The First Law of Thermodynamics (AQA A Level Physics) : Revision Note

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)