# Chapter 2.1.1: Physical Quantities and Units

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## Units

In 1960, the international scientific community agreed to adopt a single unit for each quantity. These are known as S.I. units.

Quantity:                        Unit:           Symbol:

mass                              kilogram     kg

length                             metre         m

time                                second       d

temperature                   kelvin          K

electrical current            ampere       A

amount of substance     mole           mol

luminous intensity          candela      cd

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## Derived S.I. Quantities

Quantity:                   Unit/symbol:

Acceleration                metres per second squared, m/s2

Density                     kilogram per cubic metres, kg/m3

Electric Charge             coulomb, C

Energy                      joule, J

Force                        newton, N

Momentum                  kilogram metres per second, kg m-1 s-1

Potential Difference        volt, V

Power                       watt, W

Pressure                    pascal, P

Velocity                    metres per second, m s-1

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## Homogeneity of equations

You can use base units to check whether an equation representing physical quantities could be correct. If the equation is correct, the units must be of the same type (homogenous) for each quantity in the equation. However, having homogenous units does not prove that the equation is correct.

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