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Physics behind magnetic security gates?

  1. Mar 13, 2015 #1
    Hello, I recently walked through the gates of my local library with my hard drive magnets clipped onto my fingers, and I noticed that they were vibrating. I've been looking on the internet and I also found radio-frequency identification systems. Could someone please explain the physics behind these systems? Is it due to the motor effect?
  2. jcsd
  3. Mar 13, 2015 #2

    Simon Bridge

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    You mean the bit by the door that detects when you try to remove a book from the library without permission?

    I don't know what you mean by "motor effect" - it works by unducig a current in the tag, which, in turn, produces it's own radio signal, which is picked up by a radio reciever. Ever used a crystal radio set?
  4. Mar 13, 2015 #3


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    That's a bit bizarre! Whatever were you doing that for? A bet?
    Radio frequency waves have a much high frequency than you can detect directly (of course) but there may have been some regular pulsing of the current (mains frequency, perhaps) in the frame which would produce a varying force on the magnets (motor effect). I find it surprising, though, that such high power levels would be needed for a simple close-up monitoring system. You could do a simple test, using the mains lead to a high current appliance like a heater. If you could separate the L and N conductors, you could see if a magnet placed near to just one of them, gives you the same sensation. You would need to do this carefully, of course and reinstate the cable afterwards. Health and safety rules, as usual.
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