Question

In the potentiometer arrangement shown in figure, the driving cell D has emf E and internal resistance r. The cell C, whose emf is to measured, has emf $\frac{E}{2}$ and internal resistance 2r. The potentiometer wire is 100cm long. If balance is obtained, the length AJ is

• A. equal to $50cm$
• B. more than $50cm$
• C. less than $50cm$
• D. balance will be obtained only if resistance of AB is more than $r$

Answer 1

Answer: (B)

more than $50cm$

In the measurement of external emf by potentiometer, the internal resistance of external battery does not matter since no current flows through it at the null point.

Due to internal resistance of cell D, the potential drop across AB is less than $E$.

Hence, to achieve the potential difference of $E/2$ across AJ, the length AJ must be more than half the total length.