Question

For a cell, a graph is plotted between the potential difference $V$ across the terminals of the cell and the current $I$ drawn from the cell (figure). The emf and the internal resistance of the cell are $E$ and $r$ respectively. Then

• A. $E=2\text{V},r=0.5\Omega$
  
• B. $E=2\text{V},r=0.4\Omega$
• C. $E>2\text{V},r=0.5\Omega$
• D. $E>2\text{V},r=0.4\Omega$

Answer 1

The correct option is B $E=2\text{V},r=0.4\Omega$

When current is drawn from the cell, potential difference across the terminals of it is given as: $V=E-Ir$

When $I=0$, the potential reading is $2\text{V}$. Hence emf,
$E=2\text{V}$

When $V=0$, the current is $I=5\text{A}$

Using, $V=E-Ir$

$\Rightarrow 0=2-5r$

This gives $r=0.4\Omega$

Alternate method:
Slope of the $V-I$ graph gives us the resistance.

Slope $=\tan \theta =\frac{2}{5}=0.4\Omega$