Gas Laws v Kinetic Theory (AQA A Level Physics): Revision Note

Gas laws v kinetic theory

• There is a scientific distinction between the gas laws and kinetic theory

Gas laws

• The gas laws are empirical in nature which means they are based on observation and evidence

• The gas laws include Boyle's Law, Charles's Law, Pressure Law and the ideal gas equation

• These are all based on observations of how a gas responds to changes in its environment, namely volume, pressure and temperature from experiment

Kinetic theory

• Kinetic theory is based on theory (as stated in its name)

• This means it is based on assumptions and derivations from existing theories

• These are then used to explain why the gas laws behave the way they do

Ideal gas internal energy

• The internal energy of an object is intrinsically related to its temperature

• When a container containing gas molecules is heated up, the molecules begin to move around faster, increasing their kinetic energy

• If the object is a solid, where the molecules are tightly packed, when heated the molecules begin to vibrate more

• Molecules in liquids and solids have both kinetic and potential energy because they are close together and bound by intermolecular forces

• However, ideal gas molecules are assumed to have no intermolecular forces

  • This means they have no potential energy, only kinetic energy

• So the ideal gas internal energy is the sum of all the kinetic energies of the atoms

  • In equation form, this can be written as:

$U=N\times E_k$

• Where:

  • $U$ is the internal energy of the ideal gas (J)

  • $N$ is the number of particles in the ideal gas

  • $E_k$ is the average kinetic energy of a single particle (J)

• Temperature is proportional to average kinetic energy, therefore, for an ideal gas, internal energy is proportional to temperature (this is covered in greater detail in Average Molecular Kinetic Energy)