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Magnets on and off

  1. Aug 10, 2010 #1
    Magnets "on and off"

    Is there an easy and cost efficient way to "turn on and off a magnet"? Is it possible? I have a 6x9 speaker magnet (6x9 an old 80's style car speaker), the magnet itself is about the size of a hockey puck and I want to be able to "push a button" so I can remove anything that's stuck to it. If it can be done will my magnet lose any of its power over time?
  2. jcsd
  3. Aug 10, 2010 #2
    Re: Magnets "on and off"

    Your speaker magnet is a permanent magnet. But you can have an electromagnet that turns on with the electricity is turned on, and it turns off when the electricity is turned off.

    The only time I have ever encountered the question of turning a magnet on/off is with regards to a proposed perpetual motion machine. If that is the case, let me cut the the chase - it wont work. The electromagnet consumes more energy than you will get by turning it off/on in whatever scheme you envision.
  4. Aug 10, 2010 #3
    Re: Magnets "on and off"

    Thank you for the quick response. I wish the answer were different though. In a nut shell, if I had a battery powered electromagnet it wouldnt be magnetic for long because it would require too much electricity to keep it "on" right?
  5. Aug 10, 2010 #4
    Re: Magnets "on and off"

    Not if you build your coils from a superconductor.
  6. Aug 10, 2010 #5
    Re: Magnets "on and off"

    Is that cost efficient?
  7. Aug 10, 2010 #6
    Re: Magnets "on and off"

    Depends on the application. In the LHC, it is. (:
  8. Aug 10, 2010 #7
    Re: Magnets "on and off"

    You could put your magnet in a box. And with a suitable mechanism, have an on/off switch that would actually move the magnet near/away a surface.
  9. Aug 10, 2010 #8
    Re: Magnets "on and off"

    I think I've also seen magnet lifters that work by having two magnets and turning one of them around. "off" they cancel out, "on" they combine.
  10. Aug 10, 2010 #9
    Re: Magnets "on and off"

    Uh... turning one(or both) of the magnets around does not cancel any fields from either magnet. Combining works, though.
  11. Aug 10, 2010 #10
    Re: Magnets "on and off"

    Before World War II, the speaker magnet was a coil powered by the high voltage power supply (for the vacuum tubes).

    Bob S
  12. Aug 11, 2010 #11
    Re: Magnets "on and off"

    A quick google search shows something like what I've seen before: http://www.industrydepot.com/PermanentLiftingMagnets.htm"

    Scroll down to "how it works".

    Please add to this thread an explanation of what really happens.
    Last edited by a moderator: Apr 25, 2017
  13. Aug 11, 2010 #12
    Re: Magnets "on and off"

    Ok, I see what your saying.
    Be aware, however, that in that arrangement(or any permanent magnet arrangement) the magnetic field is NEVER off.
    It is simply shielded or redirected with a then negligible influence on the "object"
    The magnetic field itself DOES NOT REDUCE. Just the spacial influence.

    But, I see what your saying now.
    Think "influence reduction"
    Last edited by a moderator: Apr 25, 2017
  14. Aug 12, 2010 #13
    Re: Magnets "on and off"

    heat the stuck object with a torch until it reaches the curie temp. then remove it with pliers , when they make slabs of steel at the mill they can't pick up the hot steel with an eltro-magnet until it cools , This idea is little crazy but im a pyro .
  15. Aug 12, 2010 #14
    Re: Magnets "on and off"

    True perpetual motion, I agree, however I have been able to create a device with magnets that continues to push a magnet back and forth between two other magnets by using a type of pulley gizmo that slides in and out some magnetic shielding. The device seemingly appears to run forever, and I have even considered hooking it up to some kind of device to generate electricity, but it's not true perpetual motion because the magnetic fields are in-fact degenerating and the magnet is the fuel source. Also, the force of the entire device may not be enough to power any kind of energy generation that could be usable in anything, so it may not matter anyways. It may, however, be an effective way to power robotics at a micro level.
  16. Aug 12, 2010 #15
    Re: Magnets "on and off"

    Could we please see a picture or, ideally, a video of your contraption?
  17. Aug 12, 2010 #16
    Re: Magnets "on and off"

    It was a sophomore project, several years ago. I'll explain it to you.

    You have two magnets at the ends of a short tube that had long faces cut out of it (both facing the same direction). Then inside you put a magnet facing the opposite direction so that its + side is facing the + side of the outer magnet and the - side is facing the - side of the opposite outer magnet. This, by itself, will give you a magnet floating between the other two.

    I then attached a gear to the floating magnet which controlled a very small balance on the side of the tube. The balance had arms reaching toward the top and bottom of the tube and had very thin magnetic shielding sheets on the ends.

    To get the experiment started, you manually force one end of the balance into the opening on the tube, placing the magnetic shielding between the floating magnet and an end magnet. The opposite end magnet would then push the floating magnet up to the magnetic shielding. However, as the floating magnet approaches the magnetic shielding, the gear attached to the floating magnet would turn the balance causing the magnetic shielding that the floating magnet was approaching to slide out of the tube. Simultaneously, the opposite side of the balance would insert the opposite piece of magnetic shielding into the tube. The floating magnet would, at this point, be influenced by the magnet it was approaching (that was shielded) and would change directions returning to the other side of the tube, which was now shielded allowing the floating magnet to approach the end magnet. Then as it returns, the gear would swap shielding again and would continue to repeat the process indefinitely.

    This experiment was only done on a small-scale. The magnets were about 1/4" in diameter and the entire tube was only about 1" long. I've never had the resources to test it on a larger scale. It's also just an illusion, really, because I doubt the force of the magnets would be strong enough to produce any kind of real applicable energy. It's also, as I mentioned already, NOT perpetual motion because the magnets are consuming their fields as they interact with each other. Also, several other factors can decrease the strength of the magnet, so it really wouldn't last forever. I do wonder how long it would last though.

    Also, I have since seen small rare earth magnets that are the same as the ones I used, but with tiny holes drilled in the center. They may make a better solution than the modified tube I was using, as you could just put a rod down the center of them.
  18. Aug 12, 2010 #17
    Re: Magnets "on and off"

    Oh, so you made a fancy hand-driven pendulum at best. Nice.
  19. Aug 12, 2010 #18
    Re: Magnets "on and off"

    I think you misunderstand. The floating magnet cannot naturally approach either of the end magnets, so the only way to start such a device is to manually insert the FIRST shielding. It's not hand-driven, but manually started. However, since the magnetic field does decay then eventually the device would stop, so in a way it would be like an extremely long-lasting pendulum. That is why I said multiple times that it is NOT perpetual motion.

    Regardless, this device could potentially supply a long-lasting power supply (not indefinite, as the magnets would have to be replaced occasionally). The actual energy created would be very small. I've never actually tested how much energy could be created from such a device, but it could potentially (on a larger scale) be cheaper than several forms of energy production.
  20. Aug 12, 2010 #19
    Re: Magnets "on and off"

    Do you think it requires zero mechanical work to insert/extract ideal magnetic shielding material from a magnetic field?
  21. Aug 12, 2010 #20


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    Staff: Mentor

    Re: Magnets "on and off"

    1. If you can extract energy from it, then it is a perpetual motion machine, specifically a type 1 PMM. You misunderstand what the term "perpetual motion machine" means: http://en.wikipedia.org/wiki/Perpetual_motion#Classification
    2. The fact that permanent magnets eventually lose their magnetism has nothing whatsoever to do with whether such a device is a PMM.
    3. No, you haven't created such a device. (flaw pointed out above)
    4. Forwarding of perpetual motion theories is forbidden here and the OP hasn't been back in a few days.... Therefore, thread locked.
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