How do charge, current, potential difference, and resistance relate — and what determines how devices behave in circuits?
Current is the rate of flow of charge: Q = It. Potential difference is the energy transferred per unit charge that moves between two points. Resistance opposes current flow — Ohm's Law (V = IR) holds for ohmic conductors at constant temperature, but not for diodes, filament bulbs, or thermistors. The I–V characteristic (graph of current against voltage) is the standard way to identify component behaviour: a straight line through the origin indicates an ohmic conductor; a curve indicates non-ohmic behaviour.
In series circuits, current is the same everywhere; potential differences add up. In parallel circuits, current splits; the potential difference is the same across each branch. These rules come directly from conservation of energy and conservation of charge. Power dissipated in a component = VI = I²R, which is why the National Grid transmits at very high voltage and very low current — reducing I dramatically cuts I²R heating losses in cables.