Temperature and Pressure



The concept of temperature arises from two aspects:

For two bodies at different temperatures, heat will flow from the hotter to the colder until their temperatures become the same and thermal equilibrium is reached. Thus, temperature and heat, although interrelated, refer to different concepts; temperature is a property of a body and heat is an energy flow to or from a body by virtue of a temperature difference.

Temperature changes have to be measured in terms of other property changes of a substance. For example, the mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change.

Temperature Scales

The centigrade, or Celsius scale, devised by the Swedish astronomer Anders Celsius, is now very commonly used. At atmospheric pressure it assigns a value of 0 C to the freezing point (and melting point) and 100 C to the boiling point of water. It is an arbitrary temperature scale in the sense that 0 °C is not the coldest temperature. Put another way, 0 °C is not an 'absolute zero'.

The concept of an absolute zero of temperature evolved in connection with experiments with gases. When a fixed mass of gas is cooled at a constant pressure (volume), its volume (pressure) decreases with its temperature. A plot of the experimental values of volume (pressure) versus temperature can be extrapolated to cross the temperature axis when the volume (pressure) would be zero. The temperature is the absolute zero of temperature. It is -273.15 °C, or more approximately -273 °C.

The atoms and molecules of a substance at absolute zero would not be completely motionless but have minimum possible movement, and so could not transfer any heat to something else. Absolute zero cannot actually be reached, but there are ways in which it can be closely approached.

The British mathematician and physicist William Thomson, or Lord Kelvin, went on to devise a new temperature scale for which absolute zero was identified as 0 degrees (0 K). The units, called Kelvins (K), were defined as identical to Celsius degrees (1 K = 1 °C).

This is the Absolute or Kelvin temperature scale.

Convert between Absolute and Celsius temperatures...

Enter a Celsius or Kelvin temperature:
Celsius temperature: °C
Kelvin (Absolute) temperature: K


The pressure exerted by a gas is due to the collisions of the molecules upon the walls of the vessel.

The gas inside an inflated balloon exerts a greater pressure outward than the inward pressure of the atmosphere. The air inside an open container exerts a pressure equal to atmospheric pressure. Similarly, the internal pressure of the fluids in the human body equal the pressure of the atmosphere preventing it from being crushed.

Atmospheric pressure is illustrated by the 'collapsing can experiment'.

A small amount of water is heated in an open metal oilcan. The water is allowed to boil for a while so that the steam formed replaces some of the air above the water. The heat source is then removed and the lid screwed on to the can. It is now left to cool. As this happens the steam begins to condense to form liquid water. This results in the internal gas pressure becoming less than the external atmospheric pressure. The walls of the can are not strong enough to resist the external pressure and the sides of the can are crushed inwards.

How is Pressure defined?

Pressure is defined as force per unit area exerted at right angles to a surface.

Units of Pressure

The SI unit of force is the Newton (N). Unit area is 1 square metre (m2), and therefore the SI unit of pressure is Nm-2, also known as the Pascal (Pa).

Newton's Second Law of Motion relates the force acting on an object, its mass and acceleration. This is expressed as:

force = mass x acceleration

The gravitation field causes a falling object to accelerate towards the ground.

Gravity is commonly measured in terms of the amount of acceleration that the force gives to an object on the Earth. The generally accepted international value for the acceleration due to gravity is 9.8 m s-2 (approximately 10 m s-2).

Weight is a force, and so from the above equation, a 0.1 Kg mass placed on an area of 1 m2 exerts a force of 1 N, and therefore a pressure of 1 N m-2.

Because of its absolute nature, pressure is commonly expressed in several other units, in particular, atmospheres (atm) and millimetres of mercury (mm Hg). 1 atmosphere pressure is defined as equal to the pressure exerted by a column of the mercury exactly 760 mm high. It corresponds to 101325 Pa.

To understand the mercury barometer you have to know something about pressure in liquids. Pressure is transmitted equally throughout a liquid at a given height so, whatever the diameter of the barometer tube, the upward pressure at the base will be the same. The downward pressure exerted by each column of mercury must therefore be the same. In a column of larger diameter, the larger weight of mercury is spread over a larger area resulting in the same pressure. So, the downward pressure of a column of liquid depends only on its vertical height.