CBSE Class 12 Physics Current Electricity Assignment Set D

UNIT 2

CURRENT ELECTRICITY

 Important Questions for NCERT Class 12 Physics Current Electricity

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. 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. 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. Faraday law of electrolysis indirectly shows
(a) quantisation of charge 
(b) quantisation of angular momentum
(c) quantisation of current
(d) quantisation of viscosity

Answer: A

Question. Two sources of equal emf are connected to an external resistance R. The internal resistance of the two sources are R1 and R2 (R2 > R1). If the potential difference across the source having internal resistance R2 is zero, then 
(a) R = R₂ – R₁
(b) R = R2 × (R₁ + R₂)/(R₂ – R₁)
(c) R = R₁R₂/(R₂ – R₁)
(d) R = R₁R₂/(R₁ – R₂)

Answer: A

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. 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. Three equal resistors connected in series across a source of e.m.f. together dissipate 10 watt of power.    
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. A 4 mF capacitor is charged to 400 V. If its plates are joined through a resistance of 2 kW, then heat produced in the resistance is   
(a) 0.64 J
(b) 1.28 J
(c) 0.16 J
(d) 0.32 J 

Answer: D

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. 50 electric bulbs are connected in series across a 220 V supply and the illumination produced is I₁. 5 bulbs are fused. If the remaining 45 are again connected in series, the illumination produced is I₂. Which of the following is true ?
(a) I₁ = I₂
(b) I₁ < I₂
(c) I₁ > I₂
(d) It will depend on the resistance of each bulb.

Answer: B

Question. A leclanche cell supplies a current of one ampere for ten minutes. The electreochemical equivalent of hydrogen = 0.00001014 gram per coulomb. The mass of hydrogen liberated is :
(a) 0.00625 g
(b) 0.01248 g
(c) 0.01872 g
(d) 0.02496 g.

Answer: A

Question. Three equal resistors connected across a source of e.m.f. together dissipate 10 watt of power. 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) 10/3
(b) 10
(c) 30
(d) 90

Answer: D

Question. In an experiment to measure the internal resistance of a cell, by a potentiometer, it is found that the balance point is at a length of 2 m, when the cell is shunted by a 5 Ω resistance and is at a length of 3 m when the cell is shunted by a 10 Ω resistance. The internal resistance of the cell is then
(a) 1.5 Ω
(b) 10 Ω
(c) 15 Ω
(d) 1 Ω

Answer: B

Question. Twelve resistors each of resistance 16 Ω are connected in the circuit as shown. The net resistance between AB is
(a) 1Ω
(b) 2 Ω
(c) 3 Ω
(d) 4 Ω

Answer: D

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 Ω
(b) 0.2 Ω
(c) 5 Ω
(d) 0.4 Ω

Answer: A

Question. The cold junction of a thermocouple is maintained at 10ºC. No thermo e.m.f. is developed when the hot junction is maintained at 530ºC. The neutral temperature is
(a) 260ºC
(b) 265ºC
(c) 270ºC
(d) 520ºC

Answer: C

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

Answer: D

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) 15 min
(b) 8 min
(c) 4 min
(d) 25 min

Answer: B

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

Answer: C

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) a³B/6b
(b) a³B/3b
(c) a³B/2b
(d) a³B/b

Answer: A

Question. There is an infinite wire grid with cells in the form of equilateral triangles. The resistance of each wire between neighbouring joint connections is R₀. The net resistance of the whole grid between the points A and B as shown is 

(a) R₀
(b) R₀/2
(c) R₀/3
(d) R₀/4

Answer: C

Question. Two 1000 W heaters when connected in parallel across 220 V supply produced heat QP in time t. If they are connected in series across the same power supply the heat produced in the same time is QS. What is QP/QS?
(a) 4
(b) 2
(c) 0.5
(d) 0.25.

Answer: A

Question. Five equal resistances each of resistance R are connected as shown in the figure. A battery of V volts is connected between A and B. The current flowing in AFCEB will be 

(a) 2V/R
(b) 3V/R
(c) V/R
(d) V/2R

Answer: A

Question. A battery is charged at a potential of 15V for 8 hours when the current flowing is 10A. The battery on discharge supplies a current of 5A for 15 hour. The mean terminal voltage during discharge is 14V. The “Watt-hour” efficiency of the battery is
(a) 87.5%
(b) 82.5%
(c) 80%
(d) 90%

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) 20 watt
(b) 60 watt
(c) 180 watt
(d) 10 watt

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/2
(b) R
(c) 2R
(d) R/4

Answer: D

Question. In the network shown, each resistance is equal to R. The equivalent resistance between adjacent corners A and D is 
(a) R 
(b) 2/3 R
(c) 3/7 R
(d) 8/15 R

