Jump to content
Matthew99

Thought experiment about entropy

Recommended Posts

Dear community!

I've recently come across following intriguing thought experiment: Imagine a jar filled with water where infinitesimally small particles of iron dust are dissolved. The average particle diameter is small enough that they do not descend, therefore they are randomly distributed in the jar (high entropy). Now you take a magnet and hold it next to the jar - after some time has passed, the density of iron particles on the magnet side of the jar will be significantly higher than the density at the opposite side. Considering entropy, such a behavior should only be possible if the total amount of entropy increases or at least stays the same. However, this demixing/separation effect definitely has negative entropy, maybe not much but not zero. The only counteractive measure that would increase entropy I can think of is an increase in temperature. But if this is really the solution to this thought experiment, where would the energy necessary to heat up the system come from?

Thanks a lot for every input

Share this post


Link to post
Share on other sites
Posted (edited)

It's a beautiful experiment, and it seems to me that it's quite doable. But keep in mind that, as long as your jar is not a closed system, it doesn't have to display entropy increase. Same reason why the gas in a refrigerator can be made to cool by expanding adiabatically --not getting or giving heat from/to the outside--, but exchanging work. In this way, the universe as a whole would see its entropy increase, but parts of it --the jar's interior-- would see their entropy decrease.

Why don't you try it? Some people here could give you advice. A Dewar flask, a magnet, and a thermometer could do the trick.

Edited by joigus

Share this post


Link to post
Share on other sites
1 hour ago, Matthew99 said:

Dear community!

I've recently come across following intriguing thought experiment: Imagine a jar filled with water where infinitesimally small particles of iron dust are dissolved. The average particle diameter is small enough that they do not descend, therefore they are randomly distributed in the jar (high entropy). Now you take a magnet and hold it next to the jar - after some time has passed, the density of iron particles on the magnet side of the jar will be significantly higher than the density at the opposite side. Considering entropy, such a behavior should only be possible if the total amount of entropy increases or at least stays the same. However, this demixing/separation effect definitely has negative entropy, maybe not much but not zero. The only counteractive measure that would increase entropy I can think of is an increase in temperature. But if this is really the solution to this thought experiment, where would the energy necessary to heat up the system come from?

Thanks a lot for every input

 

33 minutes ago, joigus said:

It's a beautiful experiment, and it seems to me that it's quite doable. But keep in mind that, as long as your jar is not a closed system, it doesn't have to display entropy increase. Same reason why the gas in a refrigerator can be made to cool by expanding adiabatically --not getting or giving heat from/to the outside--, but exchanging work. In this way, the universe as a whole would see its entropy increase, but parts of it --the jar's interior-- would see their entropy decrease.

Why don't you try it? Some people here could give you advice. A Dewar flask, a magnet, and a thermometer could do the trick.

 

Not all all sure about either the question or the answer.

Iron particles are not soluble in water.
They may be suspended which is a different thing.

Since there is no solvation, there is no heat of solution to replace when you de-suspend them.
In fact the particles move to a lower energy state because of the introduced magnetic field.

But the jar containing the supension is definitely a closed system.

Share this post


Link to post
Share on other sites
Posted (edited)
5 minutes ago, studiot said:

 

 

Not all all sure about either the question or the answer.

Iron particles are not soluble in water.
They may be suspended which is a different thing.

Since there is no solvation, there is no heat of solution to replace when you de-suspend them.
In fact the particles move to a lower energy state because of the introduced magnetic field.

But the jar containing the supension is definitely a closed system.

You're right. I was in doubt about that too.

Maybe a ferromagnetic salt could play the role of the "iron particles"? You could package permanent magnetic dipoles, while having the particles be soluble in water... Just a thought.

https://en.wikipedia.org/wiki/Ferrofluid

Edit: Ferromagnetic salt is probably not the proper term. Paramagnetic salt is more like it.

7 minutes ago, studiot said:

Since there is no solvation, there is no heat of solution to replace when you de-suspend them.

This checks with what I was thinking, although you know much more than I do about this. My impression was that it wouldn't release/absorb much heat.

Edited by joigus

Share this post


Link to post
Share on other sites

Thank you very much for your answers! Indeed such an experiment would be very interesting but unfortunately I do not have any lab equipment at home & university labs are currently closed.

Quote

"Iron particles are not soluble in water.
They may be suspended which is a different thing"

You are right, my example probably isn't very appropriate. But as joigus mentioned, just think of some magnetic soluble substance / macro structure. Putting a magnet near the isolated jar, the entropy in the jar will decrease by the entropy of mixing (or maybe a bit less as even with the magnet on one side there will still be a few molecules in the other parts of the jar).

Quote

Not all all sure about either the question or the answer

Now, as total entropy cannot decrease in closed systems (as the mentioned perfectly isolated jar is), there should be an increase in temperature and my question is, where does the energy for this come from as the system itself is closed?

Share this post


Link to post
Share on other sites
56 minutes ago, studiot said:

But the jar containing the supension is definitely a closed system.

Yes, because the term is ambiguous; as wiki says.
A closed system is a physical system that does not allow transfer of matter in or out of the system, though, in different contexts, such as physics, chemistry or engineering, the transfer of energy is or is not allowed.

Moving a magnet near a magnetic material involves a transfer of energy

 

If I put a beaker of water on a magnetic stirrer, the water will get warm because work is done by the stir bar against the viscosity of the liquid.

Share this post


Link to post
Share on other sites
Quote

Moving a magnet near a magnetic material involves a transfer of energy

You just brought me to an idea, thank you for your input. Would it be correct to say that the increase in thermal energy inside the jar comes from the decrease of potential energy as the distance between magnet and solution decreases?

Share this post


Link to post
Share on other sites

By applying an external magnetic field to the system, haven't you introduced new potential energy terms boosting the system total energy?

As the magnetic particles descend this new energy gradient to a new equilibrium position how is this energy going to be dissipated other than in the form of heat?

As a practical example, you might compare and contrast the removal of particulates from eg coal-fired power station flue gas by electrostatic precipitators. The pretty efficient separation of gas and solids suggests an entropy decrease, but on the other hand, electricity is consumed, and that's going to end up releasing heat somewhere or other. 

Share this post


Link to post
Share on other sites
3 minutes ago, sethoflagos said:

By applying an external magnetic field to the system, haven't you introduced new potential energy terms boosting the system total energy?

As the magnetic particles descend this new energy gradient to a new equilibrium position how is this energy going to be dissipated other than in the form of heat?

As a practical example, you might compare and contrast the removal of particulates from eg coal-fired power station flue gas by electrostatic precipitators. The pretty efficient separation of gas and solids suggests an entropy decrease, but on the other hand, electricity is consumed, and that's going to end up releasing heat somewhere or other. 

You are absolutely right, thank you very much for your answer. This is the solution to my thought experiment. From my point of view a very astonishing reminder of how theoretical physics apply to such a simple practical experiment.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.