Subject knowledge and
challenging concepts
- The absolute temperature scale is measured in Kelvin (K). The particles of a material at the temperature of 0 K are completely stationary: they have no kinetic energy at all. A temperature change of 1°C is the same as a temperature change of 1K (note this is one Kelvin, not one degree Kelvin). Absolute zero, 0 K, is just below ‑273°C. Thus the conversion factor is 273 (ie subtract 273 to turn K into °C, or add 273 to convert °C into K).
- In a gas, the particles are moving around at speed. If the temperature of the gas is increased, the particles will move around at a greater speed � they will have more kinetic energy. If the temperature is measured on the (absolute) Kelvin scale then the temperature of the gas will be directly proportional to the average kinetic energy of the molecules.
- If a gas is in a closed container it will exert a pressure on the sides of the container. Students need to be able to explain this in terms of the behaviour of particles. As the particles move, they will collide with each other and with the perimeter of the container. The collisions with the perimeter are the pressure being applied. If more gas is in the container there will be more collisions per second and the pressure will be greater. If the temperature of the gas increases, the particles of the gas will have more kinetic energy, will transfer more energy to the perimeter upon colliding and so the pressure will be greater.
- If a gas is in a sealed container and its temperature is measured in absolute terms, it will be directly proportional to the pressure it exerts. This relationship can be summarised by the equation P/T =constant. It can also be written as P1/T1 = P2/T2. This is useful as it enables, for a fixed mass of gas at constant volume, the temperature change to be calculated if the pressure alters (or the pressure change to be calculated if the temperature alters).
- If a fixed mass of gas is in a container whose volume is not fixed, the formula has to be modified. It still needs the temperature to be in absolute units. The formula is P1V1/T1 = P2V2/T2.