Definitions - Electricity

Electric current

This is the rate of flow of electric charge

Efficiency of a system

useful / work put in

Potential difference

The pd between two points is the energy converted from electrical potential energy to some other form per coulomb of charge flowing from one point to the other.

Ohms Law

The current in a metal wire at constant temperature is proportional to the pd across it

Electrical Resistance

The resistance of a conductor is the pd (V) placed across it divided by the resulting current (I) through it.

Resistivity

ρ the resistivity, is a constant (at constant temperature) for the material of the wire

Superconducting transition temperature

The temperature at which a material, when cooled, loses all its electrical resistance, and becomes super-conducting. Some materials (e.g. copper) never become superconducting however low the temperature becomes.

The Law of conservation of charge

Electric charge cannot be created or destroyed, (though positive and negative charges can neutralise each other). Charge cannot pile up at a point in a circuit.

EMF

The emf of a source is the energy converted from some other form (e.g. chemical) to electrical potential energy per coulomb of charge flowing through the source.