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)

3 E

Answer 2

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 = $10 \times E=10 E$ and total resistance in ten cells $=10 \times r$ $=10 r$ Therefore current in the circuit $(I)=\frac{10 E}{10 r}=\frac{E}{r}$ or potential difference across three cells $=I \times 3 r=$ $\frac{E}{r} \times 3 r=3 E$ (Since the voltmeter is ideal, therefore it will read $3 E$ ).