(a)

\(2 \mathrm{~V}, 0.5 \Omega\)

(b)

\(2 \mathrm{~V}, 0.4 \Omega\)

(c)

\(>2 \mathbf{V}, \mathbf{0 . 5} \Omega\)

(d)

\(>2 \mathbf{V}, \mathbf{0 . 4} \Omega\)

25.

Drift velocity v_d varies with the intensity of electric field as per the relation

(a)

\(v_{d} \propto E\)

(b)

\(v_{d} \propto \frac{1}{E}\)

(c)

v_d= constant

(d)

\(\boldsymbol{v}_{d} \propto \boldsymbol{E}^{2}\)

26.

The emf of the battery shown in figure is

(a)

12 V

(b)

13 V

(c)

16 V

(d)

18 V

27.

A potential difference of 100 V is applied to the ends of a copper wire one metre long. What is the average drift velocity of electrons?
(given. σ = 5.81 x 10⁷ Ω^-1or n_cu = 8.5 x 10²⁸ m^-3)

(a)

0.43 ms ^-1

(b)

0.83 ms ^-1

(c)

0.52 ms^-1

(d)

0.95 ms^-1

28.

2 mA current is flowing in the wire of potentiometer of 5m long and 5 Ω resistance. The potential gradient is

(a)

2 x 10^-3 V/m

(b)

2.5 x 10^-2 V/m

(c)

1.6 x 10^-3 V/m

(d)

2.3 x 10^-3 V/m

29.

If each of the resistance in the network in figure is R, the equivalent resistance between terminals A and B is

(a)

5 R

(b)

2 R

(c)

4 R

(d)

30.

If 2 A current is flowing in the shown circuit, then potential difference (VB - VD) in balanced condition is

(a)

12 V

(b)

6 V

(c)

4 V

(d)

zero

31.

Which of the following draws no current from the voltage source being measured?

(a)

Meter bridge

(b)

Wheatstone bridge

(c)

Potentiometer

(d)

None of these

32.

Kirchhoff's current law is consequence of conservation of

(a)

energy

(b)

momentum

(c)

charge

(d)

mass

33.

Two batteries of emf ε₁ and ε₂, (ε₂ >ε₁) and internal resistances r₁and r₂ respectively are connected in parallel as shown in figure.

(a)

Two equivalent emf ε_eq of the two cells is between ε₁ and ε₂ i.e., ε₂ < ε_eq < ε₂

(b)

The equivalent emf ε_eq is smaller than ε₁

(c)

The ε_eqis given by εeq = ε₁ + ε₂always

(d)

ε_eq is independent of internal resistances ε₁ and ε₂

34.

In the following diagram, equivalent resistance between A and D is

(a)

5 Ω

(b)

4 Ω

(c)

3Ω

(d)

2Ω

35.

The relation between electric current density (J) and drift velocity (v_d) is

(a)

J = nev_d

(b)

J = \(\frac{n e}{v_{d}}\)

(c)

J = \(\frac{v_{d} e}{n}\)

(d)

J = nev²_d

36.

The resistivity of material is investment proportional to:

(a)

number density of electrons as well and relaxation time

(b)

number density of electrons and direction proportional to relaxation time

(c)

relaxation time and directly proportional to the number density of electron

(d)

neither relaxation time nor number density of electrons.

37.

For ohmic conductor the drift velocity v_d and the electric field applied across it are related as:

(a)

\({ v }_{ d }\propto \sqrt { E } \)

(b)

v_d \(\propto \) E

(c)

\({ v }_{ d }\propto { E }^{ \frac { 3 }{ 2 } }\)

(d)

v_d \(\propto \) E²

38.

Ohms law is valid when the temperature of the conductor is:

(a)

constant

(b)

very high

(c)

very low

(d)

varying

39.

The length and area of cross - section of a conductor are doubled, its resistance will be:

(a)

halved

(b)

unchanged

(c)

doubled

(d)

quadrupled

40.

The length of a conductor is halved. Its conductivity will be

(a)

halved

(b)

unchanged

(c)

doubled

(d)

quadrupled

41.

