Gas Laws v Kinetic Theory (AQA A Level Physics)

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:

• Where:

  • U is the internal energy of the ideal gas in joules, J

  • N is the number of particles in the ideal gas

  • E_k is the average kinetic energy of a single particle in joules, 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)

As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy