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Questions about electromagnets

  1. May 29, 2007 #1
    Allright, I'm only 17 and I have some questions about electromagnets. Okay, say I have an electromagnet that had a holding value of 1700lbs. when there was no gap between the magnet and the metal object. My question is would this magnet be able to pick up the metal object if it were around 4 inches away from it, or what about if the metal object was only 1500lbs. Would it deffinetly be able to pick the metal up? And if it could pick the metal up would it be instant, or would it take time? The metal would be iron if that helps answer my question any easier. Also, when power is given to an electromagnet, is the magnetivity instantly there? Is there any delay in magnetivity from the moment electricity is put to it?
  2. jcsd
  3. May 29, 2007 #2
    In physics we don't usually talk about magnetic field strength in terms of how much pounds it can lift? But about its magnetic flux density that has somewhat complicated units of tesla (T). Thought I mentioned it.

    The magnetic field strength diminishes with the square of the distance. So 4" away, the magnetic will be less by 4 squared or 16. But to compensate for inches you have to include a proportionality constant K to convert to proper units, so the field would be less by K/16.

    Since an electromagnet is a giant coil, it has a large inductance which has property of opposing current. That means when turning on the electromagnetic, current would be rushed through the coil but would be also opposed to itself for a short while because of inductance. The transient might take about a millisecond to reach a steady state. That time entirely depends on the inductance of the electromagnet. Since magnetic field strength is directly proportional to current, that means as the current rises throughout the millisecond, so does magnetic field.
  4. May 29, 2007 #3

    The magnet im talking about has some info on this page. Its the magnet at the bottom of the list under the 12v section. I don't know if that helps you answer my questions any?

    And what is the proportional constant k? Is it the magnetic strength in Tesla of the magnet? Sorry if im an idiot, ust trying to understand somethings.
  5. May 30, 2007 #4
    The manufactures in your link don't give the magnetic field in Teslas. There is a formula however that can calculate force (lb) of your electromagnet but you need to know the field in Teslas in the first place.

    But whether the magnetic field is in teslas or lb, its field diminishes with the square distance. The K I was talking about relates units, because how would you tell the difference if you are 1 inch away or 1 ft away? K compensates for that and most likely would have to be determined empirically.
  6. May 31, 2007 #5


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    Staff: Mentor

    So the pull-in force will diminish with the square of the gap. So if the best case pull-in force is near what you need, then the 4" pullin force will be way weak, right?
  7. Jun 2, 2007 #6
    for a single thin wire yes, for a practicable coil electromagnet, it diminishes like a dipole: inversely proportional to the cube of the distance
  8. Jun 2, 2007 #7
    Yes you are right it's a dipole, my apologies.
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