In AC and DC circuits containing purely resistive loads, like lights and heaters, Ohm's Law can be used to compute current, voltage and resistance in the circuit.

In a resistive DC circuit, both current and voltage are fixed, steady values.

In an AC resistive circuit, the current alternates exactly in step with the voltage. In either case, Ohms Law can be applied.

In a resistive circuit Ohm's Law states that: voltage is equal to current times resistance.

Ohm's Law V = A x R where:

V = voltage (volts)
A = current (amps)
R = resistance (ohms)

For example, a current of 2 amps flowing through a resistance of 3 ohms is said to produce a voltage "across" that resistance of 6 volts.

A simple diagram shows the relationship between all three values. Current will be equal to voltage divided by resistance, and resistance is equivalent to voltage divided by current.

Example:
A wire with a resistance of 10 ohm's is connected to a 9-volt battery. To determine the current flow in the wire, use ohm's law and divide 9 volts by 10 ohm's. The current flow in the wire equals 0.9 amps. Replace the 9-volt battery with a 1.5 volt battery. Using the same wire the calculated current flow is 1.5 volts divided by 10 ohms, which produces a current flow of 0.15 amps. The larger voltage results in more pressure to force more current through the given resistance of 10 ohms.