Why can't internal energy be calculated at a particular state?

[LEELA PRATHAP KUMAR]

Internal energy at a specific state can't be calculated but by kinetic theory of gases and law of equipartition of energy Average kinetic energy is directly proportional to temperature.And for an ideal gas internal energy is due to kinetic energy only for an ideal gas potential energy can be neglected It means internal energy is a function of temperature only. It gives equation
KE=(3/2)RT it is for monoatomic gas (from kinetic theory of gases)
U=(f/2)RT from law of equipartition of energy
Here f is no. Of degrees of freedom
so why can't we find internal energy at a particular state?

 

[mfb]

Internal energy at a specific state can't be calculated

Why do you think that?

 

[LEELA PRATHAP KUMAR]

Why do you think that?

Because what we calculate the most is change in internal energy but not internal energy at a specific state

 

[Chestermiller]

Are you asking why one can't determine the absolute internal energy of a material?

 

[mfb]

Because what we calculate the most is change in internal energy but not internal energy at a specific state

The total energy does not matter (unless you consider general relativity). You can calculate it, but there is no point in it.

 

[LEELA PRATHAP KUMAR]

Are you asking why one can't determine the absolute internal energy of a material?

Yes
And we always talk only change in internal energy
But U=(f/2)RT from law of equipartition of energy so why can't we determine absolute internal energy

 

[LEELA PRATHAP KUMAR]

The total energy does not matter (unless you consider general relativity). You can calculate it, but there is no point in it.

Can't we calculate internal energy at a state by using law of equipartition of energy U=(f/2)RT

 

[mfb]

Can't we calculate internal energy at a state by using law of equipartition of energy U=(f/2)RT

That is the internal energy relative to some reference point which gets defined as 0 internal energy.

 

[LEELA PRATHAP KUMAR]

That is the internal energy relative to some reference point which gets defined as 0 internal energy.

What is that reference point ?

 

[mfb]

U=0 corresponds to T=0 in your equation.

 

[Chestermiller]

Leela:

Does your equation for internal energy include chemical bond energies of polyatomic molecules? How about latent heat effects of fusion and vaporization? What about the effect of temperature on heat capacity?

 

[LEELA PRATHAP KUMAR]

U=0 corresponds to T=0 in your equation.

But in kinetic theory of gases we took absolute terms only like temperature , pressure
Where this reference point is considered in kinetic theory of gases and law of equipartition of energy

 

[LEELA PRATHAP KUMAR]

Leela:

Does your equation for internal energy include chemical bond energies of polyatomic molecules? How about latent heat effects of fusion and vaporization? What about the effect of temperature on heat capacity?

But in kinetic theory of gases we don't consider molecular interactions

 

[Chestermiller]

But in kinetic theory of gases we don't consider molecular interactions

But that is a feature of real gases.

 

[LEELA PRATHAP KUMAR]

But that is a feature of real gases.

Yes but kinetic theory of gases strictly defined for ideal gases
And internal energy for real gases are different from ideal gases.I mean it is not only a function of temperature but with some variables like volume ,entropy

 

[Chestermiller]

Yes but kinetic theory of gases strictly defined for ideal gases
And internal energy for real gases are different from ideal gases.I mean it is not only a function of temperature but with some variables like volume ,entropy

Yes. So? What exactly are you asking? As a PF expert on thermo, I'm having trouble understanding your issue. Can you please restate it more precisely?