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Some questions

  1. Jan 10, 2007 #1
    I have a few questions. They are very basic and stupid but I need to understand. Please correct me if I am using wrong terms or some of my facts are wrong. The first one is about potential energy. Now, imagine a ball is held up high above the ground. You would say it has potential energy, that it has "stored energy". Now imagine that the ground has disappeared and the ball is let go, and there are no forces acting upon the ball, but we do have a frame of reference. Would the ball move? If it does, I understand that it definitely stores energy, so where and how is this energy stored in the ball?

    My second question is about magnetism. Now everyone says that magnets store energy when the domains are tapped into place. But isn't that energy lost in moving the domains into place? So where do domains get the energy to repel or attract? Do domains exert force on each other all the time and are just cancelled out?
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  3. Jan 10, 2007 #2


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    All materials have magnetic properties. For those where the attraction/repulsion property is not apparent, its is becuase the domains are so aligned that the forces cancel out. But for materials that have prominent magnetic properties, the domains are so aligned that the "south pole" and the "north pole" point away from each other. When a "normal" material is temporarily magnetized, by say having a magnet influence it, the domains get realigned so that they no longer cancel out. This however wears off and the domains "snap back" into random orientations. This force that you are asking about comes from the microscopic ordering of electron spins.

    Last edited: Jan 10, 2007
  4. Jan 10, 2007 #3
    So where do the electron spins get that force?
  5. Jan 10, 2007 #4


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    But if there are no forces acting on the ball, how would it move?
  6. Jan 10, 2007 #5


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    I guess spin is kind of a misleading word, you should think of it more like intrinsic angular momentum. As for why we get this, I think its electromagnetic in origin i.e. results from an electromagnetic field.
  7. Jan 10, 2007 #6

    I should not have said no forces acting on the ball, I'll change that to "there is nothing other than an observer and the ball in the universe but the observer doesn't affect the ball in any way". What I am trying to find is if the ball literally stores the energy, so it moves even after the force is gone, and the ball is let go only after the force disappears.

    That sounds like an unlimited source of energy, because electrons don't seem to wear out.

    Also, now am I right if I say ordering domains doesn't give that repelling or attracting force but ordering them kind of lets it out?
  8. Jan 10, 2007 #7


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    The forces are there. In a ferromagnetic material for example, there are high degrees of magnetization within the domains, but becuase there is no magnetic field to properly align them, they are randomly oriented. It is becuase of this, forces may cancel out and what not. But apply a magnetic field to the material; the domains get realigned and the material has visible magnetic properties.
  9. Jan 10, 2007 #8


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  10. Jan 11, 2007 #9


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    Potential energy isn't stored energy, it is energy of position. Consider a spring-mass system (also a form of potential energy). The mass doesn't store the energy, the spring does. Remove the spring and the energy isn't there anymore.
  11. Jan 12, 2007 #10
    I mean the magnetic power of the domains. The domains themselves never seem to wear out, and since magnets can be used for doing work, it sounds like free energy.

    So does that mean the ball would not move in my made up experiment(after the ground disappears)? Does this also mean that when people say potential energy is stored energy they are wrong?
  12. Jan 12, 2007 #11
    To idea numero uno... SIMPLE


    You lift ball = kinetic energy into potential energy

    For the "ground to disappear" you need energy to make it disappear, and one part of that large amount of energy you need to move it will be the equivalent to the potential energy stored in the ball.

    Now that you no longer have forces acting on the ball (gravitational forces I assume), then the ball has zero joules of potenial energy.
  13. Jan 13, 2007 #12
    Even if you have a magnetic that never wears out, you still won't have free energy; what you might have is a perpetual motion. You might have a system of compass and a magnet so that the compass moves back and forth like a pendulum indefinitely. But if you make the rotating compass do work, the compass will slow down or may even stop moving. no energy is created.
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