Q 12-03-251JEE MainJEE Main 2019 (10 Apr, Shift 1)Medium
Two cells, each of emf $1.5\ \text{V}$ and internal resistance $r$, are connected in series with a $2\ \Omega$ resistor and a parallel combination of $15\ \Omega$ and $10\ \Omega$ resistors. An ideal voltmeter connected across the $10\ \Omega$ resistance reads $2\ \text{V}$. The internal resistance $r$ of each cell is:
Answer: (C) $0.5\ \Omega$
The parallel pair has $2\ \text{V}$ across it, so the currents are $\dfrac{2}{10} + \dfrac{2}{15} = \dfrac13\ \text{A}$ in total. The parallel pair is $6\ \Omega$.
$$3 = \frac13\,(6 + 2 + 2r) \;\Rightarrow\; 9 = 8 + 2r \;\Rightarrow\; r = 0.5\ \Omega$$
Solution by Sreeraj P, M.Sc Physics