Units
Imagine you had to make curtains and needed to buy fabric. The shop assistant would need to know how much fabric you needed. Telling her you need fabric 2 wide and 6 long would be insufficient — you have to specify the unit (i.e. 2 metres wide and 6 metres ...
Units
Imagine you had to make curtains and needed to buy fabric. The shop assistant would need to know how much fabric you needed. Telling her you need fabric \(\text{2}\) wide and \(\text{6}\) long would be insufficient — you have to specify the unit (i.e. \(\text{2}\) metres wide and \(\text{6}\) metres long). Without the unit the information is incomplete and the shop assistant would have to guess. If you were making curtains for a doll's house the dimensions might be \(\text{2}\) centimetres wide and \(\text{6}\) centimetres long!
It is not just lengths that have units, all physical quantities have units (e.g. time, temperature, distance, etc.).
Definition: Physical Quantity
A physical quantity is anything that you can measure. For example, length, temperature, distance and time are physical quantities.
There are many different systems of units. The main systems of units are:
- SI units
- c.g.s units
- Imperial units
- Natural units
SI Units
Image credit: YouTube
We will be using the SI units in this tutorial and others. SI units are the internationally agreed upon units.
Definition: SI Units
The name SI units comes from the French Système International d'Unités, which means international system of units.
There are seven base SI units. These are listed in the table below. All physical quantities have units which can be built from these seven base units. So, it is possible to create a different set of units by defining a different set of base units.
These seven units are called base units because none of them can be expressed as combinations of the other six. These base units are like the \(\text{26}\) letters of the alphabet for English. Many different words can be formed by using these letters.
Table: SI Base Units
|
Base quantity |
Name |
Symbol |
|
length |
metre |
\(\text{m}\) |
|
mass |
kilogram |
\(\text{kg}\) |
|
time |
second |
\(\text{s}\) |
|
electric current |
ampere |
\(\text{A}\) |
|
temperature |
kelvin |
\(\text{K}\) |
|
amount of substance |
mole |
\(\text{mol}\) |
|
luminous intensity |
candela |
\(\text{cd}\) |
Other Systems of Units
The SI Units are not the only units available, but they are most widely used. In Science there are three other sets of units that can also be used. These are mentioned here for interest only.
-
c.g.s. units
In the c.g.s. system, the metre is replaced by the centimetre and the kilogram is replaced by the gram. This is a simple change but it means that all units derived from these two are changed. For example, the units of force and work are different. These units are used most often in astrophysics and atomic physics.
-
Imperial units
Imperial units arose when kings and queens decided the measures that were to be used in the land. All the imperial base units, except for the measure of time, are different to those of SI units. This is the unit system you are most likely to encounter if SI units are not used. Examples of imperial units are pounds, miles, gallons and yards. These units are used by the Americans and British. As you can imagine, having different units in use from place to place makes scientific communication very difficult. This was the motivation for adopting a set of internationally agreed upon units.
-
Natural units
This is the most sophisticated choice of units. Here the most fundamental discovered quantities (such as the speed of light) are set equal to \(\text{1}\). The argument for this choice is that all other quantities should be built from these fundamental units. This system of units is used in high energy physics and quantum mechanics.
This lesson is part of:
Skills for Science