Question

Voltmeter reads the potential difference between the terminals of an old battery as 1.4 V while a potentiometer reads its voltage to be 1.55 V. The voltmeter resistance is 280 $\Omega$. Then,

• A. The emf of the cell is of 1.4 V
• B. The emf of the cell is of 1.5 V
• C. The internal resistance of the battery is 30 $\Omega$
• D. The internal resistance of the battery is 5 $\Omega$

Answer 1

The correct option is C The internal resistance of the battery is 30 $\Omega$

Potentiometer reads the voltage of battery and voltmeter reads the
potential across the terminals of battery
$\therefore$ battery of cell is of 1.55 V

$V_{AB}=1.4V$
$1.4=280\times \frac{1.55}{280+r}$
r $\times$ 1.4 = 280(1.55 - 1.4)
r $\times$ 1.4 = 280 $\times$ 0.15
$r=\frac{2.8}{1.4}\times 15=30\Omega$