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

10 E

3 E

13 E

7 E

Solution

3 E

Given:-

Initial number of cells (n₁ ) =10

Potential of each cell = E

Internal resistance of each cell = r

and Final number of cells ( n₂ ) = 3

We know from the Ohm's law

total voltage of ten cells = 10 × E = 10 E

total resistance in ten cells = 10 × r = 10 r

Therefore,

current in the circuit

I = $\frac{10 E}{10 r}$

I = $\frac{E}{r}$

Potential difference across three cells

V = I × 3r

= $\frac{E}{r}$ × 3r

V = 3E

(Since the voltmeter is ideal, therefore it will read 3E).

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