Alternating Current

A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6 A, the voltage across the secondary coil and the current in the primary coil respectively are

1. 300 V, 15 A
2. 450 V, 15 A
3. 450 V, 13.5 A
4. 600 V, 15 A

A coil of self-inductance L is connected in series with a bulb B and an AC source. Brightness of the bulb decreases when

1. a capacitance of reactance XC = XL is included in the same circuit.
2. an iron rod is inserted in the coil.
3. frequency of the AC source is decreased.
4. number of turns in the coil is reduced.

The primary of a transformer when connected to a dc battery of 10 Volt draws a current of 1 mA. The number of turns of the primary and secondary windings are 50 and 100 respectively. The voltage in the secondary and the current drawn by the circuit in the secondary are respectively

1. 20 V and 2.0 mA
2. 10 V and 0.5 mA
3. Zero volt and therefore no current
4. 20 V and 0.5 mA

The instantaneous values of alternating current and voltage in a circuit are given as i = 1 2 (100 t) ampere e = 1 2 (100 t + 3 ) Volt The average power in watts consumed in the circuit is

1. 14
2. 34
3. 12
4. 18

In an electrical circuit R, L, C and a.c. voltage source are all connected in series. When L is removed from the circuit, the phase difference between the voltage and the current in the circuit is 3. If instead, C is removed from the circuit, the phase difference is again 3. The power factor of the circuit is

1. 12
2. 1 2
3. 1
4. 32

The r.m.s. value of potential difference V shown in the figure is [figure]

1. V0 3
2. V0
3. V0 2
4. V02

A coil has resistance 30 ohm and inductive reactance 20 ohm at 50 Hz frequency. If an ac source, of 200 volt, 100 Hz, is connected across the coil, the current in the coil will be

1. 2.0 A
2. 4.0 A
3. 8.0 A
4. 20 13 A

An ac voltage is applied to a resistance R and an inductor L in series. If R and the inductive reactance are both equal to 3 Ω, the phase difference between the applied voltage and the current in the circuit is

1. /6
2. /4
3. /2
4. zero

In the ac circuit an alternating voltage e = 200 2 sin 100t volts is connected to a capacitor of capacity 1 . The r.m.s. value of the current in the circuit is

1. 10 mA
2. 100 mA
3. 200 mA
4. 20 mA

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.

A transistor-oscillator using a resonant circuit with an inductor L (of negligible resistance) and a capacitor C in series produces oscillations of frequency f. If L is doubled and C is changed to 4C, the frequency will be

1. f/2
2. f/4
3. 8f
4. f/2 2

In a circuit L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45 . The value of C is

1. 1 f(2 fL - R)
2. 12 f(2 fL - R)
3. 1 f(2 fL + R)
4. 12 f(2 fL + R)

For a series LCR circuit the power loss at resonance is

1. V2 [ L - 1 C ]
2. I2L
3. I2R
4. V2C

A capacitor of capacity C has reactance X. If capacitance and frequency become double then reactance will be

1. 4X
2. X/2
3. X/4
4. 2X

The value of quality factor is

1. LR
2. 1 RC
3. LC
4. L/R