Question

In an experiment to measure the internal resistance of a cell by potentiometer, it is found that the balance point is at a length of 2 m when the cell is shunted by a 5 Ω resistance; and is at a length of 3 m when the cell is shunted by a 10 Ω resistance. The internal resistance of the cell is, then

• A. 1.5 Ω
• B. 10 Ω
• C. 15 Ω
• D. 1 Ω

Answer 1

Answer: (B)

10 Ω

Answer 2

By using $r = \left( \frac{l_{1} - l_{2}}{l_{2}} \right)R'$ ⇒ $r = \left( \frac{l_{1} - 2}{2} \right) \times 5$ …… (i)

and $r = \left( \frac{l_{1} - 3}{3} \right) \times 10$……. (ii)

On solving (i) and (ii) r = 10 Ω