Q 12-03-057JEE MainJEE Main 2026 (28 Jan, Shift 1)Medium
For the two cells having same EMF $E$ and internal resistance $r$, the current passing through the external resistor $6\Omega$ is same when both the cells are connected either in parallel or in series. The value of internal resistance $r$ is ______ $\Omega$.
Answer: (B) 6
Series: $I_s = \dfrac{2E}{6 + 2r}$
Parallel: equivalent EMF $E$, internal resistance $r/2$: $I_p = \dfrac{E}{6 + r/2}$
Setting $I_s = I_p$:
$$2\left(6 + \frac r2\right) = 6 + 2r \Rightarrow 12 + r = 6 + 2r \Rightarrow r = 6\ \Omega$$
Solution by Sreeraj P, M.Sc Physics