First slide

Alternating Current and voltage

Question

An ac ammeter is used to measure current in a circuit. When a given direct current passes through the circuit, the ac ammeter reads 3A. When another alternating current passes through the circuit, the ac ammeter reads 4A. then the reading of this ammeter if dc and ac flow through the circuit simultaneously, is

Difficult

A

3A
B

4A
C

7A
D

5A

Solution

Quantity of heat liberated in the ammeter of resistance R
due to direct current of $3 A = [{(3)}^{2} R / J]$

ii. due to alternating current of $4 A = [{(4)}^{2} R / J]$

total heat produced second

$= \frac{{(3)}^{2} R}{J} + \frac{{(4)}^{2} R}{J} = \frac{25 R}{J}$

Let the equivalent alternating current be I virtual A, then $\frac{I^{2} R}{J} = \frac{25 R}{J}$

Or $I$ =5A

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