Resistors

Non Ohmic Resistors

Some resistors do not follow Ohm’s law, these are called non-Ohmic resistors.

This means that the resistance changes when the current changes.

There are four non-Ohmic resistors that you need to know.

Examples of Non Ohmic Resistors

The first is a filament lamp/bulb.

The graph (which you need to be able to recognise and draw) is below.

Resistors, figure 1

The resistance increases as the temperature increases. Remember how resistance is caused when electrons collide with the ions? Well, as the filament gets hot the ions move and vibrate more or faster. This causes more collisions with the flowing electrons, hence an increased resistance.

Second is a diode.

The graph (which you need to be able to recognise and draw) is below.Resistors, figure 2

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

Third is a thermistor.

The graph (which you need to be able to recognise and draw) is below.

Resistors, figure 3

You can see from the graph that the resistance falls and the temperature increases. Thermistors can be used in the home for thermostats to control the central heating.

Fourth is an LDR - light dependent resistor.

The graph (which you need to be able to recognise and draw) is below.

Resistors, figure 4

You can see from the graph that the resistance falls and the light intensity increases.

LDRs can be used for switching lights on automatically when it gets dark.

Name an application for thermistors?
Your answer should include: Thermostats / Thermostat
What happens to the current in an LDR when the light intensity increases?
Your answer should include: Current / Increase / Increases