Question

The length of potentiometer wire is $600 \, cm$ and a current of $40 \, mA$ is flowing in it. When a cell of emf $2 \, V$ and internal resistance $10 \, \Omega$ is balanced on this potentiometer the balance length is found to be $500 \, cm$ . The resistance of potentiometer wire will be

• A. $20 \, \Omega$
• B. $40 \, \Omega$
• C. $60 \, \Omega$
• D. $80 \, \Omega$

Answer 1

Answer: (C)

Potential difference of external circuit $=$ Potential gradient $\times $ Balance length.
$2=\frac{I R}{L}\times l_{0}$
$R=\frac{2 L}{I l_{0}}=\frac{2 \times 600}{40 \times 1 0^{- 3} \times 500}$
$R=\frac{12 \times 1 0^{3}}{200}$
$R=60 \, \Omega$