Ohm's Law

Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

The formula

V = I R

where V is potential difference in volts (V), I is current in amperes (A), and R is resistance in ohms.

This can be rearranged to: I = V / R or R = V / I

Ohmic and non-ohmic conductors

A component that obeys Ohm’s law is called an ohmic conductor. Its V-I graph is a straight line through the origin. The gradient of this line equals the resistance (which stays constant).

Examples of ohmic conductors: a metal wire at constant temperature, a resistor.

Non-ohmic components have V-I graphs that are not straight lines:

• Filament lamp: resistance increases as temperature rises, so the graph curves.
• Diode: current flows in one direction only; the graph shows a sharp rise after the threshold voltage and is flat (zero current) in the reverse direction.
• Thermistor: resistance decreases as temperature increases.

Experimental verification

To verify Ohm’s law for a resistor:

1. Set up a circuit with a power supply, variable resistor, ammeter (in series), and voltmeter (in parallel with the test resistor).
2. Adjust the variable resistor to change the current.
3. Record the ammeter and voltmeter readings for each setting.
4. Plot V (y-axis) against I (x-axis).
5. If the graph is a straight line through the origin, the resistor obeys Ohm’s law. The gradient equals the resistance.