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Electric Fields

  1. Dec 1, 2004 #1
    I need help understanding electric fields. I basically suck at answering questions related to them (I get 95% of them wrong :cry:).

    What I do know about them is:
    The electric field vector moves from a positive charge (+q) to a negative charge (-q). (Is this the same case in a capacitor? I saw the vector pointing for -q to q through the dielectric of two plates.)
    I also know a few equations:


    [tex]\Delta U_{e} = -\Delta W_e = - Fd = -Eqr[/tex]

    [tex]W = kqQ (\frac{r_{a}-r_{b}}{r_{a}r_{b}})[/tex]

    [tex]\Delta U_{e} = k\frac{qQ}{r}[/tex]

    [tex]\Delta V = \Delta \frac{U_{e}}{q} = k\frac{Q}{r}[/tex]

    [tex]\Delta V = \frac{\Delta W_{e}}{q} = \frac{Fd}{q} = \frac{Eq}{q}d = Ed[/tex]

    [tex]\Delta V = \frac{q}{C}[/tex]

    [tex]C = \frac{\epsilon_{0}A}{d}[/tex]

    I appologize if this is the wrong forum.
  2. jcsd
  3. Dec 1, 2004 #2

    Doc Al

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    Staff: Mentor

    Your request is rather broad, to say the least. :smile:

    I presume you have a textbook? And have attempted to solve problems?

    If you have specific conceptual questions, feel free to ask them in this forum. But if you need help in solving problems, post them in the appropriate homework help forum--you'll get loads of help. Be sure to show your work--that's the only way we can pinpoint exactly where your confusions lie.

    If you need a site to browse to get a different take on electric fields than your textbook may provide, a good place to start is here: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html#c1 (Poke around on that site and you will find most, if not all, of the equations you listed explained.)
  4. Dec 2, 2004 #3
    Always be cautious with this relation. It is assumed that when this relation is applied that the charge is stationary. If the charge is moving then F is given by

    F = q(E + vxB)

    and in this case E does not equal F/q.

    Last edited by a moderator: Dec 2, 2004
  5. Dec 2, 2004 #4
    Just to make it clear, vxB here is the cross product of v and B.
  6. Dec 2, 2004 #5
    Thanks for the link Doc_Al. I went and got a better textbook, because I found out mine sucked. I think I understand electric fields alright now. :smile:

    And pmb_phy: if the charge is moving, then it has a magnetic field, correct?

    Speaking of magnetic fields, I have a question:

    If you are given one of those V-B-F vector diagrams, for example one with the vector lines of velocity (b) and field strength (v) and were asked to find the direction of the magnetic force. The book suggests that I use the right-hand rule, but it's kind of difficult to apply it, so I used the left-hand rule and it worked on all questions. I read that the left-hand rule was specifically meant for I-B-F vector diagrams, but can I also used it for V-B-F diagrams too, or was that a fluke?

    edit: V = charge's velocity
    Last edited: Dec 2, 2004
  7. Dec 2, 2004 #6
    I can't say unless I know exactly what you're doing since it seems that you're doing something wrong. The left hand rule will give the wrong result so all I can say without further info is that your'e applying it incorretly.

  8. Dec 2, 2004 #7
    I'm using my thumb to point in the direction of the magnetic force, my index finger to point in the direction of magnetic flux, and using my remaining fingers (or palm) to point in the direction of current/velocity.
  9. Dec 2, 2004 #8

    Doc Al

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    Staff: Mentor

    right hand rule

    Here are two links that may help with the right hand rule:
    http://www.physics.brocku.ca/faculty/sternin/120/slides/rh-rule.html [Broken]
    http://physics.syr.edu/courses/video/RightHandRule/ [Broken]
    Last edited by a moderator: May 1, 2017
  10. Dec 4, 2004 #9
    This is what I don't get: just because your hand is made the way it is, the magnetic force moves that way? My teacher never gave me an explanation for the reason WHY the magnetic force moves the way it does.
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