Question

The potential difference in open circuit for a cell is $2.2$ volts. When a $4$ ohm resistor is connected between its two electrodes the potential difference becomes $2$ volts. The internal resistance of the cell will be

• A. 1 ohm
• B. 0.2 ohm
• C. 2.5 ohm
• D. 0.4 ohm

Answer 1

Answer: (D)

The potential difference in open circuit for the cell is $2.2$ volts, it means the emf of the battery should be $2.2$ volts.
Now a 4 ohm resistor is connected between its two electrodes, then the potential difference across the battery becomes 2 volts.
$V=\frac{ER}{R+r}$
$2=\frac{2.2\times 4}{4+r}$
$r=0.4\Omega$