Sectieoverzicht

    • Fundamental Quantities

      Fundamental quantities are independent physical quantities that are not possible to be expressed in terms of any other physical quantity.

      These quantities are used to derive all other physical quantities.  

      It is important to remember these seven fundamentals (or 'base') units of measurement:

      • metre (m) - unit of length
      • kilograms (kg) - unit of mass
      • second (s) - unit of time
      • ampere (A) - unit of electrical current
      • kelvin (K) - unit of temperature
      • mole (mol) - unit of the amount of substance
      • luminous intensity (cd) unit of candela

      SI Unit Prefixes

      The SI system utilizes a standard system of prefixes to the basic units, allowing them to be more relevant and descriptive of relative magnitude. Prefixes are used to identify the original unit’s multiples or fractions. There are 20 accepted prefixes. The table below lists some the standard prefixes for the SI units of measurement.

      Approximate length of time in seconds

      Events

      1018

      Expected lifetime of the sun

      1017

      Age of the Earth

      1015

      Time since dinosaurs lived

      1013

      Time since the earliest human

      1010

      Time since Isaac Newton lived

      109

      Average human life span

      107

      A school term

      105

      One day

      100

      One second

      10-2

      Time for sound to cross the room

      10-7

      Time for an electron to travel through a TV tube

      10-8

      Time for light to cross a room

      10-11

      Time for light to pass through spectacles

      10-22

      Time for some events inside an atom


      For very large or very small numbers, we use a short way of writing them by counting the numbers of zeros.  

      For example:
      1 million = 1 000 000 (6 zeros) = 106
      0.000 001 = 1/1 000 000 =  1 millionth = 10-6

      Derived units

      There are many quantities scientists measure that come from the base units. These derived units are very useful to quote as measurements, but they are not fundamental as they come from fundamental units.

      For example, frequency is the number of times something happens per unit of time. This is a useful quantity, but it is a division into the time unit. The standard unit for frequency is considering the number 'per second', which is called 'hertz, Hz', but this comes from the fundamental unit 'second'.

      Derived units include:

      Name Unit Abbreviation
      Frequency hertz Hz
      Force newton N
      Energy joule J
      Power watt W
      Pressure pascal Pa
      Electric charge coulomb C
      Electric potential difference volt V
      Electric resistance ohm Ω
      Magnetic flux density tesla T