Current-voltage graphs

Different devices have different resistance characteristics. These can be investigated using graphs that show:

These are known as current-voltage graphs or I–V graphs.

Components which show a straight line I–V graph are linear circuit elements. They are also sometimes called 'ohmic circuit elements'. This means that current is directly proportional to potential difference for that component. This is a mathematical statement of Ohm's Law which states that the current in a conductor is proportional to the potential difference between its ends, provided temperature remains constant. It is true for a fixed resistor or resistance wire at a constant temperature.

Graph plotting potential difference against current for a fixed resistor. Line is directly proportional.The I-V graph for a linear circuit element

Non-linear circuit elements

Components which show I–V graphs that are not straight lines are non-linear circuit elements. The current through them is not directly proportional to the potential difference across them.

Resistors and filament bulbs

Non-linear circuit elements include fixed resistors that become hot, and filament bulbs which become hot and glow.

The shape of the I-V graph changes as temperature of the component increases.

When the component becomes hotter, the atoms in the metal lattice vibrate more. More collisions occur between the free electrons and atoms, increasing the resistance.

An increase in the potential difference will allow the current to increase, but only up to a certain point.

Graph plotting potential difference against current for a filament bulb. Line is an upward curve, that levels out and start to dip and potential difference increase.The I-V graph for a filament bulb

Diodes

A diode has a very high resistance in one direction and a low resistance in the other. This means that current can only pass in one direction.

Graph plotting potential difference against current for a diode. Line is horizontal on the x-axis for a part, and then it curves upwards sharply.The I-V graph for a diode

LDRs and thermistors

Light dependent resistors or LDRs are made of semiconductor material. Their resistance decreases as the light intensity increases. LDRs can be used to switch lights on or off automatically, such as stadium lights which come on when it gets dark.

This graph shows resistance against light intensity. Resistance decreases as the light intensity increases.Resistance vs light intensity for an LDR

Thermistors are also made of semiconductor material. Their resistance decreases as the temperature increases. Thermistors are used to control temperatures in many devices, such as ovens and central heating systems.

This graph shows resistance against temperature. Resistance decreases as the temperature increases.Resistance vs temperature for a thermistor