Alternating Current · Question 19

A step down transformer connected to an ac mains supply of 220 V is made to operate at 11 V, 44 W lamp. Ignoring power losses in the transformer, what is the current in the primary circuit ?

1. 4 A
2. 0.2 A
3. 0.4 A
4. 2 A

A series LCR circuit containing 5.0 H inductor, 80 F capacitor and 40 resistor is connected to 230 V variable frequency ac source. The angular frequencies of the source at which power transferred to the circuit is half the power at the resonant angular frequency are likely to be :

1. 42 rad/s and 58 rad/s
2. 25 rad/s and 75 rad/s
3. 50 rad/s and 25 rad/s
4. 46 rad/s and 54 rad/s

A 40 capacitor is connected to a 200 V, 50 Hz ac supply. The rms value of the current in the circuit is, nearly :

1. 2.05 A
2. 2.5 A
3. 25.1 A
4. 1.7 A

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