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Can you predict the direction of motion of a single monatomic molecule

  1. Jul 14, 2005 #1
    There are probably reasons I don't understand on why this is not a valid question, and may have made some incorrect assumptions, but...

    If single monatomic molecule was at absolute zero then, as I understand it, it would have no kinetic energy - no motion. If heat energy were applied from a known location could you predict the direction of the movement of the molecule? Think of this molecule and heat source as the only two objects in existence, there is nothing else. Also, I suppose, the heat source should be such that it is essentially a point. And, I don't really know what would make the molecule move...photon collisions?

    After doing some reading on the internet I have gathered that the movement of molecules in a gas are said to random (Brownian Movement?) and that an increase in temperature corresponds to an increase in their velocities. I guess a more fundamental question is, are the movements really random or just impossible to calculate because of the high number of radiant sources acting on the gas molecules and the ensuing collisions amongst the molecules?

    I really don't know what brought this on - I majored in in history (a long time ago) and the only physics class I ever took was in 9th grade. I just sometimes start thinking about how things work and get it reduced down to a level where I have a hard time finding answers (or probably, more correctly understanding). My real question initially was what makes molecules move (as in a gas) Put that into a search on the internet and all you get are thousands of hits that only say that heat (or increase in temperature) is what makes molecules move. To me, this is like saying pressing on the accelerator of a car is what makes it move. Yes, it's true, but how?? What are the fundamental elements that cause the movement - high velocity molecules colliding with the piston, which pushes the piston, transfering motion to crankshaft, to wheels and so on. So I have it reduced that far, but now I strart thinking - so what causes the molecules to move that are colliding with the piston; what are the underlying forces that get the molecule moving in the first place. And this is where I have trouble finding finding answers - searching on the internet can be frustrating. So I joined this forum in the hopes that some of you may be so kind as to help me out and have some patience with my laymen terms and goofs.
     
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  3. Jul 14, 2005 #2

    Gokul43201

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    In the single molecule case, you can "heat" it either radiatively (by chucking photons at it) or conductively (by chucking other molecules at it). Both will cause the molecule to move. Classically speaking, you can predict the motion of the molecule subsequent to this collision (given a knowledge of the position and momentum of the particle before the collision - without questioning how such knowledge was attained), but classical physics fails miserably when it comes to things as small as molecules.

    As for the 1001 internet sources that told you that molecules move because they have heat : they are not being entirely accurate (or you are misquoting them). Heat (or internal energy) is simply a label we use to keep track of how much the molecules are moving. The temperature of a body is just the average kinetic energy of its molecules.
     
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