Units and Prefixes

Units and Prefixes

All units in science are derived from seven base units:

Mass kilogram kg
Distance metre m
Time second s
Current ampere A
Amount mole mol
Temperature Kelvin K
Light Intensity candela cd

There are many other units that we use, but all of these are derived by multiplication or division of some combinations of the base units.

You can think of it like letters and words. We have 26 letters in the alphabet but we have thousands of words in our language. Here are some of the derived units:

Quantity Unit Symbol Base unit equivalent
Velocity metre per second ms-1 ms-1
Acceleration metre per second squared ms-2 ms-2
Force Newton N kg ms-2
Work or Energy joule J kg m2s-2
Power watt W kg m2s-3
Pressure Pascal Pa kg m-1s-2
Frequency hertz Hz s-1
Charge coulomb C A s

Now you have units, you often need to group these into larger or smaller numbers to make them more manageable. For example, you don't say that you are going to see someone who lives 100,000 m away from you, you say they live 100 km away from you.

Here a quick list of the common quantities used:

Name Symbol Scaling factor Common example
tera T 1012 1,000,000,000,000 Large computer hardrives can be terabytes in size.
giga G 109 1,000,000,000 Computer memories are measured in gigabytes.
mega M 106 1,000,000 A power station may have an output of 600 MW (megawatts).
kilo k 103 1,000 Mass is often measured in kilogrammes (i.e. 1000 grammes).
deci d 10-1 0.1 Fluids are sometimes measured in decilitres (i.e. 0.1 litre).
centi c 10-2 0.01 Distances are measured in centimetres (i.e. 100th of a metre).
milli m 10-3 0.001 Time is sometimes measured in milliseconds.
micro µ 10-6 1,000,000th micrometres are often used to measure wavelengths of electromagnetic waves.
nano n 10-9 nanometres are used to measure atomic spacing.
pico p 10-12 picometres used to measure atomic radii.

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