State the law of Ohm.
Fig. 5.1 gives the current $I$ against potential difference $V$ for a filament lamp. The lamp’s resistance increases as the potential difference increases. State how Fig. 5.1 shows this.
Explain why the resistance changes in this pattern.
Fig. 5.2 shows a circuit containing a battery of e.m.f. $12.0\,\text{V}$ connected to a linear potentiometer AB and two identical filament lamps P and Q. The battery has negligible internal resistance, and both lamps have the same $I$-$V$ characteristic shown in Fig. 5.1. With the slider at the midpoint, as shown in Fig. 5.2, the battery current $I$ is $1.78\,\text{A}$. Determine (i) the current in lamp P, (ii) the total power dissipated in lamps P and Q, (iii) the resistance of the potentiometer between its ends A and B.
The potentiometer slider in (c) is moved to end A. State and explain the change in brightness of lamps P and Q.