Joule's Law and Enthalpy - Thermodynamics:

Joule's Law

Joules law experiment is based on the constant volume process, and it state that I.E. of perfect gas is a function of absolute temperature only.That is, 

U = f(T)

dU = dQ - dW; It define constant volume that is
dw = 0 dU = dQ; but dQ = mC_vdT, at constant volume

dU = mC_vdT; for a perfect gas

Enthalpy

It is sum of I.E. (U) and pressure - volume product.

H + pv

For unit mass pv =RT

  h = C_VT + RT = (C_V  + R)T = C_PT = (dQ)_P

  H = mC_PT

  dH = mC_PdT