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A coil of inductance 300 mH and resistance 2Ω is connected to a source of voltage 2V. The current reaches half of its steady-state value in………

1) 0.15 sec 2) 0.3 sec 3) 0.05 sec 4) 0.1sec Answer: 4) 0.1sec Solution: I = I0(1-e-Rt/L) I0/2 = I0(1-e-Rt/L) e-Rt/L = (1/2) = 2-1 Rt/L... View Article

The time constant of an LR circuit is 20 ms The circuit is connected at t = 0 and the steady-state current is found to be 4A. Find the current at 80 ms.

1) 0.98 A 2) 1 A 3) 0.44 A 4) 0.88 A Answer: 4) 0.88 A Solution: I = I0(1-et/Τ) at t = 0, I = I0 At t = 0, I = 4A hence I0 = 4A I =... View Article

A coil of wire of radius r has 600 turns and a self-inductance of 108 mH. The self-inductance of a coil with the same radius and 500 turns is

1) 80 mH 2) 75 mH 3) 108 mH 4) 90 mH Answer: 4) 90 mH Solution: L ∝ N (L1/L2) = (N1/N2) L2 = L1 x (N2/N1) = 108 x... View Article

Whenever a magnet is moved either towards or away from a conducting coil, an emf is induced, the magnitude of which is independent of

1) the strength of the magnetic field 2) the speed with which the magnet is moved 3) the number of turns of the coil 4) the resistance of the... View Article

Lenz’s law of electromagnetic induction corresponds to the

1) law of conservation of charge 2) law of conservation of energy 3) law of conservation of momentum 4) law of conservation of angular... View Article

Two similar circular loops carry equal currents in the same direction. On moving coils further apart, the electric current will

1) increase in both 2) decrease in both 3) remain unaltered 4) increases in one and decreases in the second Answer: 1) increase in both... View Article

An inductor has a coefficient of self-induction of 40 mH. What is the energy stored in it, when a current of 2 A is passed through it?

1) 40 mJ 2) 80 mJ 3) 20 mJ 4) 100 mJ Answer: 2) 80 mJ Solution: U = (1/2) Li2 = (1/2) x 40 x 10-3 x 22 = 80 x 10-3 = 80 mJ... View Article

A current passing through a coil of self-inductance of 2 mH changes at the rate of 20 mAs^-1. The emf induced in the coil is

1) 10 µV 2) 40 µV 3) 10 mV 4) 40 mV Answer: 2) 40 µV Solution: e = -L(dI/dt) |e| = 2 x 10-3 x 20 x 10-3 = 40... View Article

A boat is moving due East in a region where the earth’s magnetic field is 5.0 × 10^-5 NA^-1 m^-1 due North and horizontal. The boat carries a vertical aerial 2 m long. If the speed of the boat is 1.50 ms^-1, the magnitude of the induced emf in the wire of aerial is

1) 0.75 mV 2) 0.50 mV 3) 0.15 mV 4) 1 mV Answer: 3) 0.15 mV Solution: Induced EMF, ε = Bvl = 5.0 ×... View Article

A step-down transformer transforms a supply line voltage of 2200 V into 220 V. The primary coil has 5000 turns. The efficiency and power transmitted by the transformer are 90% and 8 kW respectively. Then, the power supplied is

1) 9.89 kW 2) 8.89 kW 3) 88.9 kW 4) 889 kW Answer: 2) 8.89 kW Solution: Efficiency = Output power/Input power (90/100) = 8/Pinput Pinput... View Article

A step-down transformer is used on a 1000 V line to deliver 20 A at 120 V at the secondary coil. If the efficiency of the transformer is 80%, the current drawn from the line is

1) 3 A 2) 30 A 3) 0.3 A 4) 2.4 A Answer: 1) 3 A Solution: Efficiency of a transformer = (Output Power/Input Power) ... View Article

A transformer has an efficiency of 80%. It is connected to a power input of 5 kW at 200 V. If the secondary voltage is 250 V, the primary and secondary currents are respectively

1) 25 A, 20 A 2) 20 A, 16A 3) 25 A, 16 A 4) 40 A, 25 A 5) 40 A, 16 A Answer: 3) 25 A, 16 A Solution: Input power = 5 kW = 5000 W... View Article

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