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Why do Molecules move?

  1. Mar 13, 2010 #1
    This may seem like a simple question, but I am hoping to find a rather in depth answer. My father and I were discussing a lesson I had learned in Chemistry on Absolute Zero, when we got to the topic of why Molecules move. I answered him, that it was because they receive heat, and express/transfer that heat through movement. Apparently that wasn't good enough, and now we are both wondering WHY that transfer of energy takes place. He seems to think that it may perhaps take place on a sub-atomic level, but I have a hard time believing that. Is this question just to easy? Or is it like questioning why every action has an equal and opposite reaction... Thanks for all your help.

    -Connor
     
  2. jcsd
  3. Mar 14, 2010 #2

    tiny-tim

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    Welcome to PF!

    Hi Connor! Welcome to PF! :smile:

    A molecule moves because other molecules (or photons) hit it.

    It carries on moving until it hits another molecule, and then it doesn't stop, it changes velocity.

    Saying "because they receive heat, and express/transfer that heat through movement" obscures the fact that heat is just another word for molecules hitting each other. :wink:
     
  4. Mar 14, 2010 #3

    mgb_phys

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    Re: Welcome to PF!

    So why is the other molecule that hits it moving?
     
  5. Mar 14, 2010 #4
  6. Mar 14, 2010 #5
    Re: Welcome to PF!

    Exactly. This is really the question I am asking. Just why does everything move?
     
  7. Mar 14, 2010 #6

    tiny-tim

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    Because it isn't at absolute zero.

    Imagine balls on a pooltable … if even only one of the balls is moving, eventually they'll all be moving! :smile:
     
  8. Mar 14, 2010 #7
    That's fine, but why is anything moving in the first place? This is the question I am really asking. Why does everything above absolute zero move?
     
  9. Mar 14, 2010 #8

    Gokul43201

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    Re: Welcome to PF!

    Imagine you've got a bucket of sand. You shake the bucket ... the sand particles get tossed around a little bit ... a second or two later, they are all at rest again.

    What causes them to come to rest is the fact that collisions between sand particles are inelastic, which is to say that each time a pair of grains knock into each other, they lose some of their total kinetic energy. So, after a few dozen or so rapid collisions between each other, the sand particles have lost most of the kinetic energy you gave them by shaking the bucket.

    Atoms and molecules are different from sand particles in that their collisions with each other are typically elastic, which means that they merely transfer kinetic energy from one to the other, keeping the total energy between them constant. So, even if you were able to somehow find a box of molecules, all more or less stationary, and gave it the slightest little disturbance, you would set all the molecules in motion ... forever! Sure, they'd keep banging into each other, changing speed and direction with each collision, but they won't really slow down - at least not if your box is built in a special way.

    And even in a poorly built box, they will only slow down so much, till the point that the molecules on the outside of the box start transferring energy inwards by banging against the outside walls. The little bit of slowing down that happens in the box is not due to the elastic collisions between the free floating molecules inside, but rather due to the mildly inelastic collisions between the molecules inside the box and its walls.

    To summarize, molecules keep moving today, because they were all set in motion "at the beginning", and they do a pretty good job of retaining their energies even after gazillions of collisions.
     
  10. Mar 14, 2010 #9
    Interesting... so now my next question is, why does temperature effect those molecule's movements... or vice versa. If I biol water, why do those molecules move MORE as they receive more energy. Your explanation troubles me, as I feel that would call for all molecules to move at a uniform constant speed....
     
  11. Mar 14, 2010 #10
    Kinetic energy = 1/2 mv^2...increasing the kinetic energy of a molecule increases its velocity.

    Temperature is, roughly speaking, a measure of the average energy per particle.
     
  12. Mar 15, 2010 #11
    Re: Welcome to PF!

    Molecules are also excited into moving by electromagnetic fields and gravitation. But then you're going to ask from whence cometh EM fields and enormous collections of atoms and molecules that acquire significant gravitational mass :cool:
     
  13. Mar 15, 2010 #12

    Born2bwire

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    They do not all move at the same speed. In fact, there is a large distribution of speeds by which the molecules move. However, if we have a LOT of molecules, then the statistical spread of the individual velocities/kinetic energies becomes consistent. The statistical measure of the kinetic energy of the particles is related to the temperature. As for boiling water, well you are inputting energy into the water. Outside of providing the activation energy for a chemical process, the absorbed energy just goes into the kinetic energy of the particles. Hence they move faster the more energy we input into the system.
     
  14. Mar 15, 2010 #13
    Why? How does that work?
     
  15. Mar 16, 2010 #14

    Born2bwire

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    Depends. If the heat is transferred via light (radiation), then the photon is absorbed into the molecule and gives the molecule a kick by transferring its momentum. If the heat is transferred via conduction or convection, then high energy molecules will collide with other molecules again transferring part of their energy.
     
  16. Mar 16, 2010 #15
    dear conor d...,,
    "like charges repel and opposite charges attract"
    "accelerated charges produce magnetic field besides electric field"
    "momentum of any system of particles is conserved if no net force acts on the system"
    "total energy of any isolated system is always conserved"
    ...................................................................................... if you consider all these laws and put it in ur analysis you will be able to understand the situation which you are trying to summarize..
     
  17. Mar 16, 2010 #16
    Because everything always did. The Big Bang was hot.
     
  18. Mar 16, 2010 #17
    One way to measure movement is to measure speed and speed can have values ranging from zero to c.If things did not move everything would be bunched together at one end of the speed range in other words everything would not be moving.How strange that would be.
     
  19. May 13, 2010 #18

    fluidistic

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    My guess (I haven't learn any QM yet) is that if you have a closed system with say a certain number (>1) of molecules initially at rest, they will move due to the electric, magnetic and gravitational forces. These forces are due to the fields associated with the particles themselves. You cannot dissociate the gravitational field from any particle with a mass.
     
  20. May 13, 2010 #19

    russ_watters

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    Re: Welcome to PF!

    Because when the universe was created at the big bang, it was created, having a lot of energy.
     
  21. May 17, 2010 #20
    Classically there is a very small probability at ANY temperature that a given molecule in a gas will be at rest during a very small period of time. But in classical theory it's assumed that the molecule's position can be exactly known. In quantum theory the probability that a molecule will have zero momentum (and hence not be moving) is inconsistent with the idea that the molecule's position is exactly known. In quantum theory the idea is NOT that everything is at rest at absolute zero; the idea is that, at absolute zero, every system (e.g. molecule) is in the lowest possible, NON-zero energy state (called the ground state).

    From the perspective of classical theory, your question is a legitimate one. According to the Maxwell/Boltzmann speed distribution for gas molecules, the fraction of the molecules with a nonzero velocity is zero at absolute zero; that is, at absolute zero it is (according to M/B) a virtual certainty that all the molecules will be at rest.
     
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