• 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