Q.1.

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Q.2.
Lenz\'s law applies to
Q.3.
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
Q.4.
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
Q.5.
A step-down transformer has 50 turns on secondary and 1000 turns on primary winding. If a transformer is connected to 220 V and 1 A AC source, what is output current of the transformer ?
Q.6.
A transformer works on the principle of

Explanation

Q.7.
The core of a transformer is laminated to reduce

Explanation

Q.8.
For a large industrial city with much load variations, the DC generator should be

Explanation

Q.9.
Induction furnace is based on the heating effect of

Explanation

Q.10.
Turn ratio is 1.25. The step-up transformer operates at 230 V and current through secondary is 2 A. Then, current in primary is
Q.11.
A six pole generator with fixed field excitation developes an emf of 100 V, when operating at 1500 rpm. At what speed must it rotate to develop 120 V ?
Q.12.
A moving conductor coil in a magnetic field produces an induced e.m.f. This is in accordance with
Q.13.
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 transformer is approximately
Q.14.
Which types of losses do not occur in the transformer ?

Explanation

Q.15.
The turn ratio of a transformer is 1 : 2. An electrolytic DC cell of emf 2 V is connected to its primary. The output voltage across transformer is

Explanation

Q.16.
Quantity that remains unchanged in a transformer is

Explanation

Q.17.
Energy required to establish a current of 4 A in a coil of self inductance L= 200 mH is
Q.18.
In 0.1s, the current in a coil increases from 1 A to 1.5 A. If inductance of coil is 60 mH, then induced current in external resistance of 3 Ω will be
Q.19.
Induced emf in the coil depends upon
Q.20.
A solenoid of inductance 2 H carries a current 1 A. What is the magnetic energy stored in a solenoid ?
Q.21.
Magnetic flux of 10 μWb is linked with a coil, when a current of 2mA flows through it. What is the self inductance of the coil ?
Q.22.
What should be the value of self inductance of an inductor that should be connected to 220 V, 50 Hz supply, so that a maximum current of 0.9 A flows through it ?
Q.23.
When the current changes from + 2 A to -2 A in 0.05 s, an emf of 8V is induced in a coil. The coefficient of self induction of the coil is
Q.24.
If coil is open, then L and R become
Q.25.
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 coil ?
Q.26.
Energy associated with a moving charge is due to

Explanation

Q.27.
The inductance of a coil is L =10 H and resistance R = 5 Ω . If applied voltage of battery is 10 V and it switches off in 1 ms, find induced emf of inductor.
Q.28.
If the coefficient of mutual induction of the primary and secondary coils of an induction coil is 5 H and a current of 10 A is cut-off in 5 × 10-4 s, the emf inducted (in volt) in the secondary coil is
Q.29.
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
Q.30.
A transformer is often filled with oil. The oil used should have

Explanation

Q.31.
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
Q.32.
A generator at a utility company produces 100 A of current at 4000 V. The voltage is stepped-up to 240000 V by a transformer before it is sent on a high voltage transmission line. The current in transmission line is
Q.33.
A current of 5 A is flowing at 220 V in the primary coil of a transformer. If the voltage produced in the secondary coil is 2200 V and 50% of power is lost, then the current in secondary will be
Q.34.
In step-up transformer, relation between number of turns in primary (Np) and number of turns in secondary (Ns ) coils is
Q.35.
The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux ϕ linked with the primary coil is given by ϕ = ϕ0 + 4t , where ϕ is in weber, t is in second and ϕ0 is a constant, the output voltage across the secondary coil is
Q.36.
The transformation ratio in the step-up transformer is
Q.37.
Fleming's left and right hand rule are used in

Explanation

Q.38.
A step-down transformer reduces the voltage of a transmission line from 2200 V to 220 V. The power delivered by it is 880 W and its efficiency is 88%. The input current is
Q.39.
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
Q.40.
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
Q.41.
In an AC generator, a coil with N turns, all of the same area A and total resistance R, rotates with frequency ω in a magnetic field B. The maximum value of emf generated in the coil is
Q.42.
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
Q.43.
The figure shows three circuits with identical batteries, inductors and resistances. Rank the circuits according to the currents through the battery just after the switch is closed, greatest first
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Q.44.
A copper disc of radius 0.1 m is rotated about its centre with 20 rev/s in an uniform magnetic field of 0.1 T with its plane perpendicular to the field. The emf induced across the radius of the disc is
Q.45.
In circular coil, when no. of turns is doubled and resistance becomes 1/4 th of initial, then inductance becomes
Q.46.
A circular coil of radius R = 10 cm having 500 turns and total resistance 2 Ω is placed initially perpendicular to the earth's magnetic field of 3 × 10-5 T. The coil is rotated about its vertical diameter by an angle 2π in 0.5 . The induced current in the coil is
Q.47.
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
Q.48.
A horizontal straight wire 20 m long extending from East to West is falling with a speed of 5.0 ms-1 , at right angles to the horizontal component of the earth's magnetic field 0.030 ×10-4 Wbm-2 . The instantaneous value of the emf induced in the wire will be
Q.49.
Electric fields induced by changing magnetic fields are

Explanation

Q.50.
A square loop of wire, side length 10 cm is placed at angle of 45° with a magnetic field that changes uniformly from 0.1 T to zero in 0.7 s. The induced current in the loop (its resistance is 1 Ω ) is