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Acceleration and Magnets

  1. Jul 3, 2014 #1
    If I put a pice of metal close enough to a magnet it moves towards the magnet and they stick together, I can understand the force that keeps the metal and the magnet together, but where did the energy that moved one towards the other came from?, who lost energy? which kid of energy?, and how did it lost that energy?
     
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  3. Jul 3, 2014 #2

    Simon Bridge

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    You should be able to figure that out by considering other energy transformation that you are probably more used to:

    When you hold a ball over your head, and let go, it accelerates down.
    Where did the kinetic energy come from?
    Who loses energy? What kind?

    When you wave your arm - it moves, but it started out still.
    Where did that energy come from?
    Who loses energy? What kind?
     
  4. Jul 3, 2014 #3
    Hello Simon, I've been thinking about it and I think I got it, just tell me if I'm right: The piece of metal has potential energy by being away from the magnet, for it takes energy to take them apart, am I right?
     
  5. Jul 3, 2014 #4

    Simon Bridge

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    That is correct :)
     
  6. Jul 4, 2014 #5

    sophiecentaur

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    Sometimes it is hard to see the chain of events that produce a situation where two objects have somehow acquired a difference in potential between them. The magnet, in this case, needed to be formed and that would have involved perhaps a massive solenoid being turned on for a short while. The presence of the extra piece of metal would have affected the amount of magnetic energy put into magnetising the solenoid. There's where the PE came from.
     
  7. Jul 4, 2014 #6

    Simon Bridge

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    First causes are a bit of an issue.
    In the common table-top experiment, the direct cause of the potential difference between the two positions would be due to the work needed to bring the magnet and bit-of-metal to their initial configuration.

    As sophiecentaur has noted, that could be quite a complicated process in the long run. In the short-run, it was provided by your muscles moving the stuff around. Whatever the process to get to the initial situation on the table-top, the potential difference between that start and end configurations is no more than the work needed to shift them back.

    There is a nice demonstration, in fact, where an uneven magnetic field accelerates a metal ball along a track.
    If you try to loop the track to return the ball to it's initial position, you find the ball stops moving before it gets there. The energy needed to return to the initial position - no matter how big the loop is - can never be less than the energy gained in the initial acceleration. Lots of people don't notice that the start position needed work to set up...

    Donald Simanek has a series of puzzles of the "where does the energy come from" variety, in different guises, in his museum of unworkable devices.
     
  8. Jul 7, 2014 #7
    The way I see it is that sometimes, when something doesn't make sense, it may be because we are trying to see that phenomenon as something independent, while it isn't and can't be independent, and if you see a wider picture, in this case the proceses needed to set up it all, everything makes sense
     
  9. Jul 8, 2014 #8

    Simon Bridge

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    Well - sometimes you don't realize that what you are looking at is not a closed system.
    The conservation laws only apply for closed systems.

    Sometimes it is a misunderstanding of the physical laws themselves though.
    Definitely read the Unworkable Devices pages - you'll find the mental discipline useful for thinking about things like this in future even if just by working though the common mistakes. Some of the connections are very hard to see.
     
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