6.4.2 The First Law of Thermodynamics

• The First Law of Thermodynamics, which is based on the principle of conservation of energy, states:

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa)

• This is important when thinking about the expansion or compression of a gas
  • The First Law of Thermodynamics applies to all situations, not just for gases

• When a gas expands (its volume increases), work is done by the gas on the surroundings
  • This decreases the internal energy of the gas

• When a gas is compressed (its volume decreases), work is done on the gas by the surroundings
  • This increases the internal energy of the gas

• The 'gas' is sometimes referred to as the 'system'
• A gas can have work done on or by it when it is in a cylindrical container with a moveable piston

Work is done on the gas when it is compressed and by the gas when it expands

• When the piston moves down the cylinder, it compresses the gas molecules (work is done on the gas)
  • The molecules are pushed closer together
  • Therefore, they have higher kinetic energy as they move faster
  • This increases the internal energy of the gas

• When the piston moves up the cylinder, it expands the gas molecules (work is done by the gas)
  • The molecules are spread further apart
  • Therefore, they have lower kinetic energy as they move slower
  • This decreases the internal energy of the gas

• The same increase in internal energy can be achieved by not doing work (i.e. no expansion or contraction), but by heating the gas instead
  • Increasing the temperature of the gas means the molecules move around faster
  • They, therefore, have higher kinetic energy and increased internal energy