10: thermal physics 10.3 second law of thermodynamics and entropy

10: Thermal Physics

10.3 Second Law of Thermodynamics and Entropy

The Second Law of Thermodynamics

A simple statement of the Second Law is that...

... you cannot have a process whose only effect is to turn heat energy into work.

Thus a true isothermal process cannot really happen. In reality the heat flowing into a gas will cause the average KE of the molecules to increase which in turn increases the pressure, enabling the gas to do work on an external system.

Energy Degradation

High grade forms of energy:

- Useful forms of energy that can be used to do work.

- Ordered

- E.g. Chemical and electrical energy.

Low grade forms of energy:

- Less useful forms of energy that cannot easily be transferred.

- Less ordered

- E.g. Internal energy is a low grade form of energy.

Entropy

A different way of stating the Second Law of Thermodynamics is to say that...

...energy will always spread out.

Entropy is a way of measuring the degree of disorder in a system.

i.e. the less order, the more entropy.

(See PhET simulation)

The Second Law of Thermodynamics (again).

E.g. Falling rain.

The rain initially has gravitational potential energy: the energy is ordered.

After it has hit the ground the energy becomes internal energy of the surroundings (e.g. due to heat transfer as a result of friction). Thus the energy is less ordered and entropy has increased.

During any thermodynamic process the total entropy of the system must increase.

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