AQA A Level Physics

Revision Notes

6.4.2 The First Law of Thermodynamics

The First Law of Thermodynamics

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

When energy is transferred into a system by heating it or doing work on it, its internal energy must increase

  • Another way of writing this is as follows:

The increase in internal energy = Energy supplied by heating + Work done on the system

  • This is sometimes represented with symbols:

U = q + W

  • This is important when thinking about the expansion or compression of a gas
  • When the gas expands, it transfers some energy (does work) to its surroundings
    • This decreases the overall energy of the gas
  • Therefore, when the gas expands, work is done by the gas (−W)

When a gas expands, work done W is negative

  • When the gas is compressed, work is done on the gas (+W)

When a gas is compressed, work done W is positive

Positive and negative work done

Positive or negative work done depends on whether the gas is compressed or expanded

Graphs of Constant Pressure & Volume

  • Graphs of pressure p against volume V can provide information about the work done and internal energy of the gas
    • The work done is represented by the area under the line
  • A constant pressure process is represented as a horizontal line
    • If the volume is increasing (expansion), work is done by the gas and internal energy increases
    • If the arrow is reversed and the volume is decreasing (compression), work is done on the gas and internal energy decreases
  • A constant volume process is represented as a vertical line
    • In a process with constant volume, the area under the curve is zero
    • Therefore, no work is done when the volume stays the same

Constant Pressure and Volume Graphs

Work is only done when the volume of a gas changes

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