Previously: The Zeroth Law, The First Law.

Today: The Second Law!

To truly understand the import of the Second Law you first need to be aware of a somewhat abstract quantity called Entropy. Entropy can be as simple or as complicated as you want to make it.

The most accessible way to think about Entropy is to consider it as a measurement of disorder: the higher the Entropy of a system the more disordered it is. This implies that Entropy is also a measure of how much useful energy a system contains, and so another way of interpreting Entropy is to say: “when the entropy of a system is at a minimum then the useful energy is at a maximum, and vice versa”.

There are many versions of the Second Law, though they all mean the same thing in the end. The most easily accessible version is this:

Any process that occurs in a system increases the overall Entropy of the Universe.

If the entropy is increasing then this means that the level of disorder is increasing and that there is less useful energy available. The second law is therefore also stating that energy tends to disperse over time, introducing the idea of equilibrium, which is best seen in another version of the second law:

Heat cannot spontaneously flow from a material at lower temperature to a material at higher temperature.

We are intuitively familiar with this statement, which is essentially saying that hot things cool down while cold things don’t become hot by themselves.

It is, of course, possible to force heat to flow from cold to hot (as is done in the case of fridges, for example), but this is only possible when work is being done. The actual object cooling down is experiencing a decrease in entropy, but the universe is still experiencing an increase (the electrical energy that powers the fridge is being converted to heat which is dumped out the back… an increase in entropy).

Finally, the third popular way of phrasing the second law is a one concerning efficiency:

It is impossible to convert heat completely into work.

Think back to the first statement above, if all the heat available in a system is converted to nothing but useful work then the overall Entropy will not increase (there would have been no loss of useful energy and no increase in disorder). This is the definition of an efficiency of 100%, in a heat engine for example (though the principle applies to all processes), which, as you can see, is impossible.

The Second Law demands that some energy is ‘lost’ to an increase in overall Entropy, and so there cannot be a 100% conversion to work. Alas.

What does it all mean?

Though there are so many seemingly different versions of the Second Law, they all mean exactly the same thing and each statement can be used to arrive at any of the others.

The key points to this law are:

• There can be no such thing as 100% efficiency!
• Eventually all the useful energy in the Universe will have been used up!