Question # 1

The magnitude of emf of a conductor of length L and velocity V is equal to:

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VB/ L

VL/ B

VBL

qVBL

During each cycle, alternating voltage reaches a peak value:

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One time

Two time

Four-time

A number of times depending on the frequency

When no power is drawn in the secondary coil of an ideal transformer, the power of the primary coil of an ideal transformer, is:

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one

zero

½

∞

The average value of A.C. current and voltage over a complete cycle is:

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Maximum

Zero

Neither zero nor maximum

None of the above

A step-up transformer operates on 230V line and supplies a load of 2A. The ratio of primary and secondary turns is 1:25. The current in the primary is:

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50A

25A

15A

12.5A

The number of turns in the primary and secondary coils of a transformer are 100 and 300 respectively. If the input power is 60 W then the output power is:

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180 W

3 kW

60 W

90 W

The induced current can be increased by:

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Increasing the number of turns of coil

By moving the coil faster through the field

By increasing the strength

All of the above

Transformer works on the principle of:

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Self-induction

Mutual induction

Back emf

None of the above

An aero plane with a wing span of 50m flies at 540km/hr. The component of the earth’s magnetic field perpendicular to the velocity of the plane is 0.2 x 10-4 Wbm-2 . The potential difference between the tips of the wings is:

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1500V

150V

15V

0.15V

When a transformer is used to stepping up A.C voltage from 100 V to 200 V is connected across 50V D.C mains, then:

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Output voltage is 100 V

Output voltage is 250 V

No voltage is developed across the secondary

None of these

In a transformer, the number of primary coil and secondary coil are 5 and 4 respectively. If 240 V is applied on the primary coil, then the ratio of current in primary and secondary coil is:

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4:5

5:4

5:9

9:5

Such a value A.C current which produces the same heating effect as is produced by the D.C of equal magnitude is called:

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r.m.s current

Peak current

Effective current

None of the above

When the power output in watts equals one half the power input, the efficiency of the transformer becomes:

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100%

200%

50%

Zero

A coil having 500 square loops, each of the side 10cm is placed normal to a magnetic field which increased at the rate 1T/sec. The emf induced is:

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0.1V

0.5V

1V

5V

Direction of induced current always:

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Oppose the cause

Remains same as that of cause

equal to cause that produces it

Directs the cause

Why does a transformer have a core made of iron

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iron has a high melting point

iron is a conductor of head

iron is a conductor of electricity

iron is a magnetic material

A step-up transformer

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Increases power-leve

Increases voltage -level

Decreases current -level

Both B and C

A transformer is used to light a 100 W, 110 V lamp from a 220V supply. If the supply current is 0.5A, the efficiency of the transformer is:

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30%

50%

80%

90%

In a transformer, the immediate cause of the induced alternating current in the secondary coil is:

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a varying magnetic field

a varying electric field

the iron core of the transformer

a motion of the primary coil

Transformer obeys the law of conservation of:

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Flux

Momentum

Power

emf

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