Alternating currents

This MCQ is based on the topic of the Alternating currents(The electric mains supply in our homes and offices is a voltage that varies like a sine function with time. Such a voltage is called alternating voltage (ac voltage) and the current driven by it in a circuit is called the alternating current (ac current).

Start Quiz

Two coils X and Y are connected in series across a 240 V, 50 Hz supply. The resistance of X is 5Ω and inductance of Y is 0.02 H. The power consumed is 3 kW and the power factor is 0.75. The impedance of the circuit is:

0.144 Ω 1.44 Ω 14.4 Ω 144 Ω

The current through an inductor of 1H is given by I = 3t sin t. The voltage across the inductor of 1 H is:

3 sin t + 3 cos t 3 sin t + 3t cos t 3 cos t + t sin t 3t cos t + sin t

If the rms current in a 50 Hz ac circuit is 5 A, the value of the current 1/300 second after its value becomes zero is

5√2A 5√3 /2 A 5/6 A 5/√2 A

The output of a step-down transformer is measured to be 24 V when connected to a 12 watt light bulb. The value of the peak current is

1/√2A √2A 2A 2√V2A

In an alternating current circuit consisting of elements in series, the current increases on increasing the frequency of supply. Which of the following elements are likely to constitute the circuit?

Only resistor. Resistor and an inductor. Capacitor and an inductor. Only a capacitor.

As the frequency of an ac circuit increases, the current first increases and then decreases. What combination of circuit elements is most likely to comprise the circuit?

Inductor and capacitor. Resistor and inductor. Resistor and capacitor. None.

Energy dissipates in LCR circuit in :

L only C only R only All of the above

An iron core transformer with a turns ratio of 8 : 1 has 120 V applied across the primary. The voltage across the

15 V 120 V 180 V 960 V

What is the ratio of inductive and capacitance reactances in an ac circuit?

1 ω²L ω²LC l

Determine the impedance of a series LCR-circuit if the reactance of C and L are 250 Ω and 220 Ω respectively and R is 40 Ω.

250 Ω 150 Ω 50 Ω 80 Ω

If a 0.5 H inductor, 80 μF capacitor and a 40 Ω resistor are connected in series with a 150 V, 60 Hz supply. Calculate the impedance of the circuit.

100 Ω 160.3 Ω 50 Ω 65 Ω

An inductor coil joined to a 6 V battery draws a steady current of 12 A. This coil is connected in series to a capacitor and ac source of alternating emf 6 V. If the current in the circuit is in phase with the emf, find the rms current.

12 A 15 A 25 A 19 A

A series circuit with L = 0.12 H, C = 0.48 mF and R = 25 Ω is connected to a 220 V variable frequency power supply. At what frequency is the circuit current maximum?

79 Hz 19 Hz 21 Hz 93 Hz

A capacitor, resistor of 10 Ω, and an inductor of 70 mH are in series with an ac source marked 120 V, 70 Hz. If it is found that voltage is in phase with the current, then find out the impedance of the circuit.

5 Ω 10 Ω 100 Ω 900 Ω

An acceptor circuit is :

Series resonant circuit

Parallel resonant circuit

LCR circuit

None of these

In series resonant circuit:

Reactance is zero

Current is zero

Voltage is zero

None of these

To reduce the resonant frequency in an LCR series circuit with a generator