Resistivity of a conductor depends upon its:

(a)

resistance

(b)

length

(c)

area of cross - section

(d)

none of the above characteristics

42.

For higher sensitivity which of the following is essential for the potentiometer?

(a)

Higher emf of quxiliary battery

(b)

Higher resistivity of the wire

(c)

Larger length of the wire

(d)

None of the above

43.

Why the wheatstone bridge is more accurate than the other methods of measuring resistance?

(a)

It has four resistor arms

(b)

It is based on Kirchhoff's laws

(c)

It does not involve ohm's law

(d)

It is a null method

44.

According to the Kirchhoff's law the sum of the products of current and resistance as well as emfs in a closed loop is:

(a)

greater than zero

(b)

zero

(c)

less than zero

(d)

determained by the emf

45.

What is the resistance between A and B in the fig.

(a)

R/2

(b)

(c)

(d)

46.

What is the resistance across A and B in the fig?

(a)

\(\frac{R}{2}\)

(b)

(c)

(d)

47.

What is the resistance across A and B in the fig

(a)

(b)

(c)

R/3

(d)

None of the above

48.

Four wires each of same length, diameter and material are connected to each other to form a square. If the resistance of each wire is R, then equivalent resistance across the opposite corners is:

(a)

R/4

(b)

R/2

(c)

(d)

none of the above

49.

Two resistance when connected in parallel have equivalent resistance of 3\(\Omega\). When one of the resistance is burnt and broken, the net resistance is 12\(\Omega\). What is the resistance of the burnt resistor?

(a)

4\(\Omega\)

(b)

8\(\Omega\)

(c)

12\(\Omega\)

(d)

16\(\Omega\)

50.

The equivalent resistance of n resistors each of same resistance when connected in parallel is R_p. If they are connected in series, the equivalent resistance will be:

(a)

R_p/n²

(b)

R_p/n

(c)

nR_p

(d)

n²R_p

51.

When a current I is set up in a wire of radius r, the drift speed id v_d. If the same current is set up through a wire of radius 2r the drift speed will be

(a)

v_d/4

(b)

v_d/2

(c)

2v_d

(d)

4v_d

Assertion and reason

16 x 1 = 16

52.

Assertion: A voltmeter must be connected in parallel in a circuit and it should have a high resistance.
Reason: The introduction of the voltmeter in the circuit must not effect the P.D. it is to measure.
Codes:
(A) If both Assertion & Reason are true & the Reason is a correct explanation of the Assertion.
(B) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.
(C) If Assertion is true but the Reason is false
(D) If Assertion & Reason both are false.

53.

Assertion: An ammeter is always connected in series whereas a voltmeter is connected in parallel.
Reason: An ammeter is a low-resistance galvanometer while a voltmeter is high-resistance galvanometer
Codes:
(A) If both Assertion & Reason are true & the Reason is a correct explanation of the Assertion.
(B) If both Assertion and Reason are true but Reason is not a correct explanation of the Assertion.
(C) If Assertion is true but the Reason is false
(D) If Assertion & Reason both are false.

54.

Assertion (A) : Higher the range, lower is the resistance of an ammeter.
Reason (R) : To increase the range of an ammeter, additional shunt is added in series to it
(a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.

55.

Assertion (A) : As the temperature of a 147 B conducting wire increases, the drift velocity of the electrons also increases.
Reason (R) : With an increase in temperature, the average time of collision decreases.
(a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.

56.

(a) Both Assertion and Reason are true and Reason is the correct explanation ofAssertion.
(b)Both Assertion and Reason are true but Reason is not the correct explanation ofAssertion.
(c) Assertion is true but Reason is false.
(d) Assertion is false but Reason is true.
Assertion (A) The equivalent resistance between' points A and B in the given network is 2R.
Reason (R) All the resistors are connected in

57.

Assertion (A) When electrons drift in a conductor, it does not mean that all free electrons in the conductor are moving in the same direction.
Reason (R) The drift velocity is superimposed over large random velocities of electrons.
(a) If both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
(b) If both Assertion (A) and Reason (R) are true but Reason (R) is not the correct explanation of Assertion (A).
(c) If Assertion (A) is true and Reason (R) is false.
(d) If both Assertion (A) and Reason (R) are false.

