The potential difference, current and resistance of a component are linked by one equation: potential difference = current × resistance, or V = I R. The p.d. is in volts (V), the current in amperes (A) and the resistance in ohms (Ω).
The equation can be rearranged. Current = potential difference ÷ resistance (I = V ÷ R), and resistance = potential difference ÷ current (R = V ÷ I). For example, a 12 V supply across a 4 Ω resistor gives a current of 3 A. A current of 0.5 A through a 24 Ω resistor means a p.d. of 12 V.
For a fixed resistor at constant temperature the resistance stays constant, so doubling the p.d. doubles the current. In other components the resistance changes as the current changes. In a filament lamp the resistance increases as the current increases. In a metal wire, a longer wire has a greater resistance, which can be investigated by measuring the p.d. and current for different lengths.