# Energy - Equations and Definitions

efficiency = useful output energy transfer (J) / input energy transfer (J)

The law of conservation of energy - energy cannot be created or destroyed, just converted from store to store.  The total energy of the Universe is a constant.  There are loads of ways to say this, just remember that the energy before something happens is the same as the energy afterwards.

Energy stores - a calculable number to represent the energy associated with a situation.  Best expressed by an equation.

Chemical energy store - energy equivalent to the total possible enthalpy change of a chemical reaction.

Thermal energy store - energy equivalent to the product of the mass, specific heat capacity and temperature.

Kinetic energy store - energy stored in an object by virtue of the fact it is moving.   Proportional to the mass and velocity squared.

Gravitational energy store - energy stored in an object by virtue of its being raised up in a gravitational field.

Elastic energy store - energy stored in an object due to its being deformed and exerting a restoring force to return to its original shape.

Nuclear energy store - sum of all the possible energy changes in the possible nuclear reactions.

Electrostatic energy store - energy stored because two charged objects are at a certain distance from one another.

Magnetic energy store - energy stored because two magnetic objects are at a certain distance from one another.

Energy transfers - a tool for analysis where one equation can be related to another, applying the law of conservation of energy.

Mechanical working - a transfer of energy due to resultant forces causing displacement.

Electrical working - a transfer of energy due to resultant electrostatic forces on charged particles.

Heating - a transfer of energy due to one object being at a higher temperature than another.

Electromagnetic Radiation - a transfer of energy by electromagnetic waves.

Power - the rate of transfer of energy.

Kilowatt-hour - a large unit of energy used for billing electrical energy consumers.

Dissipation - the spreading out of energy into thermal energy stores, (energy is dispersed and so not in any useful concentration.)

Lubrication - one way to avoid wasted energy due to the heating effect of friction.  Usually by applying liquids to surfaces.

Insulation - a way to slow energy transfer by heating, most usually by trapped air to avoid conduction and convection.

Thermal conductivity - a measure of the rate of transfer of energy by heating through a material.  Has units Watts per metre per Kelvin, (W/mK).

Efficiency - the ratio of useful energy (or power) transferred to total energy (or power) transferred.

Sankey diagram - a scale diagram which shows energy pathways.  The vertical scale represents a volume of energy (or power.)

Energy sources - a concentration of stored energy which can be usefully transferred thermally, mechanically or electrically.

Renewable - returns as quickly as it can be used.

Non-renewable - will not return as quickly as it can be used.

Biofuel - energy source which was recently living.

Fossil fuel - energy source which was living millions of years ago and has now fossilised to become coal, oil or gas.

Nuclear Fuels - metals containing a high percentage of radioactive isotopes.

National Grid - the system of transformers, pylons and power cables by which electricity can be efficiently transferred from production to consumer.

Mains electricity - the 50Hz 230V a.c. supply that we receive as domestic or commercial consumers.

Live wire - the wire in a plug which alternates from a high positive to a high negative potential.

Neutral wire - at zero electrical potential and so completes the circuit through the wiring of the device back to the live wire.

Fuse - a small piece of wire which melts when current is higher than it is rated.  Manufactured in typical ratios, 1A, 3A, 5A, 10A and 13A.

Earth wire - a safety wire which, in the event of a fuse melting provides a sink for electrical charge/current.  Usually a very low resistance path to a buried conductor.