What Does Enthalpy Represent in Thermodynamics?

In summary: P\,\int_{R_-}\,dV\ -\ P\,\int_{R_+}\,dVwhich is just the energy of the displaced mass, and doesn't depend on the surface S at all.
  • #1
ahmedbadr
29
0
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 what's 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 can't say there's work in the same state ?so i can't 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 can't get what critical point has to do with this because basically i don't get exactly physical meaning of critical point)?
 
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  • #2
ahmedbadr said:
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 what's 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 can't say there's work in the same state ?so i can't inteprete what's the meaning of h or what's the difference between u & h?

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.

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

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).

ahmedbadr said:
(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 can't get what critical point has to do with this because basically i don't get exactly physical meaning of 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
work done

ahmedbadr said:
… 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 can't say there's work in the same state ?

Hi ahmedbadr! :smile:

I'll just add:

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 [itex]P[/itex] 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:

[tex]\int_S\,P\,\bold{x}\cdot\hat{\bold{n}}\,dA[/tex]

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+:

[tex]\int_{R_-}\,P\,dV\ -\ \int_{R_+}\,P\,dV[/tex]
 

FAQ: What Does Enthalpy Represent in Thermodynamics?

What is thermodynamics?

Thermodynamics is a branch of physics that deals with the study of energy and its transformation from one form to another. It also involves the relationships between heat, work, temperature, and energy.

What are the laws of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, but can only be transferred or converted from one form to another. The second law states that in any energy conversion, some energy will be lost as heat. The third law states that as temperature approaches absolute zero, the entropy of a pure perfect crystal will approach zero.

What is the difference between heat and temperature?

Heat is a form of energy that is transferred from a higher temperature object to a lower temperature object. Temperature, on the other hand, is a measure of the average kinetic energy of particles in a substance. In simpler terms, heat is the transfer of energy, while temperature is a measure of the amount of energy in a substance.

What is the significance of entropy in thermodynamics?

Entropy is a measure of the disorder or randomness of a system. In thermodynamics, it is related to the second law which states that the total entropy of a closed system can never decrease. This means that in any energy conversion process, some energy will always be lost as heat, increasing the entropy of the system.

How is thermodynamics used in real life?

Thermodynamics is used in many real-life applications, such as in the design of engines, refrigerators, and power plants. It is also used in environmental studies to understand energy transfer in natural systems. Additionally, thermodynamics plays a crucial role in the development of new materials and technologies for energy conversion and storage.

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