Thursday, 1 March 2007

How to ... disobey the second law of thermodynamics

OK well this post is very late, it was due up a week or so ago, but things have been hectic and since my laptop died working from home has become impossible, but I am endeavouring to catch my self up with these posts

Well the second law of thermodynamics (SLoT) gets quite a bum rap from all sides of the anti-science. For starters there is perpetual motion, infinite amounts of free energy, but the is also a horrible use of the SLoT as an argument about evolution.

One reason I suspect as to why SLoT is so mis-understood is that while it conveys a fairly simple physical law, when it is applied to different situations it tends to need to be stated in very different ways. To quote P.W. Bridgman

There are almost as many formulations of the second law as there have been discussions of it.
For example:

The entropy of an isolated system not in equilibrium will tend to increase over time, approaching a maximum value at equilibrium.

Heat cannot of itself pass from a colder to a hotter body.

A transformation whose only final result is to convert heat, extracted from a source at constant temperature, into work, is impossible.

The first two statements are from Rudolf Clausius, and the last one from Lord Kelvin. And they are all equivalent, but generally phrased to high-light what the law means with reference to a particular situation. For example Lord Kelvin's statement above is a very mechanical one, essential stating that while we can use heat to do work we can never covert all the heat to work, so there is always some energy lost as heat.

The other difficulty with SLoT is that it is a statistical law, it is based on probability. Essential every system is in a certain state (macro-state) that is comprised of all the states of the atoms (micro-states) that make up the system. The more atom states that correspond to a particular system state then the more likely that system state is to exist. Naturally there are many more disordered (macro-) states than ordered (macro-) states, so systems will tend to wards disorder (an increase in entropy).

The big caveat to this statement is that all this applies only if you do not start adding energy to the system. In other words the law only applies to isolated systems.

So the Earth is not an isolated system, it receives a lot of energy from the sun. So the Earth and everything on it does not obey the SLoT, however if we take the Universe to be our system then this is clear isolated (it is all there is by definition) then the SLoT is obeyed.

And our magnetically levitated spinning top in the foyer of the lecture theatre next to the Physics department here will only spin perpetually if the power is plugged in other wise the interaction of the magnetic fields (and some air resistance) will cause to stop spinning and fall.