For many materials, current and voltage are directly proportional to each other over a wide range of values, with the resistance as the constant of proportionality, so

 voltage = current × resistance

Such materials are said to obey Ohm’s law and are said to be ohmic. However, not all materials are ohmic in nature.

As current flows through a circuit, it transfers energy. When it flows through a material that has a non-zero resistance, this energy is used (for instance, to light a bulb or run a motor) or dissipated as heat (which is why electronics circuits sometimes feel warm, and why computers need cooling fans). As in every other context, the rate of change of energy is known as power, and can be measured in watts (symbol W).

In the context of an electrical current flowing through a resistor, the power used or dissipated can be calculated by multiplying the current flowing through it by the potential difference across the resistor, giving

 power (watts) = current (amps) × potential difference (volts)