58.

Assertion: The current in a wire is due to flow of free electrons in a definite direction.
Reason: A current carrying wire should have non -zero charge.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

59.

Assertion: A person touching a high power line gets stuck with the line.
Reason: The current carrying wire attracts the man towards it.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

60.

Assertion: Two electric bulbs of 50 and 100 watt are given. When connected in series 50 watt bulb glows more but when connected in parallel 100 watt bulb glows more .
Reason: In series combination, power is directly proportional to the resistance of circuit. But in parallel combination, power is inversely proportional to the resistance of the circuit.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

61.

Assertion: In a chain of bulbs, 50 bulbs are joined in series. One bulb is removed now the circuit is again completed. If the remaining 49 bulbs are again connected in series across the same supply, then light gets decreased in the room.
Reason: The resistance of 49 bulbs will be more than 50 bulbs.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

62.

Assertion: Fuse wire must have high resistance and low melting point.
Reason: Fuse is used for small current flow only.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

63.

Assertion: Heater wire must have high resistance and high melting point.
Reason: If resistance is high, the electric conductivity will be less.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

64.

Assertion: kWhr is a commercial unit used for expressing consumed electric energy.
Reason: Kilo-watt hour is the unit of electric power.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

65.

Assertion: Material used in the construction of a standard resistance is constant an or manganin.
Reason: Temperature coefficient of constantan is very small.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

66.

Assertion: If the length of the conductor is doubled, the drift velocity will become half of the original value (keeping potential difference unchanged).
Reason : At constant potential difference, drift velocity is inversely proportional to the length of the conductor.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

67.

Assertion: The drift velocity of electrons in a metallic wire will decrease, if the temperature of the wire is increased.
Reason: On increasing temperature, conductance of metallic wire decreases.
Codes:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

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Answers Key:

Multiple Choice Question

51 x 1 = 51

(b)
\(\frac{m l}{n e^2 \tau A}\)

(a)
6.0 × 10⁴ C

\(\begin{array}{ll} R=\rho \frac{l}{A} & R^{\prime}=\rho \frac{2 l}{\pi(2 r)^2} \\ R=\rho \frac{l}{\pi r^2} & R^{\prime}=\rho \frac{2 l}{\pi 4 r^2} \\ H=\frac{V^2}{R} t \quad \& H^{\prime}=\frac{V^2}{R^{\prime}} t \end{array}\)

V= constant

\(\begin{aligned} \frac{H^{\prime}}{H} & =\frac{V^2}{R^{\prime}} \frac{R}{V^2} \frac{t}{t} \\ & =\frac{R}{R^{\prime}}=\rho \frac{l}{\pi r^2} \frac{2 \pi r^2}{\rho l} \\ \frac{H^{\prime}}{H} & =\frac{2}{1} \\ H^{\prime} & =2 H \end{aligned}\)

We know,

\(\begin{aligned} & v_d=\frac{e \mathrm{E}}{m} \tau \\ & v_d=e \frac{\mathrm{~V}}{m l} \tau \end{aligned}\)

If temperature is kept constant, relaxation time \(\tau-\) will remain constant, and e, m are also constants.

\(\begin{aligned} v_d & \propto \mathrm{~V} \\ 2 v_d & \propto 2 \mathrm{~V} \end{aligned}\)

Thus,correct option is (c).

(d)
conservation of charge and conservation of energy

10.

(a)
low resistance in parallel

11.

(a)
25 W

12.

(b)
1536 J

13.

(d)
\(n e v_d\)

14.

15.

(d)
copper decreases and silicon increases

16.

17.

(b)
\(5.0 \times 10^{17}\)

18.

(a)
increase.

19.

20.

(d)
1 A from a to b through e

21.

(d)
the fact that there is no accumulation of charges at a junction.

22.

23.

(a)
4/3 ohm

24.

(b)
\(2 \mathrm{~V}, 0.4 \Omega\)

25.

