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Homework Help: Electric Field Between Two Parallel Plates

  1. May 25, 2008 #1
    [SOLVED] Electric Field Between Two Parallel Plates

    My textbook says that the magnitude of the electric field at any point between two plates (except near the edges) depends only on the magnitude of the charge on each plate.

    For example, if the magnitude of the electric field between two plates was 3.2 x 10^2 N/C, the field magnitude would not differ if the plate separation were to triple.

    However, in the next section, it talks about electric potential difference, and it gives a formula:

    Epsilon = Delta V / r, which implies that the magnitude of the electric field between two large parallel plates is dependent on the distance between them.

    Could someone please explain to me how these two seemingly contradictory statements make sense?

    Thanks in advance. =D
    Last edited: May 25, 2008
  2. jcsd
  3. May 25, 2008 #2

    Doc Al

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    Staff: Mentor

    If you kept the surface charge on the plates fixed, then the field would not change as you separated the plates (within the limits of the approximation). But the voltage difference certainly changes. Delta V = E*d applies here.

    If you keep the voltage fixed but change the distance you end up changing the charge on the plates.
  4. May 25, 2008 #3
    Oh, I get it now. =P Thanks so much!
  5. May 25, 2008 #4
    New Question

    Oh yeah, I also have a question, but this one's actually dealing with numbers.

    A negative charge of 2.4 x 10^-6 C experiences an electric force of magnitude 3.2 N, acting to the left. Calculate the magnitude of the electric field at that point.

    I used the formula
    Epsilon = Electric Force / Charge and i got the numerical answer to be 1.3 x 10^6 N/C. But, the answer also includes the direction, which is supposed to be
    . How does that work? I thought it would be to the left.​
  6. May 25, 2008 #5
    Electric field at a point P is defined