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Delay of free fall with electromagnets

  1. Mar 24, 2015 #1
    When you have a metallic ball hanging from a electromagnet and you disconnect the electromagnet, there is a delay between the moment you unplug the machine, and the ball starts falling. Why does this happen?
  2. jcsd
  3. Mar 24, 2015 #2
    Are you sure there is a delay, or does it just appear to hang there briefly?
    If there really is a delay I would think it has something to do with some brief residual magnetism somehow stored within the material of the electromagnet, but it is quickly lost.
  4. Mar 24, 2015 #3
    There is a delay because, when I change the voltage, the time that I measure changes as well. I need to add a correction into my calculations so that I can get a better result, but I don't know how.
  5. Mar 24, 2015 #4
    I can't really offer any more than my first idea, that somehow the material of the electromagnet, a lot of which will be iron, temporarily retains some magnetism as if it were a permanent magnet.
    Purely a guess, I'm sure somebody will be able to state categorically what is happening.
    I'd be interested to know about it too.
  6. Mar 24, 2015 #5


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    Staff: Mentor

    What is the switch circuit like? Does it have a snubber circuit to keep the switch-opening arc to a minimum? What is the inductance of the electromagnet? Can you post the snubber circuit and any other components that are part of the circuit?

    Do you have access to an oscilloscope? If so, and if there is a snubber circuit on the switch, you can measure the time constant of the collapsing field...
  7. Mar 24, 2015 #6
    Lenz's Law is applicable here. When you open the circuit the magnetic field will begin to collapse and this changing magnetic field will induce a back emf such as to oppose this change. The self inductance L determines how long this back emf will persist. Thus after opening the circuit the magnetic field will decay exponentially with a decay constant of L/R where R is the resistance of the coil. Also depending on the core there may also be a residual magnetization left in the core too. So depending on the self inductance, resistance, the core material and weight of the object there will be a delay in dropping.
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