The resistance of a component is how strongly it opposes the flow of current. For a given component, the potential difference across it, the current through it and its resistance are linked by one equation: potential difference (V) = current (A) × resistance (Ω), or V = I × R. Potential difference is measured in volts (V), current in amperes (A) and resistance in ohms (Ω).
The equation can be rearranged to find either of the other quantities: current = potential difference ÷ resistance (I = V ÷ R) and resistance = potential difference ÷ current (R = V ÷ I). Worked examples: a current of 3 A through a 4 Ω resistor needs 3 × 4 = 12 V across it. A 12 V supply across a 6 Ω resistor gives 12 ÷ 6 = 2 A. A component with 9 V across it and 0.5 A through it has a resistance of 9 ÷ 0.5 = 18 Ω.
The equation also shows what happens when one quantity changes. If the potential difference stays the same and the resistance is doubled, the current is halved. If the resistance stays the same and the potential difference is doubled, the current is doubled.