# Basics of thermodynamics

1. Sep 17, 2008

well, i've some questions abt basics of thermodynamics
(1)what is the meaning of enthalpy(h) i know h=u+pv but i don't know what it means, i know what internal energy means but i don't know whats the difference between internal energy and enthalpy,what's the meaning of quantity (pv) what does it indicate for? i know p is pressure and v is volume nut my question how can it be an energy? as i think it doesn't represent work because h is property for state and u cant say there's work in the same state ?so i cant inteprete what's the meaning of h or what's the difference between u & h?

(2)2nd question abt diagrams of pure substances (t-v diagram, p-v diagram) what is the meaning of critical point in these diagrams?

(3)3rd question abt ideal gases,my question here is when can i consider the gas is ideal? i can consider it the gas behaves an ideal gas when its density is low?or when its pressure is low and its temperature is high relative to its critical point(indeed cant get what critical point has to do with this because basically i dont get exactly physical meaning of critical point)?

2. Sep 17, 2008

### stewartcs

Enthalpy is a term of convenience. It conveniently groups two frequently used terms into one term. The Pv term is the flow work. The difference between the internal energy and enthalpy is just the addition of the flow work.

The critical point is the point at which the phase transition is no longer discrete (i.e. it could be a compressed liquid or superheated vapor beyond the critical point).

A gas will behave as an ideal gas if one of the two conditions are met:

1) Pr < 10 and Tr > 2, or P < 10Pcr and T > 2Tcr
2) Pr << 1 or P << Pcr

CS

3. Sep 17, 2008

### tiny-tim

work done

pressure = force/area,

so pressure x volume = force x distance = work done …

to be precise:

Pressure = work done per displaced volume:

Imagine a particular mass of fluid, occupying a region R, whose surface is S. After a short time, it will occupy a slightly different region R'.

R and R' will mostly overlap, but there will be a region R- at the back of R which has been vacated by the mass, and a new region R+ at the front, into which the mass has moved and displaced other fluid.

The work done by the pressure $P$ on the whole mass is the surface-integral, over every part of the surface, of pressure times area "dot" the distance through which that part has moved:

$$\int_S\,P\,\bold{x}\cdot\hat{\bold{n}}\,dA$$

which, since the pressure always acts inward, and therefore acts against the displacement on R+, is simply the volume-integral of the pressure over R- minus its volume-integral over R+:

$$\int_{R_-}\,P\,dV\ -\ \int_{R_+}\,P\,dV$$