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So here we are on the moon

  1. Jul 20, 2008 #1
    ... with all this drilling equipment and an idea, albeit a dumb one.

    Big tube, through the moon. Big permanent mag lev tube... packed with solenoids... and a brick, covered in magnets.

    Drop the brick in the tube, vacuum of space, no resistance that I can think of...

    Hooke says it will oscillate back and forth forever.

    Why wont it?
  2. jcsd
  3. Jul 20, 2008 #2


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    Why do you think you need a solenoid and magnets, it will oscillate with gravity alone.....

    Would be best if your hole was on the axis of rotation.
  4. Jul 20, 2008 #3
    Because dropping a magnet through a solenoid creates electricity... which would violate conservation... so why won't it work?
  5. Jul 20, 2008 #4


    Staff: Mentor

    The same reason that you can't just start a generator spinning like a top and generate free energy forever.
  6. Jul 20, 2008 #5
    Friction is why you can't do that.

    Why can't you do it in the above scenario?
  7. Jul 20, 2008 #6


    Staff: Mentor

    Assume an ideal generator with no frictional or heat losses. The generator would still not spin forever generating power.

    Do you know how a generator works? In a typical well-designed generator the friction losses are negligible. I would start with the http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html" [Broken].
    Last edited by a moderator: May 3, 2017
  8. Jul 20, 2008 #7
    Evidently I don't know how GOOD generators work.

    It requires energy to generate that electricity... through whatever processes that allow efficient motors to operate, it cannot breach 0 point.

    Gravity is not electricity, or energy even I don't think... it is nothing without mass. It is a force... it is a constant force... which is a source of potential energy.

    I guess what you are saying is that the conversion of that potential energy TO energy will result in something "magic" that prevents it from continuing to do so.

    I am asking, specifically, what will occur that prevents gravity from having it's effect, or from permanent magnets from having their effect, or from Faraday's law what keeps the coils from energizing. "It doesn't work that way" is NOT what I am asking for. What specifically will keep THAT from working? There is no electricity being used, only generated. There is no "energy" being used, just potential energy being converted... as near as I can see.

    I prefaced this with the fact that I am an idiot. I just need to know why. I can't sleep. It has been almost a week now. Please help me sleep.
    Last edited: Jul 20, 2008
  9. Jul 20, 2008 #8
    I am thinking it has something to do with Lenz, but being an idiot with only most of a public school education about a millennium ago, I don't understand Lenz, or Faraday, so I make that claim with no understanding whatsoever. I just believe it.
  10. Jul 21, 2008 #9


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    A generator converts mechanical kinetic energy to electromagnetic energy. It is electromagnetic forces that oppose the spinning of the generator and the motion of your device on the moon.
  11. Jul 21, 2008 #10


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    Oops, I didn't test my link and the hyperphysics one did not work. Please read the HyperPhysics pages on:

    A motor is just a generator run in reverse. If you still have questions on how they work, then please post a follow-up and I will be glad to help. The bottom-line is that magnetic fields exert forces on currents. In generators those forces oppose the external force such that the external force does mechanical work to produce electrical energy.
    Last edited by a moderator: Apr 23, 2017
  12. Jul 21, 2008 #11


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    In order to generate electricity you will have to have a magnet moving through a magnetic field. The magnetic field will exert a force on the magnet, eventually slowing to a stop. That's why such a "perpetual motion machine", even under ideal circumstances, cannot work.

    In practice, there is a slight amount of atmosphere on the moon so even without trying to draw energy out of this, it would eventually stop. But the answer to your question is that in order to generate energy, you must have some kind of resistance.
  13. Jul 21, 2008 #12
    So... as the magnet passes through the center of a solenoid, the magnetic field changes direction? Is that where the resistance comes from? Is there a way to calculate that resistance?

    Also, does the velocity of the magnet have any effect on the amount of electricity generated?

    I suppose using electromagnets on one end to pump it back up to the top would use more electricity than it generated eh?

    Lastly, would the weight of the object have any effect? It seems to take more energy to get something heavier off the ground (like a loaded airplane vs an empty one requires more energy, with the same resistance) while it will be drawn towards the center of gravity at the same speed (the pound of feathers pound of lead in a vacuum thing).

    I really appreciate all of your help thus far. My wife will too now that I will abandon this concept. I wanted to build a 3000 mile pipe that rested on the continents and on the ocean floor. 42 minute trips across 3000 miles seemed like a lot of juice.
  14. Jul 21, 2008 #13


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    What happens is that as the magnet passes through the coil, it induces a current in the coil (This is the electricity you hope to tap). When this happens, the coil itself becomes an electromagnet, and the polarity of that electromagnet will be such that its magnetic field will oppose the motion of the magnet.

    To calculate the retarding force you would need to know the strength of the magnet, the parameters of the coil and the load on the coil.

    Yes, and it also effects the resistance to the movement of the coil. If you've ever experimented with a hand cranked generator, you will notice that, under load, it becomes harder to crank the faster you try to crank it
    most definitely
    The weight of the object will determine how quickly the system will damp out and how much energy you can get from it before it completely damps out.
    Last edited: Jul 21, 2008
  15. Jul 21, 2008 #14


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    There is a neat little demo I saw in high school, where the teacher drops a magnet down a pipe and the magnet slowly makes its way down the pipe due to the current it induces in the pipe and the accompanying resistance to its ability to fall. It is precisely what you are trying to do.

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