11.2.1 The First Law of Thermodynamics

• The first law of thermodynamics is based on the principle of conservation of energy
  • This applies to heating, cooling and work done
• The first law can be expressed in different ways, depending on the sign convention used
• For AQA, the first law of thermodynamics is therefore defined as:

• Where:
  • ΔU = increase in internal energy (J)
  • Q = energy supplied to the system by heating (J)
  • W = work done by the system (J)
• The 'system' is a region of space containing a quantity of gas
• The system is open if gas or vapour flows into and out of the region
  • Examples include a gas expanding through the nozzle of an aerosol can or steam passing through a turbine
• The system is closed if gas or vapour remains within the region, although the boundary can expand or contract with changes in the volume of the gas
  • Examples include gas expanding in a cylinder by moving a piston or air in a balloon being heated

Open and closed systems

An open system is where gas can flow in and out, whilst in a closed system, gas can only remain within the boundary

• In both systems, work can 'cross' the boundary

• The first law of thermodynamics applies to all situations, not just to gases
  • There is an important sign convention used for this equation

• Amount of heat transfer Q is:
  
  • Positive if heat energy is added
  • Negative if heat energy is removed
• The change in internal energy ΔU is:
  • Positive if the internal energy increases
  • Negative if the internal energy decreases
• The work done, W is:
  • Positive if work is done by the gas (the gas expands)
  • Negative if the work is done on the gas (the gas is compressed)