Board-style MCQs and past-paper numericals in the BIEK / Sindh Board pattern. Tap an option to check yourself instantly. Solved questions are at the bottom.
Multiple-choice questions
Solved numericals (past papers)
Force on a current-carrying conductor
A straight wire of length 0.20 m carries a current of 5 A at right angles to a uniform field B = 0.40 T. Find the force on it.
F = BIL sinθ = 0.40 × 5 × 0.20 × sin90° = 0.40 × 5 × 0.20 × 1 = 0.40 N
Force on a moving charge
A proton (q = 1.6 × 10⁻¹⁹ C) moves at v = 2 × 10⁶ m/s perpendicular to a field B = 0.50 T. Find the magnetic force.
F = qvB sinθ = 1.6 × 10⁻¹⁹ × 2 × 10⁶ × 0.50 × 1
= 1.6 × 10⁻¹³ ≈ 1.6 × 10⁻¹³ N
Magnetic field inside a solenoid
A solenoid has 1000 turns over a length of 0.50 m and carries 2 A (μ₀ = 4π × 10⁻⁷ T·m/A). Find B inside.
n = N/L = 1000/0.50 = 2000 turns/m
B = μ₀ n I = 4π × 10⁻⁷ × 2000 × 2 = 16π × 10⁻⁴
= 5.0 × 10⁻³ T (≈ 5 mT)
Magnetic field of a long straight wire
A long straight wire carries 10 A. Find B at a perpendicular distance of 0.05 m (μ₀ = 4π × 10⁻⁷).
B = μ₀ I / (2πr) = (4π × 10⁻⁷ × 10) / (2π × 0.05)
= (2 × 10⁻⁷ × 10) / 0.05 = 2 × 10⁻⁶ / 0.05 = 4.0 × 10⁻⁵ T
Torque on a current-carrying coil
A rectangular coil of 50 turns and area 2 × 10⁻³ m² carries 3 A in a field of 0.40 T, its plane parallel to B. Find the maximum torque.
τ = NBIA = 50 × 0.40 × 3 × 2 × 10⁻³
= 50 × 0.40 × 3 × 0.002 = 0.12 N·m
Radius of a charged particle's circular path
An electron (m = 9.1 × 10⁻³¹ kg, q = 1.6 × 10⁻¹⁹ C) enters a field B = 1.0 × 10⁻³ T at v = 1.0 × 10⁷ m/s, perpendicular to B. Find the radius of its circular path.
r = mv / (qB) = (9.1 × 10⁻³¹ × 1.0 × 10⁷) / (1.6 × 10⁻¹⁹ × 1.0 × 10⁻³)
= (9.1 × 10⁻²⁴) / (1.6 × 10⁻²²) = 5.7 × 10⁻² m (≈ 5.7 cm)