## Time

The SI unit of time is the second, symbol s.

Three non-SI units of time; the minute, hour and day are accepted for use with the metric system.

1000 | milliseconds | = | 1 | second |

1000 | seconds | = | 1 | kilosecond |

Three non-SI units of time; the minute, hour and day are accepted for use with the metric system. |
||||

60 | seconds | = | 1 | minute |

3600 | seconds | = | 1 | hour |

SI prefixes are not used with the non-SI units of time.

## Speed

The SI unit of speed is the metre per second, symbol m/s.

The kilometre per hour, symbol km/h, is not an SI unit, but is accepted for use with the metric system.

3.6 | kilometres per hour | = | 1 | metre per second |

## Force

The SI unit of force, or thrust, is the newton, symbol N.

One newton is equal to the force needed to accelerate a mass of one kilogram at the rate of one metre per second squared in the direction of the applied force.

## Pressure

The SI unit of pressure is the pascal, symbol Pa.

One pascal is equal to the pressure exerted by a perpendicular force of one newton on an area of one square metre.

Atmospheric pressure is approximately 101 325 Pa, or 1013 hPa.

## Energy

The SI unit of energy is the joule, symbol J. It is used to measure energy in all its forms; potential, kinetic, mechanical, electrical, chemical, heat, etc.

One joule is equal to the energy transferred to an object when a force of one newton acts on it in the direction of its motion through a distance of one metre.

One joule is equal to the energy required to accelerate a mass of one kilogram at a rate of one metre per second squared through a distance of one metre.

One joule is equal to the energy required to move an electric charge of one coulomb through an electrical potential difference of one volt.

One joule is equal to the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

One joule is equal to the energy required to produce one watt of power for one second, or one watt second, symbol W s.

It follows that, at a constant power of one kilowatt (kW), the amount of energy transferred in 3600 seconds, or one hour (h), is equal to 3.6 megajoules (MJ), or one kilowatt hour (kW h).

The kilowatt hour is a non-SI unit of energy commonly used in household energy bills.

## Power

The SI unit of power is the watt, symbol W. It is used to measure power in all its forms; mechanical, electrical, etc.

One watt is equal to the power required to transfer energy, or do work, at a rate of one joule per second.

e.g. A 2 kilowatt electric fire dissipates heat at a rate of 2 kilojoules per second.

One watt is equal to the power required, or the rate at which work is done, to hold an object at a constant velocity against a constant opposing force of one newton.

One watt is equal to the power required, or the rate at which electrical work is done, when a current of one ampere flows across an electrical potential difference of one volt.

One watt is equal to the power required when energy is dissipated as heat when an electric current of one ampere passes through a resistance of one ohm.

## Electricity

The four most common units for measuring electricity are the ampere, the volt, the ohm and the watt:

### Current

The SI unit of electric current is the ampere, symbol A. The ampere is one of the seven SI base units.

One ampere is equal to the electric current in a conductor when charge passes through it at a rate of one coulomb per second.

1000 | milliamperes | = | 1 | ampere |

1000 | amperes | = | 1 | kiloampere |

### Voltage

The SI unit of electric potential difference, or voltage, is the volt, symbol V.

One volt is equal to the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those two points.

### Resistance

The SI unit of electric resistance is the ohm, symbol Ω.

One ohm is equal to the electrical resistance between two points of a conductor when a constant potential difference of one volt, applied to these points, produces in the conductor a current of one ampere.

## Light

The SI unit of luminous flux is the lumen, symbol lm.

Luminous flux is a measure of the total quantity of visible light emitted by a source per unit time.

The brightness of domestic light bulbs is measured in lumens.