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Author Topic: What is the origin of the energy that keeps particles moving?  (Read 2631 times)

Offline thedoc

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Ned Benjamin  asked the Naked Scientists:
   
Hello Dr. Chris,

Firstly, let me thank you for your podcasts which are a constant source of information and entertainment. They have accompanied me on many an otherwise dull car journey.

Secondly, I would be very grateful if you could answer a question which has been bugging me for some time: As I understand it, molecules in any of the four states of matter are in constant motion - unless at absolute zero - with those in a gaseous state moving much more rapidly and being further apart than those in a solid state and those in a liquid state being somewhere in between (I'm not sure where plasma fits in). My question is that surely this motion requires energy - where does this energy come from and why doesn't it run out?

Kind regards,

Ned Benjamin
Canary Islands

What do you think?
« Last Edit: 18/09/2012 03:30:03 by _system »


 

Offline lightarrow

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Ned Benjamin  asked the Naked Scientists:
Secondly, I would be very grateful if you could answer a question which has been bugging me for some time: As I understand it, molecules in any of the four states of matter are in constant motion - unless at absolute zero
This is not correct; the motion comes to a minimum at 0K, but it doesn't completely stop:
http://en.wikipedia.org/wiki/Zero-point_energy
Quote
- with those in a gaseous state moving much more rapidly and being further apart than those in a solid state and those in a liquid state being somewhere in between (I'm not sure where plasma fits in). My question is that surely this motion requires energy - where does this energy come from and why doesn't it run out?
Actually in the gaseous state molecules don't necessary move more rapidly than in the liquid or solid state, the difference is in their freedom of movement.
Anyway, the energy comes from the environment, as heat: when you put a cold system A (be it a portion of gas, liquid or solid matter, but even an empty region of space) in a hotter environment B, with thermo-conductive walls between the two, the system B gives, as heat, energy to the object A.

This process happens in two ways:
1. collision of faster molecules of B with slower molecules of A (remember that "faster" and "slower" is on average);
2. propagation of electromagnetic energy from a region of space with greater energy density (region B) to a region of space with lower energy density (region A).

Which of the two prevails depends on the kinds of systems and on the conductive walls/region of separation between A and B.

The energy acquired from A is equal to the energy lost by B and is called "internal energy" that is, A increases its "internal energy" and B decreases its "internal energy" of the same amount, so total energy is always conserved.
« Last Edit: 18/09/2012 12:56:25 by lightarrow »
 

Offline simonhudson

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All true, of course, but the question is an interesting one, i.e. why does thermal energy manifest as the motion of particles? I've never really thought about that aspect before, vibration = thermal energy, but why.

Energy is conserved, so they don't intrinsically slow down over time (though loss of thermal energy to other forms of energy can reduce the risk thermal energy of a system, plus loss to entropy).
 

Offline chris

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Ned Benjamin  asked the Naked Scientists:
"Secondly, I would be very grateful if you could answer a question which has been bugging me for some time: As I understand it, molecules in any of the four states of matter are in constant motion - unless at absolute zero"

This is not correct; the motion comes to a minimum at 0K, but it doesn't completely stop:

Is that a theoretical prediction? Because I can't see how anyone could test it...
 

Offline Phractality

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You are asking why does mass have inertia. I could answer that question in the New Theories section, but not here. I don't believe mainstream scientists have a clue.
 

Offline lightarrow

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This is not correct; the motion comes to a minimum at 0K, but it doesn't completely stop:
Is that a theoretical prediction? Because I can't see how anyone could test it...
Of course is theoretical, since no one has been able to reach exactly 0K yet. I don't know much about it, but I'm sure there are experiments which confirm it indirectly trough qm predictions, it's too important for qm; if that mathematical result weren't true, a lot of things would be different in the reality.
« Last Edit: 19/09/2012 15:48:36 by lightarrow »
 

Offline lightarrow

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All true, of course, but the question is an interesting one, i.e. why does thermal energy manifest as the motion of particles? I've never really thought about that aspect before, vibration = thermal energy, but why.
If you think more about it and give the definition of "thermal energy" you can probably find yourself the answer.

If you don't want to do it alone, you can look up wikipedia:
http://en.wikipedia.org/wiki/Thermal_energy
« Last Edit: 19/09/2012 15:51:53 by lightarrow »
 

Offline yor_on

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It's a result from the Heisenberg uncertainty principle Chris. One could look at it as stating that you can't find something with all properties observable simultaneously. And if something is defined as being at 'absolute zero' it should also be simultaneously observable in all its properties classically. But in QM there is no such thing as far as I've seen.
« Last Edit: 20/09/2012 06:45:04 by yor_on »
 

Offline Soul Surfer

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The simple answer to this question is that the motion IS the energy and it cannot run out unless something actually stops it.

Newtons first law of motion describes it precisely

Newton's First law of motion: (from Wikipedia) If an object experiences no net force, then its velocity is constant: the object is either at rest (if its velocity is zero), or it moves in a straight line with constant speed (if its velocity is nonzero).

WE are so used to friction and air resistance slowing things down on the earth's surface (these are resisting forces) that his appears to be the natural state of everything.  This is NOT TRUE  in outer space where these energy losses are very small this clearly does not happen and things continue to move for very long periods

This was probably the greatest intuitive leap in Newton's thinking.  Before that is was always believed that things were kept in motion by the will of God.
 

Offline evan_au

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In the macroscopic world with which we are most familiar, everything "slows down" and "stops" due to inelastic collisions, friction, fluid viscosity, electrical resistance, etc. These forces eventually turn all energy into Heat, which is the "lowest" form of energy. This process is described by thermodynamics.
 
However, in the quantum world (close to the atomic level), after matter reaches thermal equilibrium, energy does not degrade any further in this manner, but is merely transferred from one atom to another by collisions.

The common analogy of a helium atom may help here: Two negative electrons spinning around a positive nucleus.

As you cool helium gas (remove energy from the system), the velocity of the atoms drop, it condenses into a liquid, and the electrons fall into lower and lower orbitals (less energy).

If you could cool a Helium atom to absolute zero, the electrons would not end up in the nucleus as a neutral proton/electron/neutron composite - the electrons stay "spinning" around the nucleus in the lowest energy shell.

Due to Heisenberg's uncertainty principle, the position of the electrons is actually "smeared out" around the atom, rather than at a particular position on an orbit, but it stays in this lowest energy shell (rather than collapsing into the nucleus)  because of quantum effects. 
 

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