Question # 1

Identify the phenomenon by which an induced emf could be generated:

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By moving a conductor across a magnetic field

By rotating a coil faster through the field

By increasing the strength

All the above

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%

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

∞

Frequency of A.C. used in Pakistan is:

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100 CPs

60 CPs

50 CPs

120 CPs

The loss of power in transformer occurs due to:

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The shape of coils

Size of coils

Magnetic hysteresis and Eddy current

None of these

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 magnet is allowed to fall through a metal ring. Its acceleration A), during the fall is:

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a < g

a > g

a = g

a = 0

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

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

As per Faraday’s laws of electromagnetic induction, an e.m.f is induced in a conductor whenever it:

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Lies perpendicular to the magnetic field

Cuts magnetic field

Lies in a magnetic field

Moves parallel to the direction of the magnetic field

The motional of emf depends upon the:

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Length of conductor

Strength of magnetic field

Speed of conductor

All the above

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

Transformer obeys the law of conservation of:

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Flux

Momentum

Power

emf

If the magnetic flux linked with a coil varies at the rate of 1wb/min, the induced emf is:

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1V

1/60V

60V

None of these

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

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

In general, in an ac circuit:

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The average value of current is zero

The average value of the square of the current is zero

The average power dissipation is zero

The phase difference between voltage and current is zero

The primary winding of a transformer has 500 turns whereas its secondary has 5000 turns The primary is connected to an a.c supply of 20V,50Hz. The secondary will have an output of

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2V, 50Hz

200V, 500Hz

200V, 50Hz

2V, 5Hz

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

In the step up transformer, when the alternating voltage increase then the alternating current will

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Increase

Decrease

not change

Not depend on core

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