(a)[1]
Define the term electric potential difference.
(b)[3]
A cell with electromotive force (e.m.f.) $1.8\,\text{V}$ and internal resistance $r$ is connected in parallel with a resistor of resistance $6.0\,\Omega$ and a filament lamp, as shown in Fig. 7.1. The switch S is open. The ammeter reads $0.25\,\text{A}$. Determine the internal resistance $r$ of the cell.
(c(i))[1]
At time $t_1$, switch S in Fig. 7.1 is closed. Fig. 7.2 shows how the ammeter reading $I$ varies with time $t$. State whether the e.m.f. of the cell after $t_1$ is greater than, less than or the same as it was before $t_1$.
(c(ii))[3]
Using the effect of the lamp on the total resistance of the circuit, explain the variation in the ammeter reading shown in Fig. 7.2.