Previous Year Questions – Important 1 Mark Questions Chapter 3 Current Electricity Class 12 CBSE Physics
Q. 1. Define the term drift velocity of charge carriers in a conductor. Write its relationship with current flowing through it. [CBSE Delhi 2014]
Ans. Drift velocity is defined as the average velocity acquired by the free electrons in a conductor under the influence of an electric field applied across the conductor. It is denoted by Vd.
Current, I = NeA. Vd
Q. 2. The emf of a cell is always greater than its terminal voltage. Why? Give reason. [CBSE Delhi 2013]
Ans. (i) In an open circuit, the emf of a cell and terminal voltage are same.
(ii) In closed circuit, a current is drawn from the source, so, V = E – Ir, it is true/valid, because each cell has some finite internal resistance.
Q. 3. Give an example of a material each for which temperature coefficient of resistivity is
(i) Positive, (ii) Negative. [CBSE Sample Paper 2016]
Ans. (i) Copper (Cu) (Temperature coefficient of resistivity (α) is positive for metals and alloys.)
(ii) Silicon (Si) (For semiconductors, α is negative).
Q. 4. The emf of a cell always greater than its terminal voltage. Why? Give reason. [CBSE Delhi 2013]
Ans. (i) In an open circuit, the emf of a cell and terminal voltage are same.
(ii) In closed circuit, a current is drawn from the source, so, V = E – Ir, it is true/valid, because each cell has some finite resistance.
Q. 5. Distinguish between emf and terminal voltage of a cell. [CBSE Patna 2015]
Ans. The emf of a cell is equal to the terminal voltage, when the circuit is open. The emf of a cell is less than the terminal voltage, when the cell is being charged, i.e., V = E + ir.
Q. 6. Two wires, one of copper and the other of manganin, have same resistance and equal thickness. Which wire is longer? Justify your answer. [CBSE Guwahati 2015]
Ans. Copper
Reason: Let l1 and l2 be lengths of copper and manganin wires having same resistance
R and thickness i.e. area of cross-section (A).
Q. 7. Two wires one of manganin and the other of copper have equal length and equal resistance. Which one of these wires will be thicker? [CBSE (AI) 2012, (South) 2016]
Ans. Resistivity ρ of manganin is much greater than that of copper, therefore to keep same resistance for same length of wire, the manganin wire must be thicker.
Q. 8. Nichrome and copper wires of same length and same radius are connected in series. Current I is passed through them. Which wire gets heated up more? Justify your answer. [CBSE (AI) 2017]
Ans. Nichrome wire gets heated up more. Heat dissipated in a wire is given by
Here, radius is same, hence area (A) is same. Also, current (I) and length (l) are same.
Q. 9. The variation of potential difference V with length l in the case of two potentiometer P and Q is as shown. Which of these two will you prefer for comparing the emfs of two primary cells and why? [CBSE (East) 2016]
Q. 10. I – V graph for two identical conductors of different materials A and B is shown in the figure. Which one of the two has higher resistivity? [CBSE (Chennai) 2015]
