CBSE Class 12 Physics Current Electricity Boards Questions Worksheet

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   B

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  C

Question. How many electrons pass through a lamp in 1 min if the current is 300mA?
(a) 1.125x10²⁰
(b) 1.875x10^-18
(c) 1.875x10¹⁸
(d) 1.125x10^-20
Answer. B

Question. Drift velocity varies with the intensity of electric field as per the relation
(a) V α E
(b) V α 1/E
(c) V α E²
(d) V α E^-2
Answer. A

Question. In a Wheatstone bridge circuit, P = 7Ω, Q =8Ω, R = 12 Ω and S = 7 Ω. Find the additional resistance to be used in series with S, so that the bridge is balanced
(a) 6.72 Ω
(b) 7.62 Ω
(c) 2.67 Ω
(d) 6.27 Ω
Answer. A

Question. A cell of emf 2V, when short circuited gives a current of 4A. What is the internal resistance of the cell in ohm?
(a) 0.5
(b) 1.0
(c) 2.0
(d) 4.0
Answer. A

Question. When a current of 0.2 A is drawn from a battery then the potential difference between its terminals is 20V and when a current of 2A is drawn, then the potential difference drops to 16V. The emf of the battery is
(a) 15.1V
(b) 20.4V
(c) 18.9V
(d) 23.3V
Answer. B

Question. If two identical cells when connected in series or in parallel, supply the same amount of current through an external resistance of 2Ω, the internal resistance of the cell is
(a) 8 Ω
(b) 2 Ω
(c) 4 Ω
(d) 1 Ω
Answer. B

Question. Kirchhoff’s second law for the electric network is based on
(a) Law of conservation of charge
(b) Law of conservation of energy
(c) Law of conservation of angular momentum
(d) Law of conservation of mass
Answer. B

Question. If percentage change in current through a resistor is 1%, then the change in power through it would be
(a) 1%
(b) 2%
(c) 1.7%
(d) 0.5%
Answer. B

VERY SHORT ANSWER QUESTIONS

Question. If a wire is stretched to double its original length without loss of mass, how will the resistivity of the wire be influenced?
Answer. No change. Resistivity depends on the nature of the material and temperature

Question. Does the value of resistance of a wire depend on the potential difference applied across it?
Answer. No. Resistance depends on the temperature, nature and dimensions of the material

Question. What are the factors affecting the internal resistance of a cell?
Answer. Nature of the electrolyte
Directly proportional to the concentration of the electrolyte
Directly proportional to the distance between the electrodes
Varies inversely as the common area of the electrodes
Increases with the decrease in temperature of the electrolyte

Question. A 100 W and 500W bulb are joined in parallel to the mains. Which bulb will glow brighter?
Answer. In parallel same voltage V is applied to both the bulbs. But 500W bulb has smaller resistance , so it will produce more heat

Question. Using the concept of drift velocity of charge carriers in a conductor deduce the relationship between current density and resistivity of the conductor
Answer. 𝐽 = 𝐸/𝜌 ( explained in the summary)

Question. Two bulbs are marked 220V,100W and 220V and 50W respectively. They are connected in series to 220V mains. Find the ratio of heat generated in them
Answer. In series connection current is same 𝐻1/𝐻2=𝑅1/𝑅2

Question. A potential difference of 6V is applied across a conductor of length 0.12m. Calculate the drift velocity of the electrons, it the electron mobility is 5.6 x 10⁶m²v^-1s^-1
Answer. 𝑉 = μ𝐸 = μ𝑉𝐿

Question. Explain why electric power transmitted at high voltages and low currents to distant places
Answer. To minimise power loss due to Joule heating

Question. A wire when connected to 220V main supply, has power dissipation P1. Now the wire is cut into two equal pieces which are connected in parallel to the same supply, power dissipation in this case is P2. Find the ratio P2/P1
Answer. 𝑃= 𝑉2/𝑅
𝑃1𝑃2=𝑅2𝑅1= 1:4

1. The materials can be classified as conductors, semi-conductors and insulators depending on their resistivities. Metals have low resistivities in the range of 10-8Ωm. At the other end are insulators like ceramic, rubber and plastics having resistivities 10-18 times greater than metals or more. In between these two are the semiconductors. These however have resistivities characteristically decreasing with a rise in temperature. The resistivities of semiconductors can be decreased by adding small amount of suitable impurities. This last feature is exploited in use of semiconductors for electronic devices.

Question. Which of the following material can be used for heating purpose in electric geyser?
(i) Copper
(ii) Aluminium
(iii) Gold
(iv) Nichrome
Answer. D

Question. The temperature of coefficient of resistance is negative for
(i) Copper
(ii) Gold
(iii) Carbon
(iv) Silver
Answer. B

Question. The resistance of a wire at 20⁰C is 20Ω and at 5000C it is 60Ω. At what temperature the resistance is the temperature is 25Ω
(i) 160⁰C
(ii) 250⁰C
(iii) 100⁰C
(iv) 80⁰C
Answer. D

Question. The product of resistivity and conductivity of a conductor depends on
(i) Area of cross section ‘
(ii) Temperature
(iii) Length
(iv) None of these
Answer. D

2. Wheatstone bridge, also known as the resistance bridge, is the setup that is used for measuring the unknown resistance. It was invented by Samuel Hunter Christie in 1833 and was later popularized by Sir Charles Wheatstone in 1843. A Wheatstone bridge comprises four arms which are termed as resistors, and among which the ratio of two resistors is kept at a fixed value and the two arms left, that is, the remaining arms are balanced, one of them can be varied while the other arm is an unknown resistor.
Then through the method of balancing or null condition, the unknown resistance is calculated. The circuit of the Wheatstone bridge provides the exact measurement of the resistance. There are many variations of the Wheatstone bridge that are utilised for the AC circuits.

Question. Why the Wheatstone bridge is more accurate than the other methods of measuring resistance?
(i) It has four resistor arms
(ii) It is based on Kirchhoff’s laws
(iii) It does not involve ohm’s law
(iv) It is null method
Answer. D

Question. In a balanced Wheatstone’s bridge network, the resistance in arms Q and S are interchanged. As a result of this
(i) galvanometer and cell must be interchanged to balance
(ii) galvanometer shows null deflection
(iii) Network is not balanced
(iv) network is still balanced
Answer. C

Question. In a Wheatstone bridge circuit P= 5Ω, Q =6Ω, R = 10Ω and S = 5Ω. What is the additional resistance to be used in series with S, so that the bridge is balanced
(i) 5Ω
(ii) 7 Ω
(iii) 10 Ω
(iv) 9 Ω
Answer. B