CBSE Class 12 Physics Current Electricity Worksheet Set C

Important Questions for NCERT Class 12 Physics Current Electricity

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. Ten identical cells connected in series are needed to heat a wire of length one meter and radius ‘r’ by 10°C in time ‘t’. How many cells will be required to heat the wire of length two meter of the same radius by the same temperature in time ‘t’?
(a) 20
(b) 30
(c) 40
(d) 10

Answer: A

Question. Two identical batteries each of e.m.f 2 V and internal resistance 1 W are available to produce heat in an external resistance by passing a current through it.
The maximum power that can be developed across R using these batteries is
(a) 3.2 W
(b) 2.0 W
(c) 1.28 W
(d) 8/9 W

Answer: B

Question. Kirchhoff’s first law, i.e. S i = 0 at a junction, deals with the conservation of
(a) momentum
(b) angular momentum
(c) charge
(d) energy

Answer: C

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. In a Wheatstone’s bridge all the four arms have equal resistance R. If the resistance of the galvanometer arm is also R, the equivalent resistance of the combination as seen by the battery is
(a) R /4
(b) R /2
(c) R
(d) 2R 

Answer: C

Question. The resistance of each arm of the Wheatstone’s bridge is 10 ohm. A resistance of 10 ohm is connected in series with a galvanometer then the equivalent resistance across the battery will be
(a) 10 ohm
(b) 15 ohm
(c) 20 ohm
(d) 40 ohm

Answer: A

Question. A resistance wire connected in the left gap of a metre bridge balances a 10 W resistance in the right gap at a point which divides the bridge wire in the ratio 3 : 2. If the length of the resistance wire is 1.5 m, then the length of 1 W of the resistance wire is
(a) 1.0 × 10^–2 m
(b) 1.0 × 10^–1 m
(c) 1.5 × 10^–1 m
(d) 1.5 × 10^–2 m

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 ar0e 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) 3 : 4
(b) 3 : 2
(c) 5 : 1
(d) 5 : 4

Answer: B

Question. A battery of e.m.f 10 V and internal resistance 0.5 W is connected across a variable resistance R. The value of R for which the power delivered in it is maximum is given by
(a) 0.5 Ω
(b) 1.0 Ω
(c) 2.0 Ω
(d) 0.25 Ω

Answer: A

Question. A potentiometer wire has length 4 m and resistance 8 W. The resistance that must be connected in series with the wire and an accumulator of e.m.f. 2 V, so as to get a potential gradient 1 mV per cm on the wire is
(a) 44 W
(b) 48 W
(c) 32 W
(d) 40 W

Answer: C

Question. A potentiometer circuit has been set up for finding the internal resistance of a given cell. The main battery, used across the potentiometer wire, has an emf of 2.0 V and a negligible internal resistance.
The potentiometer wire itself is 4 m long. When the resistance R, connected across the given cell, has values of
(i) infinity
(ii) 9.5 W
the balancing lengths on the potentiometer wire are found to be 3 m and 2.85 m, respectively. The value of internal resistance of the cell is
(a) 0.25 W
(b) 0.95 W
(c) 0.5 W
(d) 0.75 W

Answer: C

Question. A cell can be balanced against 110 cm and 100 cm of potentiometer wire, respectively with and without being short circuited through a resistance of 10 W.
Its internal resistance is
(a) 2.0 ohm
(b) zero
(c) 1.0 ohm
(d) 0.5 ohm 

Answer: C

Question. Same length of two identical wires are first connected is series and then in parallel, then the amount of heat produced in both the conditions are in the ratio : 
(a) 1 : 4
(b) 4 : 1
(c) 3 : 1
(d) 1 : 2

Answer: B

Question. Two long conductors, separated by a distance d carry current I1 and I2 in the same direction. They exert a force F on each other. Now the current in one of them is increased to two times and its direction is reversed. The distance is also increased to 3d. The new value of the force between them is 
(a) - 2F /3
(b) F/ 3
(c) –2 F
(d) - F /3

Answer: A

Question. A wire of radius r has resistance R. If it is stretched to a radius  r/2 , its resistance will be :   
(a) 16 R
(b) 2 R
(c) 4 R
(d) zero

Answer: A

Question. A potentiometer is an accurate and versatile device to make electrical measurements of EMF 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. 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 ar0e 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) 3 : 4
(b) 3 : 2
(c) 5 : 1
(d) 5 : 4

Answer: B

Question. In a metre bridge, the balancing length from the left end (standard resistance of one ohm is in the right gap) is found to be 20 cm. The value of the unknown resistance is
(a) 0.8 W
(b) 0.5 W
(c) 0.4 W
(d) 0.25 W

Answer: D

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. 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. Seven resistances, each of value 20 W, are connected to a 2 V battery as shown in the figure. The ammeter reading will be
(a) 1/10 A
(b) 3/10 A
(c) 4/10 A
(d) 7/10 A.

