Difference between enthelpy and internal energy

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Discussion Overview

The discussion centers around the differences between enthalpy and internal energy, exploring their definitions, relationships, and implications in thermodynamics. Participants seek clarification on these concepts, which are fundamental in the study of energy systems.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that internal energy is the average of the kinetic energy of the particles in a system, while others elaborate that it is energy not accounted for by position, velocity, or mass, relating it to heat energy.
  • One participant states that the difference between enthalpy and internal energy is represented by the term PV.
  • Another participant explains that enthalpy is defined as H = U + PV, indicating that it encompasses both internal energy and the work energy associated with pressure and volume.
  • There is a discussion about the nature of internal energy as an extensive property, with some participants questioning the inclusion of mass in its definition.
  • One participant emphasizes that internal energy is an extensive property, suggesting that doubling the mass would lead to a doubling of internal energy, assuming other conditions remain constant.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of internal energy and enthalpy, particularly regarding the role of mass in internal energy. There is no consensus reached on these points, and multiple competing views remain.

Contextual Notes

Some statements made by participants rely on specific definitions and assumptions that may not be universally accepted, such as the relationship between internal energy and mass. The discussion also reflects varying interpretations of thermodynamic concepts.

Taturana
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What's the difference between enthalpy and internal energy?

Internal energy is the average of the kinetic energy (linear and angular) of the particles of the body (or system), right?

Could someone explain me this clearly?

Thank you
 
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Taturana said:
What's the difference between enthalpy and internal energy?
The difference is P V.
 
Taturana said:
What's the difference between enthalpy and internal energy?

Internal energy is the average of the kinetic energy (linear and angular) of the particles of the body (or system), right?

Could someone explain me this clearly?

Thank you

That's a good question. First, consider the total energy E of a body or system:

E = KE + PE + U

Where KE is kinetic energy, PE potential energy, and U the 'internal energy'. Internal energy is energy that cannot be accounted for by specifying position, velocity, or mass. It's not a mechanical form of energy, it's (essentially) heat energy. It's related to temperature as well.

Enthalpy H = U + PV.

If U is the heat energy, PV is the 'work' energy. Enthalpy is also the total amount of energy available in a system or body, like E, but written using thermodynamic variables instead of mechanical variables. Changes in enthalpy occur during a thermodynamic process involving the conversion of heat into work (or vice-versa).
 
Andy Resnick said:
That's a good question. First, consider the total energy E of a body or system:

E = KE + PE + U

Where KE is kinetic energy, PE potential energy, and U the 'internal energy'. Internal energy is energy that cannot be accounted for by specifying position, velocity, or mass. It's not a mechanical form of energy, it's (essentially) heat energy. It's related to temperature as well.

Enthalpy H = U + PV.

If U is the heat energy, PV is the 'work' energy. Enthalpy is also the total amount of energy available in a system or body, like E, but written using thermodynamic variables instead of mechanical variables. Changes in enthalpy occur during a thermodynamic process involving the conversion of heat into work (or vice-versa).

Thank you, now it's very clear.
 
internal energy is energy that cannot be accounted for by specifying position, velocity, or mass.

I think this is a very good quote, if you exclude the mass.

Andy, are you sure you want to include mass, isn't internal energy an extensive property, ie twice the mass means twice the internal energy, all other things being equal?
 
Studiot said:
I think this is a very good quote, if you exclude the mass.

Andy, are you sure you want to include mass, isn't internal energy an extensive property, ie twice the mass means twice the internal energy, all other things being equal?

'mass' goes to the kinetic (e.g. 1/2 mv^2) and potential (e.g. mgh) energies. But yes, internal energy is an extensive property (as is enthalpy).
 
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