# NCERT Physics Class 12 Exemplar Ch 7 Alternating Current Part 3

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21 A device ‘X’ is connected to an a.c source. The variation of voltage, current and power in one complete cycle is shown in Figure.

(a) Which curve shows power consumption over a full cycle?

(b) What is the average power consumption over a cycle?

(c) Identify the device ‘X’.

Ans:

• A

• Zero

• L or C or LC

22 Both alternating current and direct current are measured in amperes. But how is the ampere defined for an alternating current?

Ans:

• An a.c current changes direction with the source frequency and the attractive force would average to zero.

• Thus, the a.c ampere must be defined in terms of some property that is independent of the direction of current.

• Joule’s heating effect is such property and hence it is used to define rms value of a.c.

23 A coil of 0.01 Henry inductance and 1 ohm resistance is connected to 200 volt, 50 Hz ac supply. Find the impedance of the circuit and time lag between max. Alternating voltage and current.

24 A 60 W load is connected to the secondary of a transformer whose primary draws line voltage. If a current of 0.54 A flows in the load, what is the current in the primary coil? Comment on the type of transformer being used.

Ans:

The transformer is step-down and has input voltage.

Hence.

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