CBSE Class 12 Physics Current Electricity MCQs Set B

Question. A heating coil is labelled 100 W, 220 V. The coil is cut in half and the two pieces are joined in parallel to the same source. The energy now liberated per second is
(a) 200 W
(b) 400 W
(c) 25 W
(d) 50 W

Answer : B

Question. A 6 volt battery is connected to the terminals of a three metre long wire of uniform thickness and resistance of 100 ohm. The difference of potential between two points on the wire separated by a distance of 50 cm will be
(a) 2 volt
(b) 3 volt
(c) 1 volt
(d) 1.5 volt

Answer : C

Question. Three copper wires of lengths and cross-sectional areas are (l, A), (2l, A/2) and (l/2, 2A). Resistance is minimum in
(a) wire of cross-sectional area 2A
(b) wire of cross-sectional area A
(c) wire of cross-sectional area A
(d) same in all three cases.

Answer : A

Question. A charged particle having drift velocity of 7.5 × 10^–4 m s^–1 in an electric field of 3 × 10^–10 V m_–1, has a mobility in m² V^–1 s^–1 of    B
(a) 2.25 × 10¹⁵
(b) 2.5 × 10⁶
(c) 2.5 × 10^–6
(d) 2.25 × 10^–15

Answer : B

Question. The mean free path of electrons in a metal is 4 × 10^–8 m.   
The electric field which can give on an average 2 eV energy to an electron in the metal will be in units V/m
(a) 5 × 10^–11
(b) 8 × 10^–11
(c) 5 × 10⁷
(d) 8 × 10⁷

Answer : C

Question. The velocity of charge carriers of current (about 1 ampere) in a metal under normal conditions is of the order of
(a) a fraction of mm/sec
(b) velocity of light
(c) several thousand metres/second
(d) a few hundred metres per second

Answer : A

Question. The resistance of a discharge tube is
(a) non-ohmic
(b) ohmic
(c) zero
(d) both (b) and (c)

Answer : A

Question. In India electricity is supplied for domestic use at 220 V. It is supplied at 110 V in USA. If the resistance of a 60 W bulb for use in India is R, the resistance of a 60 W bulb for use in USA will be
(a) R
(b) 2R
(c) R/4
(d) R/2

Answer : C

Question. An electric kettle has two heating coils. When one of the coils is connected to an a.c. source, the water in the kettle boils in 10 minutes. When the other coil is used the water boils in 40 minutes. If both the coils are connected in parallel, the time taken by the same quantity of water to boil will be
(a) 8 minutes
(b) 4 minutes
(c) 25 minutes
(d) 15 minutes

Answer : A

Question. If voltage across a bulb rated 220 volt- 100 watt drops by 2.5% of its rated value, the percentage of the rated value by which the power would decrease is
(a) 20%
(b) 2.5%
(c) 5%
(d) 10%

Answer : C

Question. Two 220 volt, 100 watt bulbs are connected first in series and then in parallel. Each time the combination is connected to a 220 volt a.c. supply line. The power drawn by the combination in each case respectively will be
(a) 50 watt, 100 watt
(b) 100 watt, 50 watt
(c) 200 watt, 150 watt
(d) 50 watt, 200 watt

Answer : D

Question. Identify the set in which all the three materials are good conductors of electricity.
(a) Cu, Hg and NaCl
(b) Cu, Ge and Hg
(c) Cu, Ag and Au
(d) Cu, Si and diamond

Answer : C

Question. Two wires of the same metal have same length, but their cross-sections are in the ratio 3 : 1. They are joined in series. The resistance of thicker wire is 10 W. The total resistance of the combination will be
(a) 40 W
(b) 100 W
(c) (5/2) W
(d) (40/3) W.

