Question

1. a) Consider our specific latent heat equations: L_v = H_v - H_w = U_v - U_w + p_wv(V_v - V_w)

L_f = H_w - H_i = U_w - U_i + p_wi(V_w - V_i)

L_s = H_v - H_i = U_v - U_i + p_vi(V_v - V_i) From these we can see that the latent heats of all three phase changes are functions of temperature. But we assume them to be constant in meteorology. Why? (Write at least one paragraph for each)

Answer 1

Latent heat is the thermal energy (heat) released or absorbed , by a body during a constant-temperature( the body temperature only, it is not the temperature of the surroundings) (remember heat and temperature are two different things).

Latent heat is a intensive property. ie, it is a material property and not depending up on the size or extent of the sample.

Latent heat (for vaporization, fusion or sublimation) can generally written as,

$L=\Delta H$ =change in enthalpy= $\Delta u+p\Delta v$    = $U_{2}-U_{1}+P(V_{2}-V_{1})$

In the equation pressure is a constant factor. When pressure changes latent heats also changes its value.

In meteorology, the atmosphere pressure is assumed to be 1bar. The specific latent heat values of different bodies usually we see in literature is at pressure 1bar.