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Decreasing Magnetism

  1. Apr 21, 2007 #1
    Hey guys.

    I am perhaps in the wrong section but the coursework help forum didn't seem right. My question is concerning the decrease in magnetism of an electromagnet.

    I have a ball hanging from an elctromagnet which will have the power source removed instantly. There may be some delay before the power to the electromagnet is cut but once this happens does the magnetism drop straight away to nothing. I understand it doesn't but does it drop off exponentially. The magnet has one of those soft iron cores so doesn't hold the magnetism very well after being turned off but what shape the curve is matters most. Magnetic Hysteresis? Also I noticed that when i turned the switch off the voltmeter didn't go to zero stright away but could but showed it drop down in more or less an exponential decrease - very quickly went from 8v to 4v, 1.5, 0.6v, 0. Do you think this means power is still getting to the electromagnet if the voltmeter is registering voltage like this.

    My intention was to show that residual magnetism has little effect once the ball is falling as the magnetism drops off and the ball is further away. Then I could put two curves next to each other and show a two fold decrese. it gets more complicated as the ball is accelerating. Then there is another factor which is that the ball is not accelerating linearly due to the pull from the electromagnet. This is probably a neglible effect and is not quantifiable with the information available. Really what i want to do is make my coursework seem a bit more in depth and add a bit of flashy maths. If I am only able to mention this effect it would still be worth it. Any help is much appreciated. Thanks very much.
  2. jcsd
  3. Apr 21, 2007 #2
    I'm not really into electromagnets, but from what you said here, it might be an interesting experiment to have two electromagnets: one with at extremely high power, and the other barely enough to hold the ball in place. You could put the iron ball on the magnet, and then immediately turn both power supplies off at the same time and see if there is any difference in velocities/acceleration.
  4. Apr 22, 2007 #3
    Good idea. When I did a preliminary experiment I didn't bother much with the voltage because I knew I wanted to suggest using just enough voltage as an improvement for the full experiment. The results weren't mcuh different but using a very high votage (maybe sticking a few power packs together) and seeing if there is a significant effect is a possibility.

    The problem is I have done the experiment so this is all theory and talking about reliability of results and the error that residual magnetism introduces. Ideally, I would be able to get a value for the delay or effect that the magnet was having but this is way too complicated. If I did use a low and a high voltage and was able to show that despite a big difference in power used the times were not noticably or hardly affected by this. Then i could write off the effect of residual magnetism as negligible.

    Also, becasue of my set up i could implement your plan very easily as i have a time and a cut off pad. All i would need to do is the drops one after the other and compare the times.

    Thank you very much for your idea. I could mention it as a possibilty for future improvements to the experiement to get more accurate results and understanding of the errors. My research thus far suggests that the magnetism does decrease more or less expoentially.
  5. Apr 22, 2007 #4
    An electromagnet is an inductance. The behavior of an inductance is well explained by its definition:
    [tex]V=L{dI\over dt}[/tex]
    L is the inductance, V the voltage across it and I the current through the inductance.
    In a real inductance you must add a term RI for the drop across the ohmic resistance of the wire.
    When you open the switch, the current do not stop. It can't. It would imply an infinite voltage across the inductance. You could ask "If the switch is open, what is the path for the current? No problem, it will found one. The first one is to charge the parasitic capacitances of the device. The voltage grows across the capacities and a spark can occur. If not, there will be an oscillation in the inductance and the parasitic capacitance (LC oscillator).
    In any case in an inductor there cannot be instantaneous change in current.
    Last edited: Apr 22, 2007
  6. Apr 22, 2007 #5
    Thinking about what you have said it makes sense and I think I understand the concept you are explaining. It is unfortunately well outside the scope of my investigation. Its quite an interesting idea and explains a few things.

    Thanks very much for your help. I have written a bit more about magnetism and i don't think i really need to add more. Cheers again guys.
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