1.3 Units  (Page 78/6)

Introduction

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 long). Without the unit the information is incomplete and the shop assistant would have toguess. If you were making curtains for a doll's house the dimensions might be 2 centimetres wide and 6 centimetres long!

It is not just lengths that have units, all physical quantities have units (e.g. time, temperature, distance, etc.).

Physical Quantity

A physical quantity is anything that you can measure. For example, length, temperature, distanceand time are physical quantities.

Unit systems

Si units

We will be using the SI units in this course. SI units are the internationally agreed upon units. Historically these units arebased on the metric system which was developed in France at the time of the French Revolution.

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 [link] . All physical quantities have units which can be built from these seven base units. So, it is possible to create adifferent 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. This is identicalto bricks and concrete being the base units of a building. You can build different things using different combinations of bricks andconcrete. The 26 letters of the alphabet are the base units for a language like English. Many different words can be formed by usingthese letters.

The 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 thatcan 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 butit means that all units derived from these two are changed. For example, the units of force and work are different. These unitsare 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 SIunits. 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 fromplace to place makes scientific communication very difficult. This was the motivation for adopting a set of internationally agreedupon units.