(a)
\(v_{d} \propto E\)

26.

(b)
13 V

27.

(a)
0.43 ms ^-1

28.

(a)
2 x 10^-3 V/m

29.

(d)
R

30.

(d)
zero

31.

32.

33.

(a)
Two equivalent emf ε_eq of the two cells is between ε₁ and ε₂ i.e., ε₂ < ε_eq < ε₂

34.

(d)
2Ω

35.

36.

(a)
number density of electrons as well and relaxation time

37.

(b)
v_d \(\propto \) E

38.

(a)
constant

39.

(b)
unchanged

40.

(b)
unchanged

41.

(d)
none of the above characteristics

42.

43.

(d)
It is a null method

44.

(b)
zero

45.

46.

(b)
R

47.

48.

(d)
none of the above

49.

(a)
4\(\Omega\)

50.

(d)
n²R_p

51.

(a)
v_d/4

Assertion and reason

16 x 1 = 16

52.

(A) If both Assertion & Reason are true & the Reason is a correct explanation of the Assertion.

53.

(A) If both Assertion & Reason are true & the Reason is a correct explanation of the Assertion.

54.

(a) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

55.

(d) Assertion is false but Reason is true.
As temperature increases, collison becomes more frequent so average time of collison decreases. More collison means less drift velocity.

56.

57.

(a) No, all the free electrons do not move in the same direction. The net movement of the electrons is from lower to higher potential. As a result, there is definite small drift velocity of electrons, which is superimposed on the random motion of free electrons.

58.

(c): The current in a wire is due to flow of free electrons in a definite direction. But the number of protons in the wire at any instant is equal to number of electrons and charge on electrons is equal and opposite to that of proton. Hence, net charge on the wire is zero.

59.

(d): Because there is no special attractive force that keeps a person stuck with a high power line. The actual reason is that a current of the order of 0.05 A or even less is enough to bring disorder in our nervous system. As a result of it, the affected person may lose temporarily his ability to control his nervous system to get himself free from the high power line.

60.

(a): Resistance of 50 watt bulb is two times the resistance of 100 watt bulb. When bulbs are connected in series, 50 watt bulb will glow more as P = i²R (current remains same in series). In parallel, the 100 watt bulb will glow more as P = V²/R (potential difference remain same in parallel).

61.

(d): Since the bulbs are joined in series, so when one bulb is removed from chain then the resistance of the chain is decreased, hence current flowing through each bulb is increased. As heat produced \(\propto i^{2}\) hence light gets increased in the room.

62.

(c): Fuse wire must have high resistance because in series, current remains same. Therefore according to Joule's law \(H=\frac{i^{2} R t}{4.2}\) heat produced is high if R is high. The melting point must be low so that wire may melt with increase in temperature. As a current larger than the specified value flows through the circuit the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit.

63.

(b): Heater wire must have high resistance and high melting point, because in series current remains same, therefore according to Joule's lawH = i²Rt / 4.2, heat produced is high if R is high. Melting point must be high, so that wire may not melt with increase in temperature.

64.

(c): 1 kWhr = 1 kW x 1 hour
= 1000 (joule/see) x 3600 see = 36 x 10⁵ joule
i.e., kWhr is the unit of electric energy and used for expressing consumed electric energy.

65.

(a): These alloys (constantan or manganin) are used for making standard resistance because they possess high resistivity and low temperature coefficient of resistance

66.

(a): Drift velocity of free electrons is given by \(v_{d}=\frac{e E}{m} \tau\)
where,\(E=\frac{\text { Potential difference }}{\text { length }}=\frac{V}{l}\)
\(\therefore \quad v_{d}=\frac{e V}{m l} \tau \text { i.e., } v_{d} \propto 1 / l \text { where, } \frac{e V \tau}{m}\)
It mean if I is doubled, the drift velocity will become half of the original value.

67.

(b): On increasing temperature of wire the kinetic energy of free electrons increase and so they collide more rapidly with each other and hence their drift velocity decreases. Also when temperature increases, resistance increase and resistance is inversely proportional to conductivity of material.