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Attraction between a charge and a magnet

  1. Apr 14, 2013 #1
    In physics lab we positively charged a metal sphere then placed a compass near it and both ends of the compass were attracted to the sphere. I know magnetic fields are created by electrons, so I made the assumption the compass is attracted to the positive charge because the electrons in the compass are attracted to the positive charge thus the interaction is purely electric. I'm not sure if magnets are negatively charged thou. Is this the basic idea? Or does it have to do with the interactions between magnets and electric charges?
     
  2. jcsd
  3. Apr 14, 2013 #2
    Was the metal sphere made of steel or iron?
     
  4. Apr 14, 2013 #3

    Simon Bridge

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    You should have repeated the experiment with an unmagentized needle ;)
    How would you determine if the compass needle carried a net charge?

    Anyway - assuming you cannot do more experiments before you need to hand in your report for this one, you can assume the compass needle had a neutral charge (but you should state that assumption in the report). It is also a metal - a conductor.

    Before you start thinking in terms of charges on the compass needle - what determined which pole was attracted to the charge? Something about the way you positioned the compass? Since neither pole was favored - what does that tell you about the magnetic field about the charged object?

    Now back to considering charges...
    What happens when a neutral conductor is brought close to a charged object?
    (You've probably seen the effect in an electroscope - but what causes it?)
     
  5. Apr 14, 2013 #4
    First, so no-one gets angry at more for posting homework in the non-homework section, this isn't for a report I'm just trying to figure out what was going on. The magnetic field on the charged object is unaffected by which end of the compass is closest to it. Because the compass is a conductor, when it's brought near the charge the electrons gather towards the positive charge creating a force on the needle to go towards the charge. So this means it doesn't matter whether the charge was positive or negative, because if it was negative the electrons in the compass would have moved as far from the charge as possible i.e 180 degrees from the charge.
     
  6. Apr 14, 2013 #5
    Another small question, does a magnetic field have an effect on a charged object that is not magnetic? and vice-versa
     
  7. Apr 14, 2013 #6

    Simon Bridge

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    Perceptive question - a changing magnetic field will affect the charges in a conductor. When you bring the compass needle close, you are changing the magnetic field at the charged sphere.

    This is why I asked you about what determined which pole was attracted and why (probably) @technicion asked if the metal sphere was iron or steel.
     
  8. Apr 14, 2013 #7
    Yes...it would be interesting to know what the charged sphere was made of.
    If it was iron or steel then the compass needle would be attracted even if the sphere was not charged
     
  9. Apr 14, 2013 #8
    But if the compass is still then the magnetic field isn't changing is it? The compass was brought towards the charged object and then held there.

    Because the magnet would induce a magnetic field in these metals? and all the lab manual says is that the sphere was conducting
     
  10. Apr 14, 2013 #9

    Simon Bridge

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    That's right - but the effect of the motion can still result in a deflection of the needle.
    If the compass was oriented so the north pole was initially closer to the charged sphere but the south pole ended up attracted to the sphere, then there could have been an induction effect.
    ... so if this was a long-answer in an exam, you'd do well to mention the possibility that the conducting sphere has some iron in it.

    You should be able to see that quite a lot of things could have been happening.
     
    Last edited: Apr 14, 2013
  11. Apr 14, 2013 #10
    Well thanks a lot there's clear's things up
     
  12. Apr 14, 2013 #11

    Simon Bridge

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    That's the fun part of asking scientists about something - you end up with more answer than you bargained for :)
     
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