## Measurement

Here are some important measurement concepts and terminology that will be encountered regularly throughout the different topics. Though the units used can vary between textbooks, we have stuck with the most commonly used SI (Systeme International) units. You can download a reference copy at the bottom of the page.

## Base Units

These are the basic units from which other more complex units are calculated from.

**Length**– Metres (m)

The length of a metre is based on the distance that light travels through a vacuum in a specified fraction of a second (1/299,792,458th if you’re interested).

**Time**– Second (s)

The second is a duration of time that is based on the frequency of radiation emitted from caesium-133.

**Mass**– Kilogram (kg)

The SI unit of a kilogram is based on the mass of a 1kg prototype held at Sevres near Paris. 1kg is very nearly equivalent to the mass 1 litre of water (1.000025 litres).

**Temperature**– Kelvin (K)

The Kelvin is the unit of temperature defined as 1/273.16th of the thermodynamic temperature of the triple point of water (the temperature when water vapour, liquid water and ice are in equilibrium, at 0.01 Degrees Celsius)

Celsius and Kelvin use the same unit intervals (1 degree Kelvin is equal to 1 degree Celsius) but have different reference points for zero.

Absolute zero is 0 K which is -273.16 Degrees Celsius.

Mole - (Mol)

A mole is the SI unit the refers to the amount of a compound. The number of molecules in any substance is far to high to be of any use as a reference point, but is still obviously an important factors. This unit acts as a conversion to a more usable value.

1 mole is defined as 12g of Carbon 12 (it is the mass number of the molecule in grams)

1 mole is equal to 6.02 x 10^23 molecules.

A mole is the SI unit the refers to the amount of a compound. The number of molecules in any substance is far to high to be of any use as a reference point, but is still obviously an important factors. This unit acts as a conversion to a more usable value.

1 mole is defined as 12g of Carbon 12 (it is the mass number of the molecule in grams)

1 mole is equal to 6.02 x 10^23 molecules.

## Derived Units

These are units that are derived from the base units and from each other. There are a huge number of them but the most important are here

**Area**– Square meter (m2)

A two-dimensional value

**Volume**– Cubic metre (m3)

A three dimensional value, the cubic meter is the SI unit for volume

**Volume**– litre (l)

One litre is perhaps a more common unit and is one thousandth of a cubic metre.

1 litre = 10-3 m3

**Speed**– Metres per second (m s-1)

Described the distance (in metres) that is travelled in one second. It is a

__as it only has a magnitude.__

*scalar*quantityVelocity is the vector quantity for speed (i.e. it has a magnitude and a direction).

**Acceleration**– Metres per second per second (m s-2)

Is the vector quantity describing the change in velocity of an object. It describes the change in velocity per second.

**Force**– Newton (N)

Force describes an influence that will cause a mass to change its velocity.

Force = mass x acceleration

The SI unit of force is the Newton. 1 Newton is the force that will cause a 1kg mass to accelerate by 1 m s-2

1N = 1 kg m s-2

**Pressure**– Pascals (Pa)

Pressure describes force that is distributed over an area. A pascal is the SI unit of pressure and is defined as the pressure of 1 Newton applied over and area of 1 metre squared.

1Pa = N m-2

This is a very small force and so the kilopascal (kPa) is more commonly used.

1kPa = 1000Pa

**Pressure**– Bar (bar)

Whilst it isn’t an SI unit it is still commonly used.

1 bar = 100 kPa

**Pressure**– Atmosphere (atm)

A more historical unit that described the pressure at sea level. Is almost exactly equivalent to 1 bar or pressure and this is now mainly used instead.

1 atm = 101.325 kPa

**Pressure**– Millimetres of mercury (mmHg)

Yet another way to measure pressure! It describes the pressure exerted by a column of mercury of a certain height (the height measured in mm).

1kPa = 7.5mmHg

**Energy**– Joule (J)

Energy is calculated based on the force applied to move an object over a certain distance. The joule is the SI unit of energy and refers to the energy expended to apply a force of 1 Newton for 1 metre.

Work = Force x Distance

1 J = 1 N m

## Other Useful Stuff

**Standard Temperature and Pressure (STP)**

The way substances behave is so strongly influenced by factors such as temperature and pressure that a single reference point is required.

STP refers to 0 degrees centigrade (273K) and atmospheric pressure (101.3 kPa)

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