Answer: D

Question. A battery of e.m.f E and internal resistance r is connected to a variable resistor R as shown. Which one of the following is true ?
(a) Potential difference across the terminals of the battery is maximum where R = r
(b) Power delivered to resistor is maximum when R = r
(c) Current in the circuit is maximum when R = r
(d) Current in the circuit is maximum when R >> r

Answer: B,C

Question. A potentiometer wire of length L and a resistance r are connected in series with a battery of e.m.f. E₀ and a resistance r₁. An unknown e.m.f. E is balanced at a length l of the potentiometer wire. The e.m.f. E will be given by: 

Answer: A

Question. In the circuit shown in figure, the 5W resistance develops 20.00 cal/s due to the current flowing through it. The heat developed in 2 Ω resistance (in cal/s) is
(a) 23.8
(b) 14.2
(c) 11.9
(d) 7.1

Answer: B

Question. If specific resistance of a potentiometer wire is 10–7Ωm current flowing through it, is 0.1 amp and cross sectional area of wire is 10–6 m2, then potential gradient will be
(a) 10^–2 volt/m
(b) 10^–4 volt/m
(c) 10^–6 volt/m
(d) 10^–8 volt/m

Answer: A

Question. Cell having an emf e and internal resistance r is connected across a variable external resistance R. As the resistance R is increased, the plot of potential difference V across R is given by 

Answer: C

Question. A milli voltmeter of 25 milli volt range is to be converted into an ammeter of 25 ampere range. The value (in ohm) of necessary shunt will be
(a) 0.001
(b) 0.01
(c) 1
(d) 0.05

Answer: A

Question. In the circuit shown the cells A and B have negligible resistances. For V_A = 12V, R₁ = 500Ω and R = 100Ω the galvanometer (G) shows no deflection. The value of V_B is
(a) 4 V
(b) 2 V
(c) 12 V
(d) 6 V

Answer: B

Question. A potentiometer wire is 100 cm long and a constant potential difference is maintained across it. Two cells are connected in series first to support one another and then in opposite direction. The balance points are obtained at 50 cm and 10 cm from the positive end of the wire in the two cases. The ratio of emf's is : 
(a) 5 : 1
(b) 5 : 4
(c) 3 : 4
(d) 3 : 2

Answer: D

Question. A potentiometer is an accurate and versatile device to make electrical measurements of E.M.F. because the method involves 
(a) Potential gradients
(b) A condition of no current flow through the galvanometer
(c) A combination of cells, galvanometer and resistances
(d) Cells

Answer: B

Question. The resistance of the four arms P, Q, R and S in a Wheatstone’s bridge are 10 ohm, 30 ohm, 30 ohm and 90 ohm, respectively. The e.m.f. and internal resistance of the cell are 7 volt and 5 ohm respectively. If the galvanometer resistance is 50 ohm, the current drawn from the cell will be
(a) 0.2 A
(b) 0.1 A
(c) 2. 0 A
(d) 1. 0 A

Answer: A

Question. A heating coil is labelled 100 Ω, 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) 25 J
(b) 50 J
(c) 200 J
(d) 400 J

Answer: D

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. If 25W, 220 V and 100 W, 220 V bulbs are connected in series across a 440 V line, then
(a) only 25W bulb will fuse
(b) only 100W bulb will fuse
(c) both bulbs will fuse
(d) None of these

Answer: A

Question. In the circuit shown in the figure, if potential at point A is taken to be zero, the potential at point B is
(a) –1V
(b) + 2V
(c) –2V
(d) + 1V

Answer: D
 

1. The electron drift speed in a metallic conductor is only a few mm s^-1 for currents in the range of the few amperes. How then is the current established almost at the instant the circuit is closed?

2. When electrons drift in a metal from lower to higher potential, does it mean that the free electrons of the metal are moving in the same direction?

3. Two conducting wires X and Y of same diameter but different materials are joined in series across a battery. If the number density of electrons in X is twice that in Y, find the ratio of drift velocity of electrons in the two wires.

4. Two wires on e of manganin and the other of copper have equal length and equal resistance. Which one of these wires will be thicker?

5. Two 120 V light bulbs, one of 25 W and the other of 200 W were connected in series across a 240 V line. One bulb burnt out almost instantaneously. Which one was burnt and why?

6. A cell of emf E and internal resistance r is connected across an external resistance R. Plot a graph showing the variation of P.D. across R, verses R.

7. Draw the graphs showing the variation of resistivity with temperature for

(i) nichrome and (ii) silicon.

8. The circuit shown in the diagram contains a battery ‘B’, a rheostat ‘Rh’ and identical lamps P and Q. What will happen to the brightness of the lamps, if the resistance through the rheostat is increased? Give reasons.

Please click the below link to access CBSE Class 12 Physics Current Electricity Assignment Set D.