Answer : A

Question. An electric kettle takes 4 A current at 220 V. How much time will it take to boil 1 kg of water from temperature 20°C ? The temperature of boiling water is 100°C.
(a) 12.6 min
(b) 4.2 min
(c) 6.3 min
(d) 8.4 min

Answer : C

Question. A 5°C rise in temperature is observed in a conductor by passing a current. When the current is doubled the rise in temperature will be approximately
(a) 20°C
(b) 16°C
(c) 10°C
(d) 12°C

Answer : A

Question. A (100 W, 200 V) bulb is connected to a 160 volts supply. The power consumption would be
(a) 100 W
(b) 125 W
(c) 64 W
(d) 80 W

Answer : C

Question. An electric bulb is rated 60 W, 220 V. The resistance of its filament is
(a) 870 W
(b) 780 W
(c) 708 W
(d) 807 W

Answer : D

Question. Three copper wires of lengths and cross sectional areas are (l, A), (2 l, A/2) and (l/2, 2A). Resistance is minimum in
(a) wire of cross-sectional area A/2
(b) wire of cross-sectional area A
(c) wire of cross-sectional area 2A
(d) same in all the three cases

Answer : C

Question. When a current I is set up in a wire of radius r, the drift velocity is vd. If the same current is set up through a wire of radius 2 r, the drift velocity will be
(a) 4 vd
(b) 2 vd
(c) vd/2
(d) vd/4

Answer : D

Question. A current of 2 A, passing through a conductor produces 80 J of heat in 10 seconds. The resistance of the conductor in ohm is
(a) 0.5
(b) 2
(c) 4
(d) 20

Answer : B

Question. Fuse wire is a wire of
(a) high resistance and high melting point
(b) high resistance and low melting point
(c) low resistance and low melting point
(d) low resistance and high melting point

Answer : B

Question. A carbon resistor of (47 ± 4.7) kW is to be marked with rings of different colours for its identification.
The colour code sequence will be
(a) Violet – Yellow – Orange – Silver
(b) Yellow – Violet – Orange – Silver
(c) Yellow – Green – Violet – Gold
(d) Green – Orange – Violet – Gold

Answer : B

Question. The solids which have the negative temperature coefficient of resistance are
(a) metals
(b) insulators only
(c) semiconductors only
(d) insulators and semiconductors.

Answer : D

Question. A flow of 10⁷ electrons per second in a conducting wire constitutes a current of   
(a) 1.6 × 10^–12 A
(b) 1.6 × 10²⁶ A
(c) 1.6 × 10^–26 A
(d) 1.6 × 10¹² A

Answer : A

Question. The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then
(a) The resistance will be doubled and the specific resistance will be halved
(b) The resistance will be halved and the specific resistance will remain unchanged
(c) The resistance will be halved and the specific resistance will be doubled
(d) The resistance and the specific resistance, will both remain unchanged.

Answer : B

Question. Specific resistance of a conductor increases with
(a) increase in temperature
(b) increase in cross-section area
(c) increase in cross-section and decrease in length
(d) decrease in cross-section area.

Answer : A

Question. Copper and silicon is cooled from 300 K to 60 K, the specific resistance
(a) decreases in copper but increases in silicon
(b) increases in copper but decreases in silicon
(c) increases in both
(d) decreases in both.

Answer : A

Question. Which of the following acts as a circuit protection device?
(a) fuse
(b) conductor
(c) inductor
(d) switch

Answer : A

Question. The charge flowing through a resistance R varies with time t as Q = at – bt², where a and b are positive constants. The total heat produced in R is   
(a) a3 R /2b
(b) a3 R /b
(c) a3 R /6b
(d) a3 R / 3b

Answer : C

Question. A wire 50 cm long and 1 mm2 in cross-section carries a current of 4 A when connected to a 2 V battery. The resistivity of the wire is
(a) 4 × 10^–6 W m
(b) 1 × 10^–6 W m
(c) 2 × 10^–7 W m
(d) 5 × 10^–7 W m

Answer : B

Question. A wire of a certain material is stretched slowly by ten percent. Its new resistance and specific resistance become respectively
(a) both remain the same
(b) 1.1 times, 1.1 times
(c) 1.2 times, 1.1 times
(d) 1.21 times, same

