## Units

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Units is a short word meaning Units of Measurement

Measurement is a process that uses numbers to describe a physical quantity. This is necessary so you can compare one thing to another. We can measure how big things are, how warm they are, how heavy they are, and lots of other features as well. Units of Measurement provide standards for our comparisons, so that the numbers from our measurements refer to the same thing.

For example, the metre is a standard unit to measure length. If a stick has a length of 2 metres it means that it is exactly twice as long as a stick which measures 1 metre.

In 1965 the UK adopted a metric system of measurement. This system is called SI (short for International System). The SI system has units such as metres and kilograms. Before 1965 the UK used its own ‘Imperial’ system with units such as feet and ounces. Not all countries in the world use SI, but most do. The UK still uses Imperial units for a few things – miles and pints included, but mostly everything is SI. All units have a symbol which indicates what unit it is, i.e the symbol for metres is m.

## SI Base Units and their symbols

What unit measures Name Symbol
length metre m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

## SI Derived Units

In addition to the seven Base SI Units there are many Derived SI Units. For electrical work we mostly use Derived SI Units. Some derived units have symbols which are formulas, others use special symbols, mostly from Greek letters.

Derived quantity Name Symbol
angle degree* °
Celsius temperature degree Celsius °C
electric charge, quantity of electricity coulomb C or Q
electric conductance siemens S
electric potential difference, electromotive force volt V
electrical power watt W
electrical resistance ohm Ω (omega)
electrical resistivity ohm meter ρ (rho)
energy, work, quantity of heat joule J
force newton N
frequency hertz Hz
inductance henry H
luminous flux lumen lm
magnetic flux weber Wb
magnetic flux density tesla T
area square metre m2
volume cubic metre m3
speed metre per second m/s
current density ampere per square metre A/m2
magnetic field strength ampere per metre A/m
* the actual SI name is radian. But measuring angles in degrees is sufficient for electrical work