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  4. A null point is found at 200 cm in potentiometer when cell in secondary circuit is shunted by 5 Ω. When a resistance of 15 Ω is used for shunting null point moves to 300 cm. The internal resistance of the cell is Ω.

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Q. A null point is found at $200\, cm$ in potentiometer when cell in secondary circuit is shunted by $5\, \Omega$. When a resistance of $15\, \Omega$ is used for shunting null point moves to $300\, cm$. The internal resistance of the cell is ___$\Omega$.

JEE MainJEE Main 2023Current Electricity

Solution:

$ \frac{\varepsilon}{ r +5} \times 5=200\, x $.....(1)
$ \frac{\varepsilon \times 15}{ r +15}=300\, x $....(2)
$ \Rightarrow r =5 \,\Omega$