11.2.9 The Second Law of Thermodynamics

• A heat engine is a system that converts heat to usable energy which is then used to do mechanical work

• The second law of thermodynamics states that

A heat engine requires a source and a sink to operate

• A source is a high-temperature reservoir
  • It has a high temperature T_H and the heat energy from it is Q_H 
• A sink is a low-temperature reservoir
  • It has a low temperature T_C and the heat energy going into 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

Or:

When extracting energy from a heat reservoir, it is impossible to convert it all into work

• If the engine reached the temperature of the source, no heat would flow as 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
• If a heat engine only obeyed the First Law (there is no friction), the source-sink diagram would look like:

Source-Sink Diagrams

Source-sink diagram that obeys only the First Law of Thermodynamics

• It is assumed that T_H remains at a constant temperature
• This engine is 100% efficient however, it is not possible to make this type of engine, due to frictional losses in real life
• Therefore, all engines obey the Second Law of Thermodynamics, and a source-sink diagram for an actual heat engine is

Actual source-sink diagram for a heat engine that obeys the First and Second Law of Thermodynamics

• 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