The Restless universe
Introduction to The restless Universe

1 The lawful Universe

3 The irreversible Universe

3.1 Thermodynamics and entropy 1/3

» 3.1 Thermodynamics and entropy 2/3

3.1 Thermodynamics and entropy 3/3

3.2 Equilibrium and irreversibility 1/2

3.2 Equilibrium and irreversibility 2/2

3.3 Statistical mechanics 1/2

3.3 Statistical mechanics 2/2

4 The intangible Universe

5 The uncertain Universe

6 Closing items

--------------------

Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

3 The irreversible Universe

3.1 Thermodynamics and entropy

Part 1 of 3 | Part 2 | Part 3

In general, the total energy gained by a system (such as the plate) is the sum of the heat and the
work transferred to it. It is worth emphasizing that heat and work are not themselves properties of a system. We cannot examine a plate and deduce that it has received so much energy from heat and so much energy from work. All that counts is that the plate has a total amount of energy, and that any increase in this energy is the sum of the heat and work transferred to the plate. This understanding of heat, work and energy is incorporated in the first law of thermodynamics.
 First law of thermodynamicsWhen all types of energy transfer, including work and heat, are taken into account, the energy of an isolated system remains constant
From a modern perspective, we can see that this is just another way of stating the law of conservation of energy with the explicit recognition of heat as a quantity of energy to be included, alongside work, in any energy audit. Inventors should take note: an engine may convert energy from one form to another, but it cannot produce energy from nothing. The kinetic energy of the piston of a steam engine, for instance, has been paid for in advance by the heat transferred to the steam.

 One of the other books in the Physical World series, Classical Physics of Matter contains an extensive discussion of thermodynamics

Given this modern understanding of heat as energy transferred in a particular way, you might wonder why we bother to distinguish between heat and work at all. The reason is that heat can be used to define another important quantity: entropy.

We cannot define entropy properly in this introductory survey. In very broad terms you can think of entropy as a measure of 'disorder' - the random motion of molecules in steam corresponds to more disorder, and hence more entropy, than the more orderly motion of molecules in ice. Interestingly enough, there is a connection between entropy and heat: whenever heat is transferred to a body, the entropy of that body increases. In the simplest case, if a small amount of heat Q is transferred gently to a body, whilst the temperature of the body is T, the entropy of the body increases by Q/T.

The term entropy was deliberately chosen to be reminiscent of energy, though the differences between the two quantities are just as important as their similarities. Entropy and energy are similar in that an isolated body may be said to have a certain 'entropy content' just as it may be said to have a certain 'energy content'. However, while the first law of thermodynamics ensures that the energy of an isolated system is always conserved, the second law of thermodynamics makes a slightly weaker assertion about entropy:
 Second law of thermodynamicsThe total entropy of an isolated system cannot decrease; it may (and generally does) increase
The requirement that the total entropy should not decrease has the effect of ruling out enormous numbers of processes that are perfectly consistent with energy conservation.
Continue on to Thermodynamics and entropy, part 2 of 3

Some highlights of physics

Featured Physicists