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A coil having an area 2m² is placed in a magnetic field which changes from 1 Wb/m² to 4 Wb/m² in an interval of 2 second. The e.m.f. induced in the coil will be

1) 4 V 2) 3 V 3) 1.5 V 4) 2 V Answer: 2) 3 V Solution: A = 2m2 (dB/dt) = [(4 - 1)]/2 = (3/2) T/s e = dΦ/dt =... View Article

A 50 turns circular coil has a radius of 3 cm, it is kept in a magnetic field acting normal to the area of the coil. The magnetic field B increased from 0.10 tesla to 0.35 tesla in 2 milliseconds. The average induced e.m.f. in the coils is

1) 1.77 volts 2) 17.7 volts 3) 177 volts 4) 0.177 volts Answer: 2) 17.7 volts Solution: N = 50 turns r = 3 x 10-2 m B1 = 0.10 B2 =... View Article

According to phenomenon of mutual inductance

1) The mutual inductance does not depend on geometry of two coils involved 2) The mutual inductance depends on the intrinsic magnetic property,... View Article

A cylindrical bar magnet is kept along the axis of a circular coil. If the magnet is rotated about its axis, then

1) A current will be induced in a coil 2) no current will be induced in a coil 3) Only an e.m.f. will be induced in the coil 4) An e.m.f. and... View Article

According to Lenz’s law of electromagnetic induction

1) The induced e.m.f. is not in the direction opposing the change in magnetic flux 2) The relative motion between the coil and magnet produces a... View Article

A coil of inductive reactance 31Ω has a resistance of 8Ω. It is placed in series with a condenser of capacitive reactance 25Ω. The combination is connected to an a.c. source of 110 volts. The power factor of the circuit is.

1) 0.80 2) 0.33 3) 0.56 4) 0.64 Answer: 1) 0.80 Solution: = 8/10 = 0.8 ... View Article

The power factor of a good choke coil is

1) Nearly zero 2) Exactly zero 3) Nearly one 4) Exactly one Answer:1) Nearly zero Solution: The phase difference 'θ' between the... View Article

The same current is flowing in two alternating circuits. The first circuit contains only inductance and the other contains only a capacitor. If the frequency of the emf of ac is increased the effect on the value of the current will be.

1) Increase in the first circuit and decrease in other 2) Increase in both the circuit 3) Decrease in both the circuit 4) Decrease in the... View Article

In an ac circuit with voltage V and current I, the power dissipated is.

1) VI 2) (1/2)VI 3) (1/√2)VI 4) Depends on the phase between V and I Answer: 4) Depends on the phase between V and I Solution: P =... View Article

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

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