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Science

Units and Measurement

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 to guess. If you were making curtains for a doll’s house the dimensions might be 2 centimetres wide  and 6 centimeters 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.

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

Definition: SI Units The name SI units comes from the French Syst`me International d’Unit ́s, which means international system of units. There are seven base SI units. These are listed below. All physical quantities have units which can be built from these seven base units. These seven units were defined to be the 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. This is identical to bricks and concrete being the base units of a building. You can build different things using different combinations of bricks and concrete. The 26 letters of the alphabet are the base units for a language like English. Many different words can be formed by using these letters.

 Base Quantity Name Unit Length Metre m Mass Kilogram Kg Time Second S Electric current Ampere A Ammount of sustance Mole mol Temperature Kelvin K Luminous intensity Candela cd

It is very important that you are aware that different systems of units exist. Furthermore, you must be able to convert between units. Being able to change between units (for example, converting from millimetres to metres) is a useful skill in Science.