Given is the graph of potential difference (V) across combination of three identical cells in series versus current (I). The emf of each cell would be
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 2 V
For cells in series the emfs and the internal resistances add, so the combination has emf \( 3E \) and internal resistance \( 3r \). Its terminal potential difference is
\( \displaystyle V = 3E - 3rI \)
This is a straight line. Its intercept on the V-axis (at \( I = 0 \)) is the total emf of the combination.
\( V\text{-intercept from the graph} = 6\,\mathrm{V} \)
\( \displaystyle 3E = 6\,\mathrm{V} \;\Rightarrow\; E = 2\,\mathrm{V} \)
The slope of the line gives \( 3r \), here \( 6\,\mathrm{V}/1\,\mathrm{A} = 6\,\Omega \), but it is not needed.