Define a volt.
A battery with electromotive force (e.m.f.) $4.5\,\text{V}$ and negligible internal resistance is joined to two filament lamps P and Q and a resistor R, as shown in Fig. 6.1. The current in lamp P is $0.15\,\text{A}$. The $I$-$V$ characteristics of the filament lamps are shown in Fig. 6.2. Use Fig. 6.2 to determine the current in the battery. Show your working.
Calculate the resistance of $R$.
The filament wires in the two lamps are formed from a material with the same resistivity at the operating temperature in the circuit. The diameter of lamp $P$'s wire is twice that of lamp $Q$'s wire. Determine the ratio $$\frac{\text{length of filament wire of lamp }P}{\text{length of filament wire of lamp }Q}.$$
The filament wire in lamp $Q$ breaks, so it no longer conducts. State and explain, qualitatively, what happens to the resistance of lamp $P$.