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Electric field Question

  1. May 19, 2009 #1
    1. The problem statement, all variables and given/known data

    A charged particle is moving in a circular path under the influence of a uniform magnetic field. Describe how the path will change if an electric field is added, in the same direction as the magnetic field.

    2. Relevant equations

    3. The attempt at a solution

    Well, I found that if intensity of the field is increased, the radius of the circle is decreased.
    and also that if they are both turned off, the particle will fly off to its tangent

    so I'm thinking that this will make the radius bigger..?

    I'm not really sure the diff between the magnetic field and electric field.

    Last edited: May 19, 2009
  2. jcsd
  3. May 19, 2009 #2


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    Can you show your work? (Well I know you can, but would you? ;-)
  4. May 19, 2009 #3
    I dont know how to show work for this one, as it's not really a calculation probrolem (if it is, I'm not really sure which equation to use), but basically I drew a particle going in a circular motion. with mag fields going across it.

    since more are being added, I drew the exact same picture with more mag.fields to it. ( so more lines)

    I'm not sure how it owuld change the path, except maybe make the radius larger because turning it off will no long make the particle travil in circle. (my assumption though)

    Hm... The textbook is really not clear on this. I've had this problem for a while now and the teacher wouldn't really help me.

    hmm haha
  5. May 19, 2009 #4


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    Well, here's one thing you got wrong: it's an electric field that's being added, not extra magnetic field. The magnetic field stays the same.

    As a sanity check, here's a question for you: if the particle is moving in a circular path (before the electric field starts up), its motion is in a plane. Is that plane parallel to the magnetic field or perpendicular to it? Or something in between?
  6. May 19, 2009 #5
    hmm.. I thought being in a circular motion negliges perpendicular or parallel, and is parallel at 2 pts and perp. at 2pts..

  7. May 19, 2009 #6


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    Just noticed this... I guess that would explain some of your confusion ;-) To sum it up, an electric field pushes a charged particle. Positively charged particles experience a force parallel to the field line, in the same direction, while negatively charged particles experience a force antiparallel to the field line, in the opposite direction.

    A magnetic field, in contrast, pushes a current - that is, it pushes a charged particle only while it's moving. The faster the particle moves, the stronger the magnetic force. This magnetic force is always perpendicular to the magnetic field, and also perpendicular to the particle's velocity. But that leaves two choices for the direction of the force. The proper one to choose, for positively charged particles, is given by the right hand rule (which I really hope you know because I'm not sure I can come up with a decent description off the top of my head ;-). As you might guess, negatively charged particles are pushed in the opposite direction.
  8. May 19, 2009 #7
    and also... a moving charge produces an electric field and magnetic field right?... this is confusing me even more.

    so the moving particle itself produces m/e fields that effect itself too?
  9. May 19, 2009 #8


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    Nope... if the problem were taking place in 2-dimensional space, that would have to be true. But it takes place in 3 dimensions.

    Can you see how, in 3D space, the particle's motion could be always perpendicular to the magnetic field?
  10. May 19, 2009 #9


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    Just forget about that, it doesn't matter for this problem. (Yes the moving particle produces its own fields, but those only affect other particles. There are no other particles in this problem.)
  11. May 19, 2009 #10
    ohhh... yes I can. Ahh shoot.

    Your explanations helped me quite a bit haha,


    If the e.field is being added in the same direction as the m.field, and the e.field pushes the charged particle along the m.field (depends on +/-)..

    does that mean, the circular path will either shrink or get bigger depending on the charge.?

  12. May 19, 2009 #11


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    Pretend it's a positive particle, just for argument's sake.

    Here's something else to think about: do you know why the particle is moving in a circle?

    That suggests that you still don't have quite the right picture. But you're getting there...
  13. May 20, 2009 #12
    I do see how the particle in circular motion is perpendicular to the m.field. (thinking 3D wise)

    but no, i do not know why the particle is moving in a circle.. :(

    This is leading me to think in so many different possibilities, which all seem to make sense due to my low knowledge haha.
  14. May 20, 2009 #13
    A question: is magnetic field just a simple plane 2-d plane ? or streams of it making an actual volume of something. (kinda like a pipe)

    my brain hurts inside :(
  15. May 20, 2009 #14


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    It's because of the magnetic field. Think about something like a ball on a string, swung around in a circle: the tension of the string is a centripetal force that pulls the ball around in the circular path. (If not for that force, the ball would fly off in a straight line) Every time something moves in a circle, there must be a force acting on it to keep it in circular motion. In this case the centripetal force is provided by the magnetic field. Think about this and make sure you understand it. (Remember what I wrote earlier about the magnetic force being perpendicular to the magnetic field and to the particle's velocity)

    A magnetic field - actually a field of any sort - is something that fills all of space. At every point in space a field has a value. That value can be just a number (like electric potential) or it can be a vector, as the magnetic field is. At every point in space, the magnetic field points in a direction that a tiny bar magnet placed at that point would align itself.

    If you want to draw a magnetic field, there are a couple of common ways to do it: one is as a "vector field", which means you pick a bunch of points (like the intersections of a grid) and at each of them, draw a little arrow to show which way the magnetic field points at that point and how strong it is. Longer arrows indicate stronger fields. You can also draw the field as magnetic field lines, which basically involves connecting the arrows into smooth curves. You draw the line such that at every point on the line, the magnetic field there is parallel to the line. There are a few examples of the latter style (magnetic field lines) on the Wikipedia page, and if you search the internet I'm sure you can find many more examples of both kinds of drawings.
  16. May 20, 2009 #15
    hmm! thank you.

    After thinking about it, i came up with this idea:

    Although the particle will still be in a circular motion (because magnetic field is uniform and e.field doesn't change the strength of it) it will move in the direction of the e.field?

    so it travels alongs the e.field and while still looping..?

  17. May 20, 2009 #16


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    Yep, sounds like you got the right idea ;-) The path is called a helix.
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