# Entropy - Physical Meaning

## Main Question or Discussion Point

Hello.

I know that entropy equals the Boltzmann's constant times the natural log of the multiplicity but I do not know the physical interpretation of what "entropy" really is. I understand the units of it are "Joules per Kelvin" but what does that really mean?

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TimNguyen said:
Hello.

I know that entropy equals the Boltzmann's constant times the natural log of the multiplicity but I do not know the physical interpretation of what "entropy" really is. I understand the units of it are "Joules per Kelvin" but what does that really mean?
The most simple and direct interpretation

$$U = TS$$

entropy is a "measure" of the increase of internal energy with temperature or

$$S = \frac{U}{T}$$

If two bodies have the same temperature but one has more energy, then it has more entropy.

If two bodies have the same energy but one has less temperature, then it has more entropy.

Last edited:
ZapperZ
Staff Emeritus
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TimNguyen said:
Hello.

I know that entropy equals the Boltzmann's constant times the natural log of the multiplicity but I do not know the physical interpretation of what "entropy" really is. I understand the units of it are "Joules per Kelvin" but what does that really mean?
Try this:

http://www.entropysite.com/

It has a good collection of basic articles, especially in dispelling the notion that entropy is nothing more than "disorder".

Zz.

ZapperZ said:
Try this:

http://www.entropysite.com/

It has a good collection of basic articles, especially in dispelling the notion that entropy is nothing more than "disorder".

Zz.

Effectively, the old (archaic) idea of entropy like disorder is not justified and would be abandoned of literature.

Moreover, the standard formula $$S = k \ ln W$$ often invoked in that old interpretation is valid only in the very special case of an isolated system at equilibrium.

That is reason that one would reasoning what is entropy from above formulas.

Hello.

I know that entropy equals the Boltzmann's constant times the natural log of the multiplicity but I do not know the physical interpretation of what "entropy" really is. I understand the units of it are "Joules per Kelvin" but what does that really mean?
ans:
it realy means how quickly the energy is spreding in medium.