Alternating Current MCQs for NEET

Alternating current is a current whose magnitude changes with time, and direction reverses periodically.

The alternating emf E at any time is given by the equation:

$E=E_0\sin \omega t$

The alternating current I at any time is given by:

$I=I_0\sin (\omega t-\phi )$

Following are a few important terms related to alternating current:

1. Peak Value – Maximum values of voltage and current in a cycle
2. Instantaneous Value – The value of alternating voltage and current at an instant t.
3. Root Mean Square Value – The value of direct current which produces the same heating effect in a given resistor as is produced by the given alternating current when passed for the same time.
4. Mean or average value – It is defined as that value of direct current which sends the same charge in a circuit in the same time as is sent by the given alternating current in its half time period.

1. What is the frequency of a 5μF capacitor that has a reactance of 1000 Ω?

1. 200 cycle/sec
2. 100/π cycle/sec
3. 5000 cycles/sec
4. 1000/π cycles/sec

2. What is the reciprocal of impedance?

2. Inductance
3. Reactance
4. Conductance

3. What is the time taken by the alternating current of 50 Hz with an r.m.s value of 10 ampere to reach from zero to a maximum value and the peak value of current?

1. 2×10-2 sec and 14.24 amp
2. 5 ×10-3 sec and 8.07 amp
3. 5×10-3 sec and 14.14 amp
4. 1 ×10-2 sec and 7.07 amp

Answer: (c) 5×10-3 sec and 14.14 amp

4. What is the voltage applied across the resonant circuit if the ac voltage across resistance R, inductance L and capacitance C is 5 V, 10 V and 10 V respectively?

1. 5 V
2. 10 V
3. 20 V
4. 25 V

5. Why can’t the DC ammeter measure an alternating current?

1. AC cannot pass through a DC ammeter
2. AC changes its direction
3. AC is virtual
4. The average value of a complete cycle is zero

Answer: (d) The average value of a complete cycle is zero

6. If the peak current is given by Ip, then how much power is dissipated by a sinusoidal ac current which flows through resistor of resistance R?

1. ${I_p}^2R\cos \Theta$
2. $\frac{4}{\pi }{I_p}^{2}R$
3. $\frac{1}{\pi }{I_p}^{2}R$
4. $\frac{1}{2 }{I_p}^{2}R$

Answer: (d) $\frac{1}{2 }{I_p}^{2}R$

7. What is the frequency of ac mains in India?

1. 120 Hz
2. 60 Hz
3. 50 Hz
4. 30 Hz

8. If a current I given by $I_0\sin (\omega t-\frac{\pi }{2})$ flows across an AC circuit with potential $E=E_0\sin \omega t$, then what is the power consumption in the circuit?

1. $P=\frac{E_0I_0}{2}$
2. $P=\sqrt{2}{E_0I_0}$
3. $P=\frac{E_0I_0}{\sqrt{2}}$
4. $P=0$

Answer: (d)$P=0$

9. What is the r.m.s current value of an alternating current given by the equation of $i=i_1\cos \omega t+i_2\sin \omega t$?

1. $\frac{1}{\sqrt{2}}(i_1^2+i_2^2)^{1/2}$
2. $\frac{1}{2}(i_1^2+i_2^2)^{1/2}$
3. $\frac{1}{\sqrt{2}}(i_1+i_2)^{2}$
4. $\frac{1}{\sqrt{2}}(i_1+i_2)$

Answer: (a) $\frac{1}{\sqrt{2}}(i_1^2+i_2^2)^{1/2}$

10. What is the peak value of the voltage of an electric lamp connected to a 220 V, 50 Hz supply?

1. 311 V
2. 320 V
3. 211 V
4. 210 V