Glossary — Electricity
Alternating current (a.c.) — a current that keeps changing direction. The UK mains supply is alternating, at 50 Hz and about 230 V.
Ammeter — an instrument that measures current, in amperes. It is connected in series with the component.
Charge — a property of matter. Charge comes in two kinds, positive and negative, and is measured in coulombs.
Coulomb (C) — the unit of charge. One coulomb is the charge that passes a point when a current of one ampere flows for one second.
Current — the rate of flow of electric charge, measured in amperes. A current of one ampere carries one coulomb each second.
Diode — a component that allows current to flow in one direction only. In the reverse direction, its resistance is very high.
Direct current (d.c.) — a current that flows in one direction only. A battery supplies direct current.
Earth wire — the green-and-yellow safety wire in a mains cable. It is at 0 V and carries a current only if there is a fault.
Electric field — the region around a charged object where it exerts a force on another charged object. The field is strongest close to the object.
Electron — a particle with a negative charge. Electrons moving through a conductor carry the charge that makes a current.
Equivalent resistance — the single resistance that would give the same current as a combination of resistors. For resistors in series, it is their sum.
Filament lamp — a lamp in which a thin wire is heated until it glows. Its resistance increases as the filament gets hotter.
LDR (light-dependent resistor) — a resistor whose resistance decreases as the light intensity increases.
Live wire — the brown wire in a mains cable. It carries the alternating potential difference from the supply and is at about 230 V relative to earth.
National Grid — the system of cables and transformers that links power stations to consumers. It transfers power at high potential difference and then at a lower potential difference for domestic use.
Neutral wire — the blue wire in a mains cable. It completes the circuit and is at, or close to, earth potential.
Ohm () — the unit of resistance. One ohm is one volt per ampere.
Ohmic conductor — a conductor whose resistance stays constant as the current changes, provided its temperature stays constant. The current is directly proportional to the potential difference.
Parallel circuit — a circuit in which components are joined across the same two points. The potential difference is the same across each component, and the total current is the sum of the branch currents.
Potential difference (pd) — the energy transferred to or from each unit of charge as it moves between two points in a circuit. It is measured in volts. Many books and papers also use the word voltage.
Resistance — a measure of how strongly a component opposes the flow of charge. It is found from resistance = potential difference ÷ current, and is measured in ohms.
Series circuit — a circuit in which components are joined one after another. The current is the same through each component, and the supply potential difference is shared between them.
Static electricity — charge that stays on an insulating material after it has been rubbed, instead of flowing away.
Step-down transformer — a transformer that decreases potential difference, so that power from the transmission cables can be used in homes and businesses.
Step-up transformer — a transformer that increases potential difference, from a power station to the transmission cables.
Thermistor — a resistor whose resistance decreases as its temperature increases.
Voltage — another word for potential difference.
Voltmeter — an instrument that measures potential difference, in volts. It is connected in parallel with the component.