# Thermodynamics: Entropy and specific heat

#### Pandris

Hi here!

(before scriptum. Sorry for my lousy English and LaTex.)

I am a bit confused about entropy change statements:

In most textbooks is given:
$$\Delta S = n (C_V ln \frac{ \ T_{2}}{T_1} + R ln \frac{ \ V_{2}}{V_1})$$

where n - moles of gas. And dimension of entropy is [J/K]

But now I have one book there entropy change is defined using specific heats:

$$\Delta S = m (c_v ln \frac{ \ p_{2}}{p_{1}} + c_p ln \frac{ \ V_{2}}{V_{1}})$$

where m - mass of gass, and dimension is [J/(kmol*K)]

I can't understand where this isobaric specific heat gets here!

Ok, I understand that these entropies essentially are given for different measures of amount of gass. First is given for moles, but second for kilograms. (am I right??)

But how to involve specific heats there is obscure for me.

Thanks!

Last edited:

#### Pandris

Oh my!

Am I blind or what?!

Everything works out if take in mind that
$$C_p - C_V = R$$
and

$$c_v = \frac { \ C_{V}}{ \mu}$$

That's it! Sorry for buzzer!

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving