A tungsten filament lamp is connected to a variable d.c. power supply so that the potential difference across the filament can be increased gradually.
(i) Describe how the current through the filament is expected to change as the applied potential difference is increased from zero to the rated value of the lamp.
(ii) Sketch the expected V–I graph for the lamp on the same axes along with the straight-line graph for an ohmic conductor of constant resistance. Label both curves.
(iii) Explain why the curve for the filament lamp deviates from the straight line even though tungsten is a metallic conductor.
(iv) If the resistance of the filament is 20 Ω at room temperature and rises to 80 Ω when glowing, explain qualitatively why the brightness of the lamp does not increase in direct proportion to the applied voltage.