The Second Law of Thermodynamics and Entropy

State that the second law of thermodynamics implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.

HEAT ENGINE: Any device which converts heat energy into mechanical energy.

Heat_engine.png
Heat engine


EFFICIENCY: The ratio of useful output energy to total energy input (often expressed as a percentage).

The equation below applies to a Carnot cycle (idealised heat engine).
n_thermal_efficiency.png


State that entropy is a system property that expresses the degree of disorder in the system.


ENTROPY: A measure of disorder of a system. It is measured in J/K.

[[image:S=Q:T.png width="72" height="47" caption="Entropy when T is constant"]]

State the second law of thermodynamics in terms of entropy changes.

A statement that the overall entropy of the universe is increasing will suffice or that all natural processes increase the entropy of the universe.

SECOND LAW OF THERMODYNAMICS: No continually working heat engine can take heat from a source and convert it completely into work.
OR: Thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.
OR: Although local entropy may decrease, the direction of a process is such as to increase the total entropy of the system and surroundings.


Discuss examples of natural processes in terms of entropy changes.

Students should understand that, although local entropy may decrease, any process will increase the total entropy of the system and surroundings, that is, the universe.

The following involve entropy changes.
1 kg of ice melting at 273K. The entropy change is equal to the latent heat of fusion.
Crystallisation: the increase in order of the atoms is balanced by the release of heat.
Life: the organisation of living systems is paid for by dissipation of heat.
The Universe: as time passes, concentrated energy such as stars will give way to a more equal spread of energy.
Mechanical changes: systems become warmer due to dissipative forces, so entropy increases.
Fossil fuels: sources of concentrated energy which is spread out when the fuel combusts.
Gases mix.
Eggs break.
Mountains erode.