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
-