CBSE 2025, Set 2

Question

A cell of emf $E$ and internal resistance $r$ is connected to an external variable resistance $R$. Plot a graph showing the variation of terminal voltage $V$ of the cell as a function of current $I$, supplied by the cell. Explain how the emf of the cell and its internal resistance can be found from it.

Edvaya · Accepted Answer

The correct answer is Subjective. The terminal voltage $V$ of a cell discharging a current $I$ is given by the equation:
$$V = E - Ir$$

This is the equation of a straight line ($y = mx + c$), where $V$ is plotted on the y-axis and $I$ on the x-axis. The slope of the line is $-r$ and the y-intercept is $E$.

![Terminal Voltage vs Current Graph](https://www.edvaya.com/images/cbse/physics/2025/set2/q17-terminal-voltage-current-graph-answer.webp)

**Finding emf and internal resistance:**
* **Emf ($E$):** The emf is the intercept of the graph on the y-axis (Voltage axis). When $I = 0$, $V = E$.
* **Internal Resistance ($r$):** The internal resistance is the negative of the slope of the $V-I$ graph. $r = -	ext{slope} = \frac{\Delta V}{\Delta I}$.

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Physics > Electrostatics And Current Electricity > Current Electricity

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