State and explain what effect, if any, the resistance of a filament wire in a lamp has as the current in the wire falls.
On Fig. 5.1, sketch the $I$-$V$ characteristic of a filament lamp.
A battery with electromotive force (e.m.f.) $E$ and negligible internal resistance is arranged in parallel with two filament lamps $A$ and $B$, as shown in Fig. 5.2. The current in the battery is $3.3\,\text{A}$ and the current in lamp $A$ is $1.5\,\text{A}$. The power dissipated in lamp $A$ is $18\,\text{W}$.
Calculate the e.m.f. $E$ of the battery.
The filament wire of lamp B has a cross-sectional area of $1.4 \times 10^{-9}\,\text{m}^2$. The number of free (conduction) electrons per unit volume in the metal of the filament wire is $3.4 \times 10^{28}\,\text{m}^{-3}$. Calculate the mean drift speed of the free electrons in the filament wire of lamp B.