A heat engine requires both a source and a sink to operate

Engineering Physics | AQA A-Level Physics

The second law

A heat engineA system that converts thermal energy into useful mechanical work. is a system that converts heat into usable energy, which is then used to do mechanical work.
The second law of thermodynamics states that:

Key Definition

Second law of thermodynamics: A heat engine requires a source and a sink to operate. Equivalently: it is impossible to extract energy from a heat reservoir and convert it entirely into work.

Source and sink

A source is a high-temperature reservoir at temperature $T_H$. The heat energy transferred from it is $Q_H$.
A sink is a low-temperature reservoir at temperature $T_C$. The heat energy transferred to it is $Q_C$.
Another way of stating the second law is: thermal energy cannot spontaneously transfer from a region of lower temperature to a region of higher temperature.

Why a sink is necessary

If the engine reached the temperature of the source, no heat would flow because they would have reached thermal equilibrium. Therefore, no work would be done.
This means it is impossible for a heat engine to work solely on the first law of thermodynamics. A temperature difference is essential.

Source-sink diagrams

If a heat engine only obeyed the first law (no friction, no sink), all the heat from the source would be converted to work: $Q_H = W$. This would be 100% efficient, but it is not possible in reality.
In a real heat engine that obeys both laws:
- Heat energy $Q_H$ is transferred from the source at temperature $T_H$
- Some of this energy is transferred into work, $W$
- The remaining energy, $Q_C$, is transferred to the sink at temperature $T_C$
Crucially, in a source-sink diagram, all the arrows flow downwards (from hot to cold), with work leaving the engine to the side. This reflects the natural direction of heat flow.

Common Mistake

Be careful with the terminology: heat engines convert thermal energy into mechanical work (source to sink). Heat pumps transfer heat energy from low temperature to high temperature (this is covered in the reversed heat engines topic). Mixing these up is a common exam error.