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Battery alone deflects compass needle?

  1. Oct 15, 2014 #1
    1. The problem statement, all variables and given/known data
    I'm an introductory electromagnetism course and in lab we were observing how an electric current could deflect a compass needle. however, i also noticed that the batter itself (with no wire attached, so no current) could also deflect the needle. (for ex, if you brought the the + end of the d cell near the compass the needle would move). These were Duracell alkaline D batteries.

    Does anyone know why this might be happening?

    2. Relevant equations


    3. The attempt at a solution

    My first thought was that since there was a potential difference across the terminals of the battery, that when i moved the battery i was moving charges, thus creating a current and therefore a magnetic field that deflected the compass. However, it can't be just from me moving the battery because I took a magnaprobe to a stationary battery saw the same effect. I also saw this with other batteries, it was not unique to one.


    What could be causing this? Let me know your thoughts!
     
  2. jcsd
  3. Oct 15, 2014 #2

    Dick

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    I think the explanation is much simpler. The battery casing is probably made out of steel (iron). What does that suggest?
     
  4. Oct 15, 2014 #3
    i thinks that...
    the battery is charged and every charged particles have a magnetic field.
     
  5. Oct 15, 2014 #4
    So your suggestion is that since this battery has at some point been connected to a circuit, that current aligned the (what are they called, territories?)in the metal and thus magnetized the casing? If this is true, would i not expect to see this from a brand new battery?
     
  6. Oct 15, 2014 #5
    only moving charges create a magnetic field!
     
  7. Oct 15, 2014 #6

    CWatters

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    I'm sure that's NOT what he is suggesting.

    There is no need for the battery to be magnetised. Do you know how a compass is made?
     
  8. Oct 15, 2014 #7
    Indeed, all the batteries I have tried are attracted by magnets.
    Not surprising after reading that the positive pole is a cylinder made of steel.:)
    See, for example, http://en.wikipedia.org/wiki/Alkaline_battery
    under "Construction".
    It has nothing to do with being connected in a circuit or with the electric charge separation in the battery. It's just some steel interacting with a magnet.
     
  9. Oct 15, 2014 #8
    yes, I understand any piece of metal is attracted to a magnet, but that would not explain why opposite end of the compass needle are attracted to opposite ends of the battery right?
     
  10. Oct 15, 2014 #9
    Yes, it does. Any end of a magnet (the compass needle is a magnet) is attracted by steel.
    And both ends of the battery have some steel. At least in my batteries. So any end of the battery is attracted by any end of the magnet.
    This is it.
    If you would observe some repulsion then you may assume that the steel casing have some proper magnetization.

    Even if the battery would have produced some proper magnetic field, it would have been most likely shielded by the steel casing.
     
  11. Oct 15, 2014 #10

    Dick

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    Iron very easily picks up magnetization from any stray magnetic field. A few strokes of a magnet on a screwdriver will let it pick up screws. And it only takes a weak magnetic field to swing a compass.
     
  12. Oct 16, 2014 #11

    CWatters

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    Does either terminal of the battery repel the needle?
     
  13. Oct 22, 2014 #12
    yes, one end of the battery attracts one end of the needle/repels the other while the opposite is true for the other end.
     
  14. Oct 23, 2014 #13

    CWatters

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    Ok then than means the battery has become slightly magnetised. Probably when the cases are manufactured.
     
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