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Fleming’s left and right-hand rules are used in

1) DC motor and AC generator 2) DC generator and AC motor 3) DC motor and DC generator 4) Both rules are the same, anyone can be used... View Article

The transformation ratio in the step-up transformer is

1) one 2) greater than one 3) less than one 4) the ratio greater or less than one depends on the other factors Answer: 2) greater than... View Article

A transformer of efficiency 90% draws an input power of 4 kW. An electrical appliance connected across the secondary draws a current of 6 A. The impedance of the device is

1) 60 Ω 2) 50 Ω 3) 80 Ω 4) 100 Ω 5) 120 Ω Answer: 4) 100 Ω Solution: η = 90/100 = 0.9 = output power/input power P0... View Article

An electric motor runs a DC source of emf 200 V and draws a current of 10 A. If the efficiency is 40%, then the resistance of the armature is

1) 5 Ω 2) 12 Ω 3) 120 Ω 4) 160 Ω Answer: 2) 12 Ω Solution: Input power = VI = 200 x 10 = 2000 W Output power = (40/100) x 2000 = 800... View Article

Energy associated with a moving charge is due to

1) electric field 2) magnetic field 3) Both (a) and (b) 4) None of these Answer: 1) electric field Solution: The energy associated... View Article

In a coil when current changes from 10 A to 2 A in time 0.1 s, induced emf is 3.28 V. What is self-inductance of the coil?

1) 4H 2) 0.4 H 3) 0.04 H 4) 5 H Answer: 3) 0.04 H Solution: Induced emf = L x (di/dt) 3.28 V = L x [(10-2)/0.1] = L x 80 ⇒ L =... View Article

If coil is open, then L and R become

1) ∞ and 0 2) 0 and ∞ 3) ∞ and ∞ 4) 0 and 0 Answer: 2) 0 and ∞ Solution: When the coil is open, the... View Article

A solenoid of inductance 2 H carries a current 1 A. What is the magnetic energy stored in a solenoid?

1) 2 J 2) 1 J 3) 4 J 4) 5 J Answer:2) 1 J Solution: Magnetic energy stored, Um = (1/2)Li2 = (1/2) x 2 x 12 = 1 J... View Article

Induced emf in the coil depends upon

1) conductivity of coil 2) amount of flux 3) rate of change of linked flux 4) resistance of coil Answer: 3) rate of change of linked... View Article

In 0.1s, the current in a coil increases from 1 A to 1.5 A. If the inductance of the coil is 60 mH, then induced current in external resistance of 3 Ω will be

1) 1 A 2) 0.5 A 3) 0.2 A 4) 0.1 A Answer: 4) 0.1 A Solution: Induced emf is, L(di/dt) = 60 x [(1.5 - 1)/0.1] = 300 mV The induced... View Article

Energy required to establish a current of 4 A in a coil of self inductance L= 200 mH is

1) 0.16 J 2) 0.18 J 3) 0.40 J 4) 1.6 J Answer: 4) 1.6 J Solution: Energy stored in a self-inductor, E = (1/2)Li2 = (1/2) x 200 x 10-3... View Article

Quantity that remains unchanged in a transformer is

1) voltage 2) current 3) frequency 4) None of these Answer: 3) frequency Solution: Frequency remains unchanged in a transformer... View Article

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of a transformer is approximately

1) 30% 2) 50% 3) 90% 4) 10% Answer: 3) 90% Solution: Power = 100 W Current in the primary, Ip = 0.5 A Voltage across the primary, Vp... View Article

A transformer rated at 10 kW is used to connect a 5 kV transmission line to a 240 V circuit. The ratio of turns in the windings of the transformer is

1) 5 2) 20.8 3) 104 4) 40 Answer: 2) 20.8 Solution: The ratio of turns in the windings of the transformer is (Np/Ns) = (Vp/Vs)... View Article

The number of turns in primary and secondary coils of a transformer is 50 and 200, respectively. If the current in the primary coil is 4 A, then the current in the secondary coil is

1) 1 A 2) 2 A 3) 4 A 4) 5 A Answer: 1) 1 A Solution: In a transformer Np/Ns = Is/Ip (50/200) = Is/1 ⇒ Is = 1 A... View Article

Turn ratio is 1.25. The step-up transformer operates at 230 V and the current through secondary is 2 A. Then, current in the primary is

1) 25 A 2) 100 A 3) 50 A 4) 20 A Answer:3) 50 A Solution: in an ideal transformer, ⇒ Ip = (25/1) x 2 = 50 A... View Article