Alternating Current · Question 39

A condenser of capacity C is charged to a potential difference of V1. The plates of the condenser are then connected to an ideal inductor of inductance L. The current through the inductor when the potential difference across the condenser reduces to V2 is

1. ( C(V1-V2)2L ) 12
2. C (V12-V22 )L
3. C (V12+V22 )L
4. ( C (V12-V22 )L ) 12

In the given circuit the readings of voltmeters V1 and V2 are 300 volts each. The readings of the voltmeter V3 and ammeter A are respectively [figure]

1. 150 V, 2.2 A
2. 220 V, 2.2 A
3. 220 V, 2.0 A
4. 100 V, 2.0 A

A 220 volt input is supplied to a transformer. The output circuit draws a current of 2.0 ampere at 440 volts. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is

1. 3.6 ampere
2. 2.8 ampere
3. 2.5 ampere
4. 5.0 ampere

Power dissipated in an LCR series circuit connected to an A.C. source of emf is

1. 2 [R2 + (L - 1C )2 ]R
2. 2 R2 + (L - 1C )2R
3. 2 R R2 + (L - 1C )2
4. 2 R [R2 + (L - 1C )2 ]

In an a.c. circuit the e.m.f. ( ) and the current (i) at any instant are given respectively by = E0 sin t, i = I0 sin( t - ) The average power in the circuit over one cycle of a.c. is

1. E0I02
2. E0I0
3. E0I02
4. E0I02

The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux linked with the primary coil is given by = 0 + 4t, where is in webers, t is time in seconds and 0 is a constant, the output voltage across the secondary coil is

1. 120 volt
2. 220 volts
3. 30 volts
4. 90 volts

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 amp, the efficiency of the transformer is approximately

1. 50%
2. 90%
3. 10%
4. 30%

What is the value of inductance L for which the current is maximum in a series LCR circuit with C = 10 F and = 1000 s-1 ?

1. 1 mH
2. cannot be calculated unless R is known
3. 10 mH
4. 100 mH

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

1. 0.33
2. 0.56
3. 0.64
4. 0.80

The core of a transformer is laminated because

1. ratio of voltage in primary and secondary may be increased
2. energy losses due to eddy currents may be minimised
3. the weight of the transformer may be reduced
4. rusting of the core may be prevented.