A voltmeter of resistance $150 \Omega$ connected across a cell of e.m.f. $3 \mathrm{~V}$ reads $2.5…

A voltmeter of resistance $150 \Omega$ connected across a cell of e.m.f. $3 \mathrm{~V}$ reads $2.5 \mathrm{~V}$. What is the internal resistance of the cell?
  1. $10 \Omega$
  2. $15 \Omega$
  3. $20 \Omega$
  4. $30 \Omega$

Solution

Internal resistance, $\begin{aligned} \mathrm{r} & =\left[\frac{\mathrm{E}}{\mathrm{V}}-1\right] \mathrm{R} \\ & =\left[\frac{3}{2.5}-1\right] 150 \\ & =30 \Omega \end{aligned}$

Asked in: MHT CET 2023 (10 May Shift 1)

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