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Wonders of Physics

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Common SI prefixes

Physical Quantity is a quantity that can be measured and it consists of a numerical magnitude (size) and a unit. (e.g. 5m where 5 is the numerical magnitude and m is the unit)

1) The seven base quantities and their SI units

Base Quantity Name of SI unit Symbol for SI unit
Length metre m
Mass kilogram kg
Time second s
Electric Current ampere A
Thermodynamic temperature kelvin K
Luminous intensity candela cd
Amount of substance mole mol

Out of the seven, for O level Physics, you are required to learn the first 5 (length, mass, time, current and temperature).

2) Some common derived quantities and units

Physical Quantity How it is derived from base quantities Symbol for SI unit
Area length x width m2
Volume length x width x height m3
Speed length/time m/s
Acceleration velocity/time m/s2
Density mass/volume kg/m3

You can think of base quantities as the smallest building blocks (like Lego bricks). When you combined any base quantities to ‘build’ into something else, they becomes a derived quantity.The derived quantity is ‘derived’ or ‘gotten from’ when 2 or more quantities are combined together.

3) Prefixes for SI unitsPrefixes are used to represent very large or small quantities.For example, instead of writing 1000000 m, it will be more convenient to write 1Mm, which means the same thing!

Factor Prefix Symbol
109 giga- G
106 mega- M
103 kilo- k
10-1 deci- d
10-2 centi- c
10-3 milli- m
10-6 micro- ยต
10-9 nano- n