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

• A. 3 amper
• B. 1 ampere
• C. 7 ampere
• D. 5 ampere

Answer 1

Answer: (D)

5 ampere

Answer 2

Quantity of heat liberated in the ammeter of resistance R

(i) Due to direct current of 3 ampere = [(3)2 R/J]

(ii) Due to alternating current of 4 ampere = [(4)²R/J]

Total heat produced per second

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

Let the equivalent alternating current be I ampere; then

$\frac { \mathrm { I } ^ { 2 } \mathrm { R } } { \mathrm { J } }$ = $\frac { 25 \mathrm { R } } { \mathrm { J } }$

or I = 5 amp