Sketch a graoh of current against potential difference for a filament lamp and explain its shape.

(draw diagram on whiteboard) As the potential difference increases, the current increases, since V=IR (where V is the potential difference, I is the current and R is the resistance). Initially the current increases rapidly with potential difference but then increases more slowly. This is because as the current increases, the temperature of the filament increases, so the resistance of the filament increases, so the current increases with potential difference less rapidly.