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 3 A. When another alternating current passes through the circuit, the AC ammeter reads 4 A. Then the reading of this ammeter, if DC and AC flow through the circuit simultaneously, is

• A. 3 A
• B. 4 A
• C. 7 A
• D. 5 A

Answer 1

Answer: (D)

5 A

Answer 2

Quantity of heat liberated in the ammeter of resistance (i) due to direct current of 3 A $= [ (3)^2 \, R \, / \, J ) ]$ (ii) due to alternating current of 4 A $= [ (4)^2 \, R \, / \, J ) ]$ $\therefore$ Total heat produced per second = $\frac{ ( 3)^2 \, R }{ J } + \frac{ ( 4)^2 \, R }{ J } = \frac{ 25 \, R }{ J }$ Let the equivalent alternating current be I virtual ampere; then $\frac{ I^2 \, R }{ J} = \frac{ 25 \, R }{ J }$ I = 5 A