Four cells, each of emf 9 V and internal resistance \( 0.1\,\Omega \), are connected as shown in the figure. What is the equivalent emf of the combination?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 36 V
When cells are joined in series, the positive terminal of one goes to the negative terminal of the next. Their emfs add, and their internal resistances also add.
From the figure, all four cells point the same way along the row. Each is joined + to − with its neighbour, so they are in series and aid one another. The external \( 10\,\Omega \) resistor does not change the emf of the combination.
\( \begin{aligned} \varepsilon_{\mathrm{eq}} &= \varepsilon_1+\varepsilon_2+\varepsilon_3+\varepsilon_4 \\ &= 4\times9\,\mathrm{V} \\ &= 36\,\mathrm{V} \end{aligned} \)
The internal resistance of the combination is \( 4\times0.1\,\Omega=0.4\,\Omega \). It limits the current but not the emf.