Glossary — Electricity

Capacitance — the charge stored on one plate of a capacitor divided by the potential difference across the capacitor, $C = \dfrac{Q}{V}$. The unit is the farad.

Capacitor — a component that stores charge on two separated conductors.

Charge carrier — a particle that carries electric charge through a material, such as an electron in a metal or an ion in a solution.

Conventional current — the direction in which positive charge would flow, from the positive terminal of a source around the circuit to the negative terminal. It is opposite to the flow of electrons.

Dielectric — the insulating material between the plates of a capacitor.

Electric current — the rate of flow of electric charge, $I = \dfrac{\Delta Q}{\Delta t}$. The unit is the ampere.

Electromotive force (emf) — the energy transferred to each unit of charge by a source, $\varepsilon = \dfrac{W}{Q}$. The unit is the volt.

Elementary charge — the smallest packet of charge, carried by one electron or one proton, $e = 1.60 \times 10^{-19}\,\text{C}$.

Farad — the unit of capacitance. One farad is one coulomb per volt.

Internal resistance — the resistance inside a source, which makes the terminal potential difference less than the emf when a current flows.

Kirchhoff's junction rule — the total current entering a junction equals the total current leaving it, because charge is conserved.

Kirchhoff's loop rule — around any closed loop in a circuit, the sum of the emfs equals the sum of the potential differences, because energy is conserved.

Light-dependent resistor (LDR) — a component whose resistance falls as the light falling on it increases. It is used in a potential divider as a light sensor.

Lost volts — the potential difference $Ir$ across the internal resistance of a source. It is the part of the emf that is not available to the external circuit.

Ohm — the unit of resistance. One ohm is one volt per ampere.

Ohm's law — for a metallic conductor at constant temperature, the current through the conductor is proportional to the potential difference across it.

Parallel — describing components connected across the same two points, so the potential difference across each component is the same.

Parallel plate capacitor — a capacitor made of two parallel plates separated by a dielectric, with $C = \dfrac{\varepsilon_0 \varepsilon_r A}{d}$.

Potential difference (pd) — the energy transferred from each unit of charge by a component, $V = \dfrac{W}{Q}$. The unit is the volt.

Potential divider — two or more components in series that share a supply potential difference. The output is $V_{\text{out}} = V_{\text{in}}\dfrac{R_2}{R_1 + R_2}$.

Resistance — the potential difference across a component divided by the current through it, $R = \dfrac{V}{I}$. The unit is the ohm.

Resistivity — the resistance of a piece of material of unit length and unit cross-sectional area, $\rho = \dfrac{RA}{L}$. The unit is the ohm metre.

Series — describing components connected in a single path, so the same current flows through each component.

Superconductor — a material whose resistance falls to zero below a critical temperature.

Terminal potential difference — the potential difference across the terminals of a source when a current flows. It equals the emf minus the lost volts.

Thermistor — a component whose resistance falls as its temperature rises. It is used in a potential divider as a temperature sensor.

Time constant — the product $RC$ for a capacitor discharging through a resistor. It is the time for the charge or the potential difference to fall to $\dfrac{1}{e}$ of its starting value.