Which statement correctly explains why the current through a tungsten filament lamp…
PHYSICS · Single correct · A Level · Physics · Electricity · Electricity
Problem
Which statement correctly explains why the current through a tungsten filament lamp is not directly proportional to the applied voltage after the lamp has been glowing for a short time?
(a) The supply voltage fluctuates continuously, so current cannot stay proportional.
(b) The filament’s temperature rises sharply, so its resistance increases with current.
(c) Thermal expansion makes the filament appreciably longer, altering its resistance and spoiling proportionality.
(d) With rising temperature, tungsten’s resistivity actually decreases, upsetting the V–I ratio.