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.

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: (d) 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

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 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

Answer: (c) 50 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

Answer: (a) 311 V

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