Alternating Current

A series LCR circuit is connected to an ac voltage source. When L is removed from the circuit, the phase difference between current and voltage is 3. If instead C is removed from the circuit, the phase difference is again 3 between current and voltage. The power factor of the circuit is :

1. 0.5
2. 1.0
3. -1.0
4. zero

An inductor 20 mH, a capacitor 100 and a resistor 50 are connected in series across a source of emf, V = 10 314 t. The power loss in the circuit is

1. 0.79 W
2. 0.43 W
3. 2.74 W
4. 1.13 W

An inductor 20 mH, a capacitor 50 and a resistor 40 are connected in series across a source of emf V = 10 340t. The power loss in A.C. circuit is

1. 0.76 W
2. 0.89 W
3. 0.46 W
4. 0.67 W

A small signal voltage V(t) = V0 t is applied across an ideal capacitor C

1. Current I(t) is in phase with voltage V(t).
2. Current I(t) leads voltage V(t) by 180
3. Current I(t) lags voltage V(t) by 90 .
4. Over a full cycle the capacitor C does not consume any energy from the voltage source.

Which of the following combinations should be selected for better tuning of an L-C-R circuit used for communication?

1. R = 20 , L = 1.5 H, C = 35
2. R = 25 , L = 2.5 H, C = 45
3. R = 15 , L = 3.5 H, C = 30
4. R = 25 , L = 1.5 H, C = 45

The potential differences across the resistance, capacitance and inductance are 80 V, 40 V and 100 V respectively in an L-C-R circuit. The power factor of this circuit is

1. 0.4
2. 0.5
3. 0.8
4. 1.0

A series R-C circuit is connected to an alternating voltage source. Consider two situations : (a) When capacitor is air filled. (b) When capacitor is mica filled. Current through resistor is i and voltage across capacitor is V then

1. ia > ib
2. Va = Vb
3. Va < Vb
4. Va > Vb

A resistance 'R' draws power 'P' when connected to an AC source. If an inductance is now placed in series with the resistance, such that the impedance of the circuit becomes 'Z', the power drawn will be

1. P ( RZ )
2. P
3. P ( RZ )2
4. P RZ

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.