Two identical cells of EMF $1.5\,\text{V}$ each, joined in parallel, supply energy to an external circuit of two $7\,\Omega$ resistances joined in parallel. A high-resistance voltmeter reads the terminal voltage as $1.4\,\text{V}$. Calculate the internal resistance of each cell.

Internal resistance of each cell $=0.5\,\Omega$.

CBSE Class 12 · 2016 · All India · Current ElectricityShort Answer

Two identical cells of EMF

1.5 V

each, joined in parallel, supply energy to an external circuit of two

7 Ω

resistances joined in parallel. A high-resistance voltmeter reads the terminal voltage as

1.4 V

. Calculate the internal resistance of each cell.

Answer: Internal resistance of each cell $= 0.5 Ω$ .

External resistance

R = \frac{7 \times 7}{7 + 7} = 3.5 Ω

. The parallel combination of the two cells has total internal resistance

r^{'} = (\frac{E - V}{V}) R = \frac{1.5 - 1.4}{1.4} \times 3.5 = 0.25 Ω

. For two identical cells in parallel

r^{'} = \frac{r}{2}

, so

r = 2 r^{'} = 0.5 Ω.

CBSE Class 12 Board · 2016 · All India · Subjective