# Chapter 3 Current Electricity -Important 1 Mark Questions Class 12 CBSE Physics PYQ

**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 V_{d}.

Current, **I = NeA. V _{d}**

**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]**