Answer: C

Question. What is the equivalent resistance between the points A and D in given figure?
(a) 10 Ω
(b) 20 Ω
(c) 30 Ω
(d) 40 Ω

Answer: C

Question. Water boils in the electric kettle in 15 minutes after switching on. If the length of heating wire is decreased to 2/3 of its initial value, then the same amount of water will boil with the same supply voltage in
(a) 8 minutes
(b) 10 minutes
(c) 12 minutes
(d) 15 minutes

Answer: B

Question. The power dissipated in the circuit shown in the figure is 30 Watts. The value of R is
(a) 20 Ω
(b) 15 Ω
(c) 10 Ω
(d) 30 Ω

Answer: C

Question. A potentiometer wire, 10 m long, has a resistance of 40Ω. It is connected in series with a resistance box and a 2 V storage cell. If the potential gradient along the wire is 0.1 m V/cm, the resistance unplugged in the box is
(a) 260 Ω
(b) 760 Ω
(c) 960 Ω
(d) 1060 Ω

Answer: B

Question. A cell when balanced with potentiometer gave a balance length of 50 cm. 4.5 W external resistance is introduced in the circuit, now it is balanced on 45 cm. The internal resistance of cell is
(a) 0.25 Ω
(b) 0.5 Ω
(c) 1.0 Ω
(d) 1.5 Ω

Answer: A

Question. Two identical batteries each of e.m.f. 2 V and internal resistance 1 Ω are available to produce heat in an external resistance by passing a current through it. The maximum power that can be developed across R using these batteries is
(a) 3.2 Ω
(b) 2.0 Ω
(c) 1.28 Ω
(d) 8/9 Ω

Answer: B

Question. A heater of 220 V heat a volume of water in 5 minutes time.
A heater of 110 V heats the same volume of water in
(a) 5 minutes
(b) 8 minutes
(c) 10 minutes
(d) 20 minutes

Answer: D

Question. The internal resistance of a primary cell is 4W. It generates a current of 0.2 A in an external reistance of 21 W. The rate of chemical energy consumed in providing the current is
(a) 0.42 J s^–1
(b) 0.84 J s^–1
(c) 1 J s^–1
(d) 5 J s^–1

Answer: C

Question. In the circuit shown, the internal resistance of the cell is negligible. The steady state current in the 2Ω resistor is
(a) 0.6 A
(b) 0.9 A
(c) 1.2 A
(d) 1.5 A

Answer: B

Question. A wire has a resistance 12 W. It is bent in the form of a circle. The effective resistance between two points on any diameter is
(a) 6 Ω
(b) 3 Ω
(c) 12 Ω
(d) 24 Ω

Answer: B

Question. For measuring voltage of any circuit, potentiometer is preferred to voltmeter because
(a) the potentiometer is cheap and easy to handle.
(b) calibration in the voltmeter is sometimes wrong .
(c) the potential draws no current during measurement.
(d) range of the voltmeter is not as wide as that of the potentiometer.

Answer: C

Question. 2, 4 and 6 S are the conductances of three conductors.
When they are joined in series, their equivalent conductance will be
(a) 12 S
(b) (1/12) S
(c) (12/11) S
(d) (11/12) S

Answer: C

Question. An ammeter has a resistance of G ohm and a range of I amp.
The value of resistance used in parallel to convert it into an ammeter of range nI amp is
(a) nG
(b) (n – 1)G
(c) G/n
(d) G/(n – 1)

Answer: D

Question. Potentiometer wire of length 1 m is connected in series with 490Ω resistance and 2 V battery. If 0.2 mV/cm is the potential gradient, then resistance of the potentiometer wire is
(a) 4.9 Ω
(b) 7.9 Ω
(c) 5.9 Ω
(d) 6.9 Ω

Answer: A

Question. A conductor carries a current of 50 μ A. If the area of crosssection of the conductor is 50 mm², then value of the current density in Am^–2 is
(a) 0.5
(b) 1
(c) 10^–3
(d) 10–6

Answer: B

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

1. What is an electric current? Give the formula to find the electric current. Define its SI unit. 

2. Name the phenomenon in which charges flow from the clouds to the earth through the atmosphere. 

3. Name any two devices where a steady current flow through the device. 

4. Name the layer of the atmosphere in which charged particles exist. 

5. What are conductors? What is the effect of an electric field on a conductor? 

6. State Ohm’s Law. 

7. Define resistance of a conductor. Give the SI unit of resistance. Give the factors on which the resistance of a conductor depends upon. . 

8. Define current density. What is its SI unit of measurement? .

9. What is resistivity of a material? Give the factors on which the resistivity of a material depends upon. How is different from conductivity? 

