Question

Voltmeter reads the potential difference across the terminal of an old battery as 1.40 volt while a potentiometer reads its voltage to be 1.55 volt. The voltmeter resistance is 280 W. Then:

• A. The emf of the battery is 1.4 V
• B. The emf of the battery is 1.55 V
• C. The internal resistance r of the battery is 30 W
• D. The internal resistance r of the battery is 5 W

Answer 1

Answer: (C)

The internal resistance r of the battery is 30 W

Answer 2

The potentiometer measures the exact value of emf of a battery.

\ E = 1.55 V

Also, 1.4 = I(280)

\ I = 0.005 A

Also, V = E – Ir

\ r ==$\frac { 1.55 - 1.40 } { 0.005 }$= $\frac { 0.15 } { 0.005 }$ = 30 W