Question: What are the Laws of Thermodynamics?

Bruce Alexander answered on 15 Jun 2012:

Technical answer…

The first law says that energy can be transformed between one form to another, but can neither be created nor destroyed. You burn coal to transfer the chemical energy in coal into heat energy, for example.

The second law says that entropy always increases. It is really difficult to easily describe entropy properly in a manner that people understand, partly because many scientists don’t understand it themselves! Some say that entropy means disorder and say that as time goes on, the amount of disorder increases. So, for example, in salt, all of the sodium and chloride atoms are arranged in a nice, ordered way and the look like a cube (which is why you get little cubes of salt crystals if you look at them closely). This is a highly ordered system. As you dissolve the salt, the atoms can move around more freely in water so the disorder increases.

Actually, this description of entropy is not strictly accurate (but it kind of works until someone asks how salt crystals form in the first place!). It is more correct to think of entropy as being the penalty you have to pay for converting one form of energy into another (back to the first law). As you convert one form of energy into another, you increase entropy as the transformation of heat, say, into movement, is never 100% efficient – you always lose something – this is entropy increasing.

The third law says that entropy stops changing if you cool a system down to absolute zero. Absolute zero, roughly -273°C, is the lowest possible temperature that we can ever physically reach. At this temperature, all atoms will stop moving (and remember that movement is an indication of entropy). So…if we could cool something down to -273°C, then motion stops and entropy is zero. But we know from the second law that entropy always increases, which means that we can never actually reach absolute zero. The closest we have ever come is just a fraction of a degree above it. Where was this achieved? In a research lab in the middle of the desert in New Mexico!!

So…non technical summary?

You cannot create energy from nothing, the best you can do is convert one form into another.
Even if you convert energy from one form into another, you are going to lose some of it as you cannot convert everything efficiently.
The only way you could convert it all, would be at absolute zero, but this is impossible to reach, so don’t even try.

How long do you have? Explaining the laws of thermodynamics in 30 minutes and explaining the laws of cricket to the Swiss are equally challenging!