A voltmeter is connected in parallel with a variable resistance $R$ which is in series with an ammeter and a cell as shown in the figure. For one value of $R$, the meters read $0.3 \,A$ and $0.9 \,V$. For another value of $R$ the readings are $0.25 \,A$ and $1.0 \,V$. What is the internal resistance of the cell is ......... $\Omega$
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(b)
$^-0.9=^-E-^+0.3 r$
$1=E-0.25 r$
$0.1=0.05 r$
$r=\frac{10}{5}=2 r$
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