Question

Two identical cells send the same current in $ 3\,\Omega $ . resistance, whether connected in series or in parallel. The internal resistance of the cell should be:

• A. $ 1\,\Omega $
• B. $ 3\,\Omega $
• C. $ \frac{1}{2}\,\Omega $
• D. $ 3.5\,\Omega $
• E. $ \frac{3}{2}\,\Omega $

Answer 1

Answer: (B)

From ohms law
$ I=\frac{2E}{2r+R} $
In parallel combination of two cells, the current through the external resistance r will be
$ I=\frac{E}{\frac{r}{2}+R}=\frac{2E}{2r+R} $
If $ I=I' $ If then $ 2r+R=r+2R $
$ \Rightarrow $ $ R=r=3\,\Omega $