Integrating a differential (Problem in thermodynamic derivation)

[Runei]

Hi,

I'm looking at a derivation of the thermodynamics of black-body radiation. My question is in regards to the math of the derivation.

Using the first law of thermodynamics and considering an adiabatic expansion of the cavity, it can be stated that

dU = -\frac{u}{3}dV

Here small u is the volumetric density of the internal energy U/V.

Small u can also be described as

u = \int_{0}^{\infty}u_fdf

Here u_f is the distribution function for how the internal energy density is distributed amongst the various frequencies (wheew!).

The steps where I'm confused goes like this:

u = \frac{U}{V} = \int_{0}^{\infty}u_fdf \Longleftrightarrow
U = \int_{0}^{\infty}Vu_fdf \Longrightarrow
dU = \int_{0}^{\infty}d(Vu_f)df

My confusion comes from the last step. I can't see how the differential of U is the same as the integral of the differential of Vu_f with respect to the frequency f.

Can anybody explain the math behind this? It's quite possible that it's just a small thing I just can't see :D But any help is most appreciated :)

Thank you.

 

[Runei]

Well I figured it out now, and as I thought, it was quite simple.

Since

U = \int_{0}^{\infty}Vu_fdf = \int_{0}^{\infty}Fdx

F is now just equal to Vu_f. I just made a change of variable names to make it more simple.

dU = Fdx

dF = fdx

\int_{0}^{\infty}dFdx = \int_{0}^{\infty}f dx dx = F dx = dU

This corresponds to the problem I was asking.