Ohm's Law

The current flowing through a conductor is proportional to the potential difference across it, provided the physical conditions remain constant.

V is the potential difference (voltage), I the current.

The value V/I is the resistance R of the conductor and is a constant.

Ohmic or linear conductors

Materials that follow Ohm's Law are often called ohmic conductors or linear conductors.

A graph of the current and voltage of an ohmic conductor is a straight line through the origin. In the graph above the gradient is equal to 1/R.

Metals and alloys follow Ohm's Law. Carbon is a non-metal that also follows Ohm's Law. For most other materials the resistance is not a constant and changes with the applied voltage.

Physical Conditions

Note that Ohm's Law only applies (i.e. the resistance is a constant) provided that the physical conditions remain constant. Physical conditions that can change the resistance of a conductor include:

temperature
strain (stretching or bending)
strong magnetic field
light

Temperature

In metals resistance increases with temperature. In most practical circuits heating is an unavoidable consequence of passing a current through a conductor. So the resistance of hot components in a circuit will be more than when the circuit is switched off and they are cold.

Strain

Applying a stretching force to a wire makes it longer and thinner. This increases the resistance of the wire.