# Explanation of entropy for a beginner please!

by kate84
Tags: beginner, entropy, explanation
 P: 1 Hello, I am looking for some clarity on the second law of thermodynamics. I am an amateur physics student and only just beginning and so my understanding is currently very basic! I have watched Brian Cox's Wonders of the Universe, where he talks about the second law of thermodynamics being that entropy always increases. My understanding of entropy is that a low entropy state has a lot more potential for decay and disorder and a high entropy state is one that is already disordered and not much can be done to alter it/ there is not much potential for decay. (Cox uses the example of a sandcastle being of low entropy, ie easily disordered, and a pile of sand being of high entropy, ie whatever you do to it does not really alter it.) If entropy is always increasing that would seem to suggest (to my understanding!) that the universe is moving from a more ordered to a more disordered state. This is obviously not the case though, as of course highly ordered and complex systems such as our own bodies have formed from stardust. While it's true on a smaller timescale that of course our bodies age and decay as time progresses, the formation of life itself is surely an example of something going from a disordered to an ordered state as time progresses. Therefore, I can be certain that I am missing something vital from my understanding of what entropy is and what the second law of thermodynamics really means! Please could someone explain what I am missing!! Thanks :)
 P: 683 Hello, kate84. Welcome to PF! The thing you're missing is the Sun. You can say a system always has to go from lower entropy to higher entropy ONLY if the system is closed. If you supply energy from outside, you can make all sorts of things with low entropy. But if you include the source of energy in your balance, you'll inevitably notice that the TOTAL entropy goes up. In other words, the 2nd law of thermodynamics describes closed systems only. It makes no sense to use it with non-closed systems. A better definition of entropy than the level of 'complexity' is the inverse of the ability to do work. The higher the entropy in the system, the less useful work can be extracted. E.g., a plant has lower entropy than its component molecules - you can eat it or burn it to get useful energy out of it. But the amount of solar energy that was 'wasted' on producing one is higher than the amount you can extract from the finished product.
Emeritus