Question

Four cells of identical $emf \varepsilon$ and internal resistance $r$ are connected in series to a variable resistor. The following graph shows the variation of terminal voltage of the combination with current. The emf of each cell used is

• A. 1.4 V
• B. 5.6 V
• C. 2 V
• D. 1 V

Answer 1

Answer: (A)

Emf of each cell $=\varepsilon$
Internal resistance of each cell $=r$
Number of cells $(n)=4$
Total $emf =n \varepsilon=4 \varepsilon$
Total internal resistance $=nr = 4r$
Terminal voltage, $V=4 \varepsilon-I(4 r)$...(i)
From the graph, When $I=0,\, V=5.6\, V$
$\therefore $ From (i),
$5.6=4 \varepsilon-0$
$\varepsilon=\frac{5.6}{4}=1.4\, V$