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Some basic questions about magnetic field force on metal objects.

  1. Jun 17, 2011 #1
    When energizing a solenoid, why does an iron object travel along the axis of the solenoid?... and why does it typically get sucked inward no matter the polarity of the solenoid windings?

    This is a similar question: Why do relays work with both polarities? How does energizing the coil cause the switch's pole to come in contact with the throw, no matter what the polarity?

    I do, however, understand the jumping ring. When the ring is placed around the outside of a solenoid, and the solenoid is energized, the ring will develop a current to oppose the suddenly-changing magnetic field caused by the solenoid. According to Ampere's law, parallel conductors with oppositely-traveling current will repel each other. The cross product formulas make sense for THAT.... still not understanding the first two though.

    Thanks for the help.
  2. jcsd
  3. Jun 19, 2011 #2
    Don't know much about relays, but I'll try on your other questions.

    When current is run through a solenoid, it becomes a magnet, with a north and south pole that depends on the direction of the current and whether the solenoid is wound left-handed or right-handed. Both sides of a magnet (North and South) attract ferrous metal, so the iron will get sucked into the solenoid no matter the polarity or direction of current through the solenoid. It gets sucked along the axis because that's where the magnetic field created by a solenoid is stronger. In terms of field lines (which can be seen by sprinkling iron fillings), the lines are tighter in the center, hence stronger.

    Eddy currents created by a changing flux can cause repulsion or attraction. An example of attraction is if the magnetic field strength through the metal object were decreasing instead of increasing (i.e., assume the field is always in the same direction, but getting weaker in magnitude). Then the eddy currents are in the opposite direction, so there would be attraction instead of repulsion.

    So evaluating the situation of a metal core sucked into a charging solenoid, there are certainly eddy currents that repel the core. However, unless the eddy currents are strong enough to reverse the direction of the magnetic field at the metal core (the magnetic field at the core is the sum of the solenoid's field and the field created by eddy currents), then domains will be flipped so that the iron core is attracted to the solenoid, while the electrons in the eddy currents are repelled. Whichever wins out (eddy currents or domains) will determine whether the core gets sucked in or repelled.
  4. Jun 20, 2011 #3
    Question Answered. Thanks, Redx.
    Isn't it amazing my "Engineering Electromagnetics" book does not define the word "electromagnet" anywhere in the book? Can't even find it in the index.
    That's why I love the internet.
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