An ac ammeter is used to measure currnet 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 4A. Then find the reading of this ammeter (inA), if dc and ac flow through the circuit simultaneously.

Options

A. 7A
B. 5A
C. 3A
D. 1A

Answer

B

Explanation

A.c. ammeter works on the heating effect of electric current.
`:.` Heat produced in the ammter of resistor R per sec due to d.c. of `3A = ((I^(2)R)/(J))=((3)^(2)R)/(J) = (9R)/(J)`
and that due to a.c. of `4 A = ((4)^(2)R)/(J) = (16R)/(J)`
`:.` Total heat prodcued`//`sec = `(9R)/J + (16R)/J = (25R)/(J)`
If is the equivalent rms current, then `(I^(2)R)/(J) = (25R)/(J)`
`:. I^(2)=25 or I=5A`.