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How and why does external magnetic field/electric effect the internal energy of a sub

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gursimran
#1
Jun15-11, 10:07 AM
P: 36
In magneto caloric effect, we see that when a substance in an adiabetic closure is exposed to external magnetic field, its temperature increases. But the internal energy of the substance has to be constant so this implies decrease in internal potential energy. However the application of magnetic field can only directly change the external potential energy of the substance as a whole. How it can affect the internal energy of the substance??

For example like gravitational field cant effect the internal energy of the substance..
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DrDu
#2
Jun15-11, 10:51 AM
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P: 3,593
Even the gravitational field can affect the internal energy of a sample although the gravitational field of earth is quite weak to make this effect observable: Think about an container of fixed volume containing water vapour just above its condensation temperature. If you bring this container at in a strong gravitational field while keeping its temperature constant, it will be energetically more favorable for water to condense and collect as a liquid at the bottom of the container where its potential energy is lower. In that process, heat will be released, just like in the magnetocaloric effect.
gursimran
#3
Jun15-11, 11:25 AM
P: 36
Quote Quote by DrDu View Post
Even the gravitational field can affect the internal energy of a sample although the gravitational field of earth is quite weak to make this effect observable: Think about an container of fixed volume containing water vapour just above its condensation temperature. If you bring this container at in a strong gravitational field while keeping its temperature constant, it will be energetically more favorable for water to condense and collect as a liquid at the bottom of the container where its potential energy is
lower. In that process, heat will be released, just like in the magnetocaloric effect.
Thanks for answer but i'm having a hard time understanding this all..

So can I generalize that all force fields can effect the internal energy of the substance (it can affect the amount in which it is distributed in potential and kinetic energy) but it in itself does not contribute to internal energy. eg - if I turn on the magnetic field/ electric does the internal energy(KE+PE) of the substance changes in amount??

Secondly, how does it effect. If applying the magnetic field alligns all the dipoles in the dierction of magnetic field. How that effect the internal energy distribution of the gas. Please justify.

Thanks in advance. ...

gursimran
#4
Jun22-11, 08:37 PM
P: 36
How and why does external magnetic field/electric effect the internal energy of a sub

I was reading a research paper on magetocalorific effect. It says magnetic component of entropy but entropy being a state function should not get affected by the external fields. Just like it does not get affected by gravitational and electric field. Also can't magneto calorific effect be explained in terms of internal energy changes. For example if we adiabetically magnetise a material the temperature should rise. It means thermal agitation has increased and because dq=0 so KE should have converted to internal potentail energy. But how is internal potentail energy affected by the application of magnetic field?

Is there a good book which explains the basic theory of magnetocaloric effect with rigor?
DrDu
#5
Jun23-11, 01:22 AM
Sci Advisor
P: 3,593
Why do you open a new thread? You asked essentially the same question before. I also showed you that a gravitational field can produce the same effects as a magnetic field.
So why do you say it doesn't?
Entropy is a state function, but the state depends on the magnetic field just like it depends on e.g. volume. When you magnetize e.g. a paramagnetic material, you create a preferred orientation for the magnetic dipoles. When the dipoles relax into these orientation, their entropy decreases, the heat capacity of the dipoles decreases, and they release energy which is taken up by the lattice vibration and the lattice temperature increases. This effect may be strongly enhanced near a ferromagnetic phase transition were you observe the giant magnetocaloric effect.
gursimran
#6
Jun23-11, 05:47 AM
P: 36
Quote Quote by DrDu View Post
Why do you open a new thread? You asked essentially the same question before. I also showed you that a gravitational field can produce the same effects as a magnetic field.
So why do you say it doesn't?
Entropy is a state function, but the state depends on the magnetic field just like it depends on e.g. volume. When you magnetize e.g. a paramagnetic material, you create a preferred orientation for the magnetic dipoles. When the dipoles relax into these orientation, their entropy decreases, the heat capacity of the dipoles decreases, and they release energy which is taken up by the lattice vibration and the lattice temperature increases. This effect may be strongly enhanced near a ferromagnetic phase transition were you observe the giant magnetocaloric effect.
oh I apologize for that. Ya you wont believe but its true that I almost forget that I had already asked this question before. Still I dont remember what were the replies. Thanks anyway for replying. Could you suggest me some books on this to get out of this dilemma or confusion..
gursimran
#7
Jun23-11, 06:01 AM
P: 36
Quote Quote by gursimran View Post
Thanks for answer but i'm having a hard time understanding this all..

So can I generalize that all force fields can effect the internal energy of the substance (it can affect the amount in which it is distributed in potential and kinetic energy) but it in itself does not contribute to internal energy. eg - if I turn on the magnetic field/ electric does the internal energy(KE+PE) of the substance changes in amount??

Secondly, how does it effect. If applying the magnetic field alligns all the dipoles in the dierction of magnetic field. How that effect the internal energy distribution of the gas. Please justify.

Thanks in advance. ...
oh here it is. I was not convinced. If you say that external gravitational field can affect the internal energy. Then so can psuedo gravitational field due to gas in a accerlating frame. But I have read it essentially many times that systematic motion of a gas as a whole cant change the internal energy of a gas. You can see that in wikipedia as well.

You can read this " It is the energy needed to create the system, but excludes the energy to displace the system's surroundings, any energy associated with a move as a whole, or due to external force fields." Taken from http://en.wikipedia.org/wiki/Internal_energy
ZapperZ
#8
Jun23-11, 06:39 AM
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Two different threads have been merged.

Zz.


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