Define electric potential difference (p.d.) in words.
A battery of electromotive force (e.m.f.) $14\,\text{V}$ and negligible internal resistance is connected to a resistor network, as illustrated in Fig. 6.1. $R_1$ and $R_2$ are fixed resistors with resistances $6.0\,\Omega$ and $12\,\Omega$ respectively. $R_3$ is a variable resistor. Switch $S$ is closed. Calculate the current in the battery when the resistance of $R_3$ is set to zero.
Calculate the current in the battery when the resistance of $R_3$ is changed to $24\,\Omega$.
Use your answers in (b)(i) to calculate the change in the total power produced by the battery when the resistance of $R_3$ is changed from zero to $24\,\Omega$.
Switch $S$ in Fig. 6.1 is now open. $R_1$ and $R_2$ are metal-wire resistors. Figure 6.2 gives some information about these resistors. Find the ratio $$\frac{\text{average drift speed of free electrons in } R_1}{\text{average drift speed of free electrons in } R_2}.$$