SI Units and calculating unit changes

This learning package is concerned with understanding SI units.

In order to complete it, you might like to have a piece of paper and a pen handy to jot down notes, but this is not essential.

Once you have completed this learning package you will be able to:

Identify SI and SI derived untis for the main untis of measurement and be able to use the appropriate prefixes when working with large or small values.
Standardising measurement

People have devised many different  systems to measure the world around them.  Historically some of these were based on parts of the body – such as thumb length.There are two problems with this. Can you think what they are?

Firstly, standardisation.  In our example, not everyone’s thumbs are the same size and secondly, having different units of measurement can be confusing.  Even recently a Mars lander crashed into the planet’s surface due to a mix up between cms and inches.  A costly mistake!

Therefore in 1960 a standardised system of measurement, the System International or SI system was launched to over come these problems.
SI Units

The SI System has now been adopted throughout most of the world.  It is based on 7 standardised units of measurement.  They are:

The metre – which is the SI unit for length or distance

The gram (or kilogram) – which is the SI unit for mass

The second – which is the SI unit for time

The Ampere – which is the SI unit for electrical current

The Candela – which is the SI unit for light intensity

The Kelvin – which is the SI unit for temperature

And 

The Mole – which is the SI unit for the amount of a sunstance

Combining SI units

These 7 SI units can be combined to form many other types of measures.

For example speed can be defined as metres per second  - ie the number of metres that a particular object moves in one second.

All other measures can be derived from these 7 basic units.  Some are given special names

For example:  Fluid volume is often measured in litres, pressure in pascals, energy in Joules , power in watts and so on.

It is possible to see how each of these are derived form the 7 basic units, but the units are so common themselves, that this is not usually necessary.
Prefixes
SI units of measurement must be able to work across a very wide range.  For example the units for the measurement of length must be able to cope with very small values such as the diameter of the human skin cell – at 0.0001 m, but  at the samle time with very large values – such as the diameter of the earth at nearly 12800000 m.  

It is obviously difficult to work with such large and small numbers, so SI units have a series of standard prefixes that act as shorthand for larger and small values.

When a quantity reaches 1000 X its units it is shortened to kilo units.  Think of  weight.  If someone weighs 70,000grams we write this as 70 kilograms.  When a quantity reaches 1000000 X its units we write this as Mega units.  When we think of the output of power stations we think in terms of megawatts of output. 

Working in the other direction, when a quantity is only one thousandth of its unit, we write this as milli units.  We often measure small lengths in millimetres.  When a quantity is only one millionth of its  unit, we write this as micro () unit.  Some drugs are given in microgram quantities, for example. You need to familiarise yourself with these prefixes and become competent at converting between them.

Well done.  You have now completed this learning pack.   

Prefix list

yotta [Y]	 1 000 000 000 000 000 000 000 000     
zetta [Z] 	1 000 000 000 000 000 000 000         
exa   [E] 	1 000 000 000 000 000 000             
peta  [P] 	1 000 000 000 000 000                 
tera  [T] 	1 000 000 000 000                     
giga  [G] 	1 000 000 000 		   
mega* [M] 	1 000 000 			(a million)
kilo  [k] 	1 000 			(a thousand)
hecto [h] 	100                              		(a hundred)
deca  [da]	10                              		(ten)
	 1
deci  [d] 	0.1                             		(a tenth)
centi [c] 	0.01                             		(a hundredth)
milli [m] 	0.001 			(a thousandth)
micro [µ]	 0.000 001 			(a millionth)
nano  [n] 	0.000 000 001 		(a thousand millionth)
pico  [p] 	0.000 000 000 001			
femto [f] 	0.000 000 000 000 001		
atto  [a] 	0.000 000 000 000 000 001		
zepto [z]	 0.000 000 000 000 000 000 001		
yocto [y] 	0.000 000 000 000 000 000 000 001	

* 1 megagram is known as I tonne

Resources
A comprehensive list of SI derived units is outside the scope of this learning package but you can use these additional resources to look up others that we have not discussed here, or to learn how each is derived from the seven basic SI units.

Simetric Metric Conversion Tables	A-Z listing of SI and SI derived units, explanation of SI prefixes and a handy tool to convert between SI and non SI units	
A Dictionary of Measures, Units and Conversions	SI and SI derived units, background, conventions and prefixes	
Units of Measurement	SI and SI derived units, background, conventions and prefixes	
Essentials of the SI: base and derived units	SI units and SI derived units, conventions, relationships, prefixes and conversion between SI and non SI units