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 4 Ω resistance and at 3 m when cell is shunted by a 8 Ω resistance. The internal resistance of cell is –

• A. 12 Ω
• B. 8 Ω
• C. 16 Ω
• D. 1 Ω

Answer 1

Answer: (B)

8 Ω

Answer 2

V_A – V_B = φ × l

E – $\frac{Er}{r + 4}$= φ × 2 … (1), E $\left( \frac{4}{r + 4} \right)$= φ × 2 ... (1)

Simillarily E $\left( \frac{8}{r + 8} \right)$= φ × 3 … (2)

$\frac{eq_{1}}{eq_{2}}$⇒$\frac{r + 8}{2(r + 4)}$= $\frac{2}{3}$ ⇒ 3r + 24 = 4r + 16 ⇒ r = 8Ω