Answer : D

Question. The masses of the wires of copper is in the ratio of 1 : 3 : 5 and their lengths are in the ratio of 5 : 3 : 1. The ratio of their electrical resistance is
(a) 1 : 3 : 5
(b) 5 : 3 : 1
(c) 1 : 25 : 125
(d) 125 : 15 : 1

Answer : D

Question. Three resistances each of 4 W are connected to form a triangle. The resistance between any two terminals is
(a) 12 W
(b) 2 W
(c) 6 W
(d) 8/3 W

Answer : D

Question. A primary cell has an e.m.f. of 1.5 volt. When short-circuited it gives a current of 3 ampere. The internal resistance of the cell is
(a) 4.5 ohm
(b) 2 ohm
(c) 0.5 ohm
(d) (1/4.5) ohm

Answer : C

Question. Two cities are 150 km apart. Electric power is sent from one city to another city through copper wires.
The fall of potential per km is 8 volt and the average resistance per km is 0.5 W. The power loss in the wire is
(a) 19.2 W
(b) 19.2 kW
(c) 19.2 J
(d) 12.2 kW

Answer : B

Question. The resistance of a wire is ‘R’ ohm. If it is melted and stretched to ‘n’ times its original length, its new resistance will be
(a) R /n
(b) n²R
(c) R /n²
(d) nR

Answer : B

Question. A wire of resistance 4 W is stretched to twice its original length. The resistance of stretched wire would be
(a) 8 W
(b) 16 W
(c) 2 W
(d) 4 W

Answer : B

Question. The drift velocity of the free electrons in a conducting wire carrying a current i is v. If in a wire of the same metal, but of double the radius, the current be 2I, then the drift velocity of the electrons will be
(a) v/4
(b) v/2
(c) v
(d) 4v

Answer : B

Question. A filament bulb (500 W, 100 V) is to be used in a 230 V main supply. When a resistance R is connected in series, it works perfectly and the bulb consumes 500 W. The value of R is
(a) 230 W
(b) 46 W
(c) 26 W
(d) 13 W

Answer : C

Question. A circuit contains an ammeter, a battery of 30 V and a resistance 40.8 ohm all connected in series. If the ammeter has a coil of resistance 480 ohm and a shunt of 20 ohm, the reading in the ammeter will be
(a) 2 A
(b) 1 A
(c) 0.5 A
(d) 0.25 A

Answer : C

Question. When a wire of uniform cross-section a, length l and resistance R is bent into a complete circle, resistance between any two of diametrically opposite points will be
(a) R/4
(b) 4R
(c) R/8
(d) R/2

Answer : A

Question. A 5 ampere fuse wire can withstand a maximum power of 1 watt in the circuit. The resistance of the fuse wire is
(a) 0.04 ohm
(b) 0.2 ohm
(c) 5 ohm
(d) 0.4 ohm

Answer : A

Question. Two bulbs are of (40 W, 200 V), and (100 W, 200 V). Then correct relation for their resistances is
(a) R40 < R100
(b) R40 > R100
(c) R40 = R100
(d) no relation can be predicted.

Answer : B

Question. Resistance n, each of r ohm, when connected in parallel give an equivalent resistance of R ohm. If these resistances were connected in series, the combination would have a resistance in ohms, equal to
(a) n2R
(b) R/n2
(c) R/n
(d) nR

Answer : A

Question. When three identical bulbs of 60 watt, 200 volt rating are connected in series to a 200 volt supply, the power drawn by them will be
(a) 60 watt
(b) 180 watt
(c) 10 watt
(d) 20 watt

Answer : D

Question. The infinity resistance plug in a post-office box has
(a) an air gap only
(b) a resistance coil of infinite resistance
(c) largest resistance available in box
(d) resistance of the coil 5000 W

