Question

Ten identical cells each of potential $E$ and internal resistance $r$ are connected in series to form a closed circuit. An ideal voltmeter connected across three cells, will read

• A. 10 E
• B. 3 E
• C. 13 E
• D. 7 E

Answer 1

Answer: (B)

Given: Initial number of cells $\left(n_1\right)=10$ ;
Potential of each cell $=E;$ Internal resistance of each cell $=r$ and final number of cells $\left(n_2\right)=3$
We know from the Ohm's law, total voltage of ten cells = $10\times E=10E$ and total resistance in ten cells $=10\times r$ $=10r$
Therefore current in the circuit $(I)=\frac{10E}{10r}=\frac{E}{r}$ or potential difference across three cells $=I\times 3r=$ $\frac{E}{r}\times 3r=3E$
(Since the voltmeter is ideal, therefore it will read $3E$ ).