Physical Quantities + Units

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  • Created by: Jane
  • Created on: 07-04-13 20:47
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Physical Quantities + Units:
There are 7 base quantities with 7 base units
Base Quantity Symbol Base Unit Symbol
time t second s
length l metre m
mass m kilogram kg
temperature T, kelvin K
electric current I ampere A
amount of substance n mole mol
(luminous intensity) - candela cd
Derived Units:
All physical quantities that aren't measured in the 7 base units are known as derived quantities. The quantity and its
unit are derived from a combination of base units using a defining equation.
Example:
Both distance and length are base quantities and metres and seconds are base units so the derived unit of velocity is
metres per second written m/s or m s-1
The derived unit of acceleration is metres per second per second or metres per second squared written m/s2 or m
s-2
Common Derived Units:
Physical Quantity Defined As Unit Special Name
density
momentum
)
force newton (N)
)
pressure pascal (Pa)
work (energy) joule (J)
power watt (W)
electrical charge coulomb (C)
potential difference volt (V)

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Homogeneity of Equations:
The base units of each part of an equation must be the same. When true, this is said to be homogeneous.
E.g. kinetic energy = ½ x mass x velocity2
Unit of kinetic energy = joule = kg m2 s-2
Unit of ½ x mass x velocity2 = kg x (m s-1) = kg m2 s-2
However an equation can be homogenous but still incorrect.…read more

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Comments

Iridann

Helpful But the derivations were a bit awkward units wise.

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