Current, potential difference and resistance

Electric current is the rate of flow of charge: Q = I × t, with charge in coulombs (C), current in amps (A) and time in seconds. Potential difference (voltage) is the energy transferred per unit of charge, measured in volts (V) with a voltmeter connected in parallel. Current is measured with an ammeter in series.

Resistance, in ohms (Ω), opposes current: V = I × R. For a resistor at constant temperature, current is directly proportional to potential difference: its I–V graph is a straight line through the origin (an ohmic conductor). A filament lamp's resistance increases as it gets hotter, so its graph curves. A diode lets current flow only one way. A thermistor's resistance falls as temperature rises, and an LDR's resistance falls as light gets brighter.