Class 12 Physics Chapter 3: Current Electricity MCQs with answers

Answer: (d) 1.0 A

Negative charge moving right to left is equivalent to positive charge moving left to right, so the net charge carried to the right is 3.0 + 1.0 = 4.0 C. I = q/t = 4.0/4.0 = 1.0 A. Subtracting the two (0.50 A) is the usual slip.

Answer: (d) Ω m

From R = ρl/A, ρ = RA/l, with unit Ω m² / m = Ω m. S m⁻¹ is the unit of conductivity.

Answer: (d) 4.0 × 10⁶ A m⁻²

j = I/A = 2.0 A / (0.50 × 10⁻⁶ m²) = 4.0 × 10⁶ A m⁻².

Answer: (b) opposite to the field, and constant in time

v_d = −(eE/m)τ: the minus sign puts it opposite to E, and it does not depend on time even though each electron accelerates between collisions. The random velocities average to zero, so the drift is due to the field alone and is far smaller than the thermal speed.

Answer: (c) 5.0 cm² V⁻¹ s⁻¹

μ = v_d/E = (0.30 × 10⁻³ m s⁻¹)/(0.60 V m⁻¹) = 5.0 × 10⁻⁴ m² V⁻¹ s⁻¹. Since 1 m² V⁻¹ s⁻¹ = 10⁴ cm² V⁻¹ s⁻¹, μ = 5.0 cm² V⁻¹ s⁻¹.

Answer: (c) copper, germanium, glass

Metals have the lowest resistivity (10⁻⁸ to 10⁻⁶ Ω m), insulators such as glass the highest, and semiconductors such as germanium lie in between.

Answer: (b) decreases

Silicon is a semiconductor; the resistivity of a semiconductor decreases as its temperature rises.

Answer: (a) manganin

Manganin, like nichrome and constantan, has a resistivity that depends very little on temperature, so it is used for wire-bound standard resistors. Copper and aluminium change appreciably; silicon is a semiconductor.

Answer: (d) 250 W

R = V²/P is fixed, so P ∝ V². Halving V gives P = 1000 × (110/220)² = 250 W. Taking P ∝ V gives the wrong 500 W.

Answer: (c) when no current is drawn from it

With I = 0, V = ε − Ir = ε: emf is the potential difference between the electrodes in an open circuit.