Specific heat and petit's law

1. Mar 18, 2012

thentangler

Hi there,
I have a quick question. According to duLong and petit's law, the heat capacity at constant volume for most solids at high temperatures is 3R. (I.e) Cv=3R.
Where R is the gas constant. Does this mean that the heat capacity is the same for all materials?
I thought each material at a unique heat capacity. Can somebody explain?
Thank You

2. Mar 18, 2012

Jano L.

Hello thentangler,
For solids in high enough temperatures above the Debye temperature, the heat capacaity is 3R per mole.

Check the table of temperatures here

http://en.wikipedia.org/wiki/Debye_model#Debye_versus_Einstein

For long organic molecules, the heat capacity can be much larger than 3R per mole, because they have many active degrees of freedom.

3. Mar 18, 2012

Staff: Mentor

Note: it is specific heat capacity per mole that is (almost) identical.

4. Mar 18, 2012

thentangler

So suppose I encounter a material that is a classical solid, would it be safe to substitute 3R for its specific heat when im calculating its Q per mole?

5. Mar 19, 2012

Staff: Mentor

Define "safe". Dulongâ€“Petit law is an approximation. Sometimes it works, sometimes it doesn't. If you can't find specific heat capacity and you use 3R you should always spell it out, as it is possible you are way off.

6. Mar 19, 2012

thentangler

Aahh, thank you Borek. The limitations of classical physics I suppose. :)