Answer : A

Question. In an household electric circuit, which of the following is / are correct?
(A) All electric appliances drawing power are joined in parallel
(B) A switch may be either in series or in parallel with the appliance which it controls
(C) If a switch is in parallel with an appliance, it will draw power when the switch is in the ‘off’ position (open)
(D) If a switch is in parallel with an appliance, the fuse will blow (burn out) when the switch is put ‘on’ closed.
(a) A, D
(b) A, C, D
(c) B, C, D
(d) A, B, D

Answer : B

Question. Three equal resistors connected in series across a source of e.m.f. together dissipate 10 watt of power. D
What will be the power dissipated in watt if the same resistors are connected in parallel across the same source of e.m.f.?
(a) 30
(b) 10 /3
(c) 10
(d) 90

Answer : D

Question. One kilowatt hour is equal to
(a) 36 × 10^–5 J
(b) 36 × 10^–4 J
(c) 36 × 10⁵ J
(d) 36 × 10³ J 

Answer : C

Question. If two bulbs, whose resistances are in the ratio of 1 : 2 are connected in series, the power dissipated in them has the ratio of
(a) 2 : 1
(b) 1 : 4
(c) 1 : 1
(d) 1 : 2.

Answer : D

Question. Which of the following characteristics of electrons determines the current in a conductor?
(a) Drift velocity alone
(b) Thermal velocity alone
(c) Both drift velocity and thermal velocity
(d) Neither drift nor thermal velocity.

Answer : A

Question. A dynamo develops 0.5 A at 6 V. The energy which is generated in one second is
(a) 0.083 J
(b) 3 J
(c) 12 J
(d) None of these

Answer : B

Question. If the resistance of a conductor is 5Ω at 50º C & 7Ω at 100º C, then mean temperature coefficient of resistance (of material) is
(a) 0.013/ ºC
(b) 0.004/ ºC
(c) 0.006/ ºC
(d) 0.008/ ºC

Answer : A

Question. If R₁ and R₂ are the filament resistances of 200 W and a 100 W bulb respectively both designed to run at the same voltage, then
(a) R₂ is four times of R₁
(b) R₁ is four times of R₂
(c) R₂ is two times of R₁
(d) R₁ is two times of R₂

Answer : C

Question. Two cells of emf ’s approximately 5 V and 10 V are to be accurately compared using a potentiometer of length 400 cm.
(a) The battery that runs the potentiometer should have voltage of 8V.
(b) The battery of potentiometer can have a voltage of 15 V and R adjusted so that the potential drop across the wire slightly exceeds 10 V.
(c) The first portion of 50 cm of wire itself should have a potential drop of 10 V.
(d) Potentiometer is usually used for comparing resistances and not voltages.

Answer : B

Question. A wire of resistance 12Ω/m is bent to form a complete circle of radius 10 cm. The resistance between its two diametrically opposite points A and B as shown in figure is
(a) 3 Ω
(b) 6 rX
(c) 6 Ω
(d) 0.6 rX

Answer : D

Question. Two heating wires of equal length are first connected in series and then in parallel to a constant voltage source. The rate of heat produced in two cases is (parallel to series)
(a) 1 : 4
(b) 4 : 1
(c) 1 : 2
(d) 2 : 1

Answer : B

Question. What is the equivalent resistance between the points A and D in given figure?

Current Electricity MCQ Class 12 Physics
(a) 10 Ω
(b) 20 Ω
(c) 30 Ω
(d) 40 Ω

Answer : C

Question. A resistance R is to be measured using a meter bridge. Student chooses the standard resistance S to be 100 Ω. He finds the null point at l₁ = 2.9 cm. He is told to attempt to improve the accuracy. Which of the following is a useful way? 
(a) He should measure l1 more accurately.
(b) He should change S to 1000 Ω and repeat the experiment.
(c) He should change S to 3 Ω and repeat the experiment.
(d) He should give up hope of a more accurate measurement with a meter bridge.

Answer : C