# 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 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

Work is only done when the volume of a gas changes

Close