The First Law of Thermodynamics (AQA A Level Physics)

Definition of the first law of thermodynamics

• The first law of thermodynamics is based on the principle of conservation of energy, and is:

The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it

• The internal energy of a system is increased when:
  • Work is done on the system (e.g. it is compressed)
  • Thermal energy is transferred to the system by heating

• The internal energy of a system is decreased when:
  • Work is done by the system (e.g. the system expands)
  • Thermal energy is transferred away from the system by heating

Work done on or by a system

• Mechanical work is done when a distance is covered against a force

• While the first law applies to all systems, work done is important when thinking about the expansion or compression of a gas
• 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

• A common scenario is a gas in a container with a moveable piston (one face of the container can move back and forth freely)
  • A gas can do work on the piston, moving it outwards and increasing the volume of the gas
  • The piston can do work on the gas, moving inwards and decreasing the volume of the gas

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

• When the piston moves down, the space occupied by the gas molecules is compressed (work is done on the gas)
  • If the system is insulated (no thermal energy is transferred away from the gas), this work is transferred to the system's internal energy
  • This increases the internal energy of the gas
  • The particles have a greater average kinetic energy

• When the piston moves up, the space occupied by the gas molecules expands (work is done by the gas)
  • If the system is insulated (no thermal energy is transferred to the gas), this work is transferred from the system's internal energy
  • This decreases the internal energy of the gas
  • The particles have a lower average kinetic energy