10. What are the limitations of Ohm’s law? 

11.  Define mobility. Derive an expression to find the mobility of a charged particle.

12. The electron drift arises due to the force experienced by electrons in the electric field inside the conductor. But force should cause acceleration. Why then do the electrons acquired steady average drift sped? 

13. Are the paths of electrons straight lines between successive collisions (with the positive ins of the metal) in the (i) presence of electric field, (ii) presence of electric field? 

14 Name the materials that are used in wire bound resistors. Give reason why these materials are used in making these resistors?

15 Give the colour code used in case of carbon resistors. Also give any two advantages of these resistors.

16 Plot a graph showing the variation resistivity of the following materials with temperature (a) copper (b) nichrome (c) semiconductor.

17 Give the formula to find the resistivity of a material. State the factors on which the resistivity or the conductivity of a material depends upon.

Important Questions for NCERT Class 12 Physics Current Electricity

Question. A set of n equal resistors, of value R each, are connected in series to a battery of emf E and internal resistance R. The current drawn is I. Now, the n resistors are connected in parallel to the same battery. Then the current drawn from battery becomes 10I.   
The value of n is
(a) 10
(b) 11
(c) 20
(d) 9 

Answer  A

Question. A constant voltage is applied between the two ends of a uniform metallic wire. Some heat is developed in it. The heat developed is double if
(a) both the length and radius of wire are halved    
(b) both length and radius of wire are doubled
(c) the radius of wire is doubled
(d) the length of the wire is doubled

Answer  B

Question. The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10 W is    
(a) 0.8 W
(b) 1.0 W
(c) 0.2 W
(d) 0.5 W

Answer  D

Question. A current of 2 A flows through a 2 W resistor when connected across a battery. The same battery supplies a current of 0.5 A when connected across a 9 W resistor. The internal resistance of the battery is    
(a) 0.5 W
(b) 1/3 W
(c) 1/4 W
(d) 1 W 

Answer  B

Question. A student measures the terminal potential difference (V ) of a cell (of emf e and internal resistance r) as a function of the current (I) flowing through it. The slope, and intercept, of the graph between V and I, then, respectively, equal    
(a) –r and e
(b) r and –e
(c) –e and r
(d) e and –r 

Answer  A

Question. A potentiometer wire, 10 m long, has a resistance of 40W. It is connected in series with a resistance box and a 2 V storage cell. If the potential gradient along the wire is 0.1 m V/cm, the resistance unplugged in the box is    
(a) 260 W
(b) 760 W
(c) 960 W
(d) 1060 W

Answer  B

Question. For a cell terminal potential difference is 2.2V when circuit is open and reduces to 1.8 V when cell is connected to a resistance of R = 5 W. Determine internal resistance of cell (r).  
(a) 10 /9Ω
(b) 9 /10Ω
(c) 11 /9Ω
(d) 5 /9Ω

Answer  A

Question. When the power delivered by a 100 volt battery is 40 watts the equivalent resistance of the circuit is :  
(a) 100 ohms
(b) 250 ohms
(c) 300 ohms
(d) 350 ohms

Answer  B

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. A car battery of emf 12 V and internal resistance 5 × 10–2 W, receives a current of 60 amp from external source, then terminal potential difference of battery is    
(a) 12 V
(b) 9 V
(c) 15 V
(d) 20 V

Answer  C

Question. Three resistances P, Q, R each of 2 W and an unknown resistance S form the four arms of a Wheatstone bridge circuit. When a resistance of 6 W is connected in parallel to S the bridge gets balanced. What is the value of S ?   
(a) 3 W
(b) 6 W
(c) 1 W
(d) 2 W

Answer  A

Question. In a Wheatstone’s bridge all the four arms have equal resistance R. If the resistance of the galvanometer arm is also R, the equivalent resistance of the combination as seen by the battery is    
(a) R /4
(b) R /2
(c) R
(d) 2R

Answer  C

Question. The resistance of each arm of the Wheatstone’s bridge is 10 ohm. A resistance of 10 ohm is connected in series with a galvanometer then the equivalent resistance across the battery will be    
(a) 10 ohm
(b) 15 ohm
(c) 20 ohm
(d) 40 ohm

Answer  A

Question. A resistance wire connected in the left gap of a metre bridge balances a 10 W resistance in the right gap at a point which divides the bridge wire in the ratio 3 : 2. If the length of the resistance wire is 1.5 m, then the length of 1 W of the resistance wire is    
(a) 1.0 × 10^–2 m
(b) 1.0 × 10^–1 m
(c) 1.5 × 10^–1 m
(d) 1.5 × 10^–2 m

Answer  B

Question. In a metre bridge, the balancing length from the left end (standard resistance of one ohm is in the right gap) is found to be 20 cm. The value of the unknown resistance is   
(a) 0.8 W
(b) 0.5 W
(c) 0.4 W
(d) 0.25 W

Answer  D