# What is entropy?

1. Jul 28, 2004

### lakshmi

what is entropy

2. Jul 28, 2004

entropy is the measure of Disorder in a system. Lets say that you have a deck of cards, when you hold these cards in your hand, the entropy is n, if you throw these cards in the air and let them flutter to the ground, then the entropy will drastically increase (entropy>>n) because the disorder has increased. In chemistry of any day life, when a substance goes from a solid to liquid, entropy increases, or from a liquid to gas.

3. Jul 28, 2004

### maverick280857

Nenad has given you the physical picture of entropy. Mathematically it is a function defined as

$$S = \int \frac{dQ_{rev}}{T}$$

where $$dQ_{rev}$$ is the (infinitesimal) heat energy change associated with a reversible process and T is the temperature. Of course you don't need all this math right now if you're beginning to understand entropy but thats just how its defined.

Think of it as follows: something that is associated with greater disorder has greater entropy. Entropy is a measure of the disorder of a system. If you know the entropy change, you know how the disorder has changed in the system. Pack of cards is a good example to do that.

Secondly, the Ludwig Boltzmann equation states that

$$S = k\ln w$$

where k is a constant called the Boltzmann Constant and w is the partition function...it is roughly the number of "microstates" available for redistribution of energy. If the number of microstates available for energy distribution is large, the entropy is large. You can easily see why the entropy increases during a change of state from solid to liquid to gas. Atoms are restricted from moving much in solid state, less restricted in liquids and least restricted in gases. So the number of microstates available for energy distribution is large in case of gases, less in case of liquids and least in case of solids.

Hope that helps...

Cheers
Vivek

Last edited: Jul 28, 2004