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:

The alternating current I at any time is given by:

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.

Question and Answer

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

Answer: (b) 100/π cycle/sec

2. What is the reciprocal of impedance?

1. Admittance
2. Inductance
3. Reactance
4. Conductance

Answer: (a) Admittance

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

Answer: (a) 5 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 I_p, then how much power is dissipated by a sinusoidal ac current which flows through resistor of resistance R?

1. $\begin{array}{l}{I_p}^{2}R\cos \mathrm{\Theta }\end{array}$
2. $\begin{array}{l}\frac{4}{\pi }{I_p}^{2}R\end{array}$
3. $\begin{array}{l}\frac{1}{\pi }{I_p}^{2}R\end{array}$
4. $\begin{array}{l}\frac{1}{2}{I_p}^{2}R\end{array}$

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

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

Answer: (c) 50 Hz

1. $\begin{array}{l}P=\frac{E_0{I}_0}{2}\end{array}$
2. $\begin{array}{l}P=\sqrt{2}{E}_0{I}_0\end{array}$
3. $\begin{array}{l}P=\frac{E_0{I}_0}{\sqrt{2}}\end{array}$
4. $\begin{array}{l}P=0\end{array}$

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

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

Answer: (a) 311 V

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