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Question about electrons

  1. Oct 10, 2007 #1
    Hi !!! First - my apologies, I'm a physics "newbie" and my question may sound quite "stupid" to most of the people here but I'm just wandering about something and hope you can help me - to understand why/if this is true or false.

    Say we have a beam of electrons pointed right into the middle of a source of magnetic field.
    One of the magnetic poles is "+" and the other is "-". The electrons from the beam should "bend" towards the positive pole since they have negative charge. I believe this was how they were discovered but may be mistaken.

    So my question is - how you can 'know' which magnetic pole 'exerted' more force - the positive one - due the attraction with the negative charge of the electrons or the negative one - due the repelling force because of the same charges ?

    Does the beam bends because of the "+" magnetic pole attraction or due the "-" magnetic pole repulsion ?

    I know that this sounds kind of philosophical and may not matter in the "real world" - say it does :) how you can know which pole is the 'reason' for the bending ? And can you know this at all ?

    Of course I assume "perfection conditions" which imply that you are actually able to point the electron beam right into the "middle" of the source of magnetic field.

    Thanks in advance !
  2. jcsd
  3. Oct 10, 2007 #2
    Firstly, your question doesn't completely make sense; physics investigates "how" electrons behave rather that "why" they do so. Except for philosophy, I guess the only thing to say is that both poles contribute (since mathematically, the fields of magnetic monopoles would add).

    Secondly, you've got the direction of bending wrong.

    Welcome to PF! :wink:
    Last edited: Oct 10, 2007
  4. Oct 10, 2007 #3


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    There's a fundamental misunderstanding here. The electrons will be bent towards a positively charged object and away from a negatively charged object (like the plates of a capacitor). In other words, they are being bent by an electric field (not a magnetic field).

    When you shoot an electron into a magnetic field (made by a horseshoe magnet, say), the electrons are bent in a direction that is perpendicular to the magnetic field (and to the path of the electron). In other words, they are not bent towards the north or south poles of the magnet.

    If you consider an electron moving between the charged plates of a capacitor, then the force felt by the electron is due to both plates. The positive plate pulls it closer and the negative plates puches it away. Initially, the force will be greater from the plate that the electron is closer to. Eventually, if the plates are large enough, or the electron is moving slowly, the attraction from the positive plate will get bigger and bigger as the electron gets closer and closer to it.

    Ref: http://images.google.com/imgres?img...netic+field&svnum=10&um=1&hl=en&safe=off&sa=G
    Last edited: Oct 10, 2007
  5. Oct 10, 2007 #4
    Inadequate question :) My apologies. Thanks again !
    Last edited: Oct 10, 2007
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