IP Lamp IV curve © Oxford University Press 2011 Lamp IV curve

IP Lamp IV curve © Oxford University Press 2011  A filament lamp is a thin coil of wire which is usually made of a metal with a high melting point such as tungsten. When a current flows through the wire, it glows brightly and becomes hot.  The glowing filament gives off useful light but it also generates a lot of wasted heat.

IP Lamp IV curve © Oxford University Press 2011  When you plot the current against potential difference you do not get a straight line like that for a resistor – instead you get a curve.  This shows that as the current increases it gets harder to push the electrons round the circuit as an increasingly higher potential difference is needed for each change in current.

IP Lamp IV curve © Oxford University Press 2011  The curve shows that the resistance is not constant; in fact the resistance of the lamp increases as the current increases. Why does the resistance of the filament lamp increase with increased current?  As the current increases the filament gives off heat and its temperature increases. This rise in temperature makes the atoms in the metal vibrate more and this in turn makes it harder for the electrons to flow through it. This raises the resistance of the filament.