Important Questions and Answers: Alternating Current

1. Why is choke coil better than resistor in electrical appliances?

Ans: Choke coil is an inductance which is used to regulate current in AC circuit. It decreases current without wasting electrical energy into heat. But resistance controls the current and waste electrical energy in the form of heat. So, choke coil is better than resistors in electric appliances.

2. What do you mean by wattles current ?

Ans: The current which does not consume any power in a circuit is called wattles current. This is possible when the phase difference between the current and voltage is π/2.

3. What do you mean by r.m.s. value of an AC current ?

Ans: The steady current which when passes through a given resistance for a given time produces the same amount of heat produced by the alternating current when it passes through the same resistance for the same time is called r.m.s. value of current. It is given by: $$\text{I}_v = \frac{\text{I}_o}{ \sqrt{2}}$$

Where Iv is the rms value and I0 is the average value of AC current.

4. Can we use ordinary moving coil galvanometer for measuring a.c.?

Ans: No, since the polarity varies every seconds in a.c., the needle of a moving coil galvanometer deflect right and left everytime. So, the average deflection will be zero.

5. What is choke?

Ans: Choke is an inductor of high inductance and negligible resistance. It is connected in series in a circuit to reduce current without loss of power.

6. Why choke cannot be used for d.c.?

Ans: Choke cannot be used for d.c. because it offers on opposition to d.c. and even draws large amount of current from d.c. source producing sufficient heat to burn it. So, choke cannot be used for d.c. .

7. What is impedence of an a.c. circuit?

Ans: The total opposition offered to the alternating current by the circuit is called impedance of an a.c. circuit. It is denoted by Z.

For R-L circuit,

$$Z = \sqrt{R^2 + X_L^2}$$

For R-C Circuit,
$$Z = \sqrt{R^2 + X_c^2}$$

For LCR Circuit,
$$Z = \sqrt{R^2 + (X_l-X_c)^2}$$

8. Why AC is more dangerous than DC ?

Ans: We know that the peak value of ac is given by √2 × rms speed. Say we have 220 volts of current. Since the magnitude of ac fluctuate every seconds, the peak value of ac becomes √2 × 220 = 311 Volts but for dc its same i.e., 220 volt. So, ac is more dangerous than dc.

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3 Responses to “Important Questions and Answers: Alternating Current”

  1. swati

    Really effective

  2. kavita meena

    a coil of 200ohm resistance and 1H inductance to an AC source of frequency 200/2Hz. phase angle between potential and current will be

  3. Shiv Kumar

    What is current??