I like very much to consider myself an educated person who is able to talk about any cultural subject, like some kind of 19th century gentleman -with the gentleman component omitted of course as I was born in a suburb to a working class family and I need to work to earn my daily bread but you get the reference.

I was debating about the meaning of culture the other day -dangerous subject- when I was asked the question that, according to C P Snow, separates an educated person from his neolithic ancestors:

– What is the second law of thermodynamics?

I started to sweat and to feel butterflies in my stomach; come on Ariel you know this! You studied physics at university, didn’t you? For Christ sake you are not only educated you are a scientist!

I stopped to think for a few seconds and then, I mumbled:

– Is it the entropy thing?

It was the entropy thing, big relief. But I felt that I had lost face: it took me a few seconds to replay and my answer came out as a question.

Many of us are people of the educated type. Perhaps we left Uni a long time ago though, and our memory is blurry. To avoid you losing face in an intellectual debate like I did, I put together this very brief post:

An easy way to remember the laws of thermodynamics according to C. P. Snow himself:

Law 1: You can’t win in the game (energy cannot be created from nothing).

Law 2: You can’t break even in the game (The universe always claims a bit of energy for itself: entropy).

Law 3: You can’t quit the game (absolute zero temperature –where entropy is zero- is not attainable).

If you are the geeky type you can continue reading, but that was enough toss a quick answer to your opponent and to swiftly move on to softer issues.

As I like the idea of the 19th century gentleman let’s continue in a way that a 19th century mechanical engineer would put it:


First law:

ΔW = ΔE – ΔQ              ΔW is, for example, the work a machine does.

ΔE is the energy in the machine (let’s say the fuel).

ΔQ is the heat (in a steam machine) absorbed by the machine.


As you can see you can produce only so much work as energy you put in.


Second law:

This is the entropy thing: the universe as a whole is inevitably drifting to a state of disorder. Entropy is a measure of that disorder and therefore:

ΔS ≥ 0                         ΔS is the change in the entropy of the universe.


There are many equations that express entropy in a mathematical way; we are using a 19th century style:


ΔS = ΔQ / T                 T is the absolute temperature (in Kelvin’s degrees)



ΔW = ΔE – T ΔS


The work you produce is always less than the energy you put in. Entropy is like the income tax you have to pay the universe for working!


Third law:


At T=0 S=0

All very well but:


“It is impossible by any procedure, no matter how idealized, to reduce the temperature of any system to zero temperature in a finite number of finite operations”.


The explanation for this escapes the mathematical skills of a biologist but my physicist friends tell me it is true. Anyway, the question is always about the second law…

That’s it. Now you are ready not only to answer the question “what is the second law of thermodynamics?” but you can ask the question to someone that accuses you of being a brute for not having read Shakespeare.




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