In part 2 we discussed Ohm’s Law where current is proportional to p.d. provided resistance is constant.

I-V Characteristics show how current varies with p.d. for different components: those that obey Ohm’s Law (i.e. resistance is constant) and those that do not (i.e. resistance varies during operation.) I-V characteristics are generally illustrated by graphs of current (I) against p.d. (V). Usually current is on the vertical axis and p.d. on the horizontal but you should be aware that sometimes V-I graphs are used where the axes are swapped!

A component which obeys Ohm’s Law is called an ohmic conductor. Commercially produced resistors have constant resistance (indicated by the coloured striped on their sides) provided you don’t exceed their operating P.d. (voltage).

For an ohmic conductor, current is directly proportional to p.d. so the graph of I against V so the I-V graph is a straight line through the origin like this: http://physicsnet.co.uk/wp-content/uploads/2010/06/VI-graph-for-resistor1.jpg

A filament lamp (commonly known as a light bulb!) is not an ohmic conductor. It does not obey Ohm’s Law because the temperature of the filament increases which in turn causes the resistance of the filament to increase.

Since the resistance is not constant, p.d. is not proportional to current and the I-V graph is not a straight line: http://physicsnet.co.uk/wp-content/uploads/2010/06/VI-graph-for-filament-lamp1.jpg

A diode is a component designed to conduct current in only one direction. It consists of two layers of semi-conducting material called n-doped (n=negative, so surplus of electrons) and p-doped (p=positive, so lacking electrons).

If the diode is connected in its reverse bias direction, its resistance is extremely high (effectively infinite) and negligible current flows. If the p.d. is increased to very high levels, eventually, the diode will break down and conduct current.

In the forward bias direction, the resistance rapidly decreases once the p.d. across the diode exceeds a certain level called the threshold voltage. Above this value, the current increases greatly for only a small additional increase in p.d.

This gives this characteristic asymmetrical graph: http://physicsnet.co.uk/wp-content/uploads/2010/08/break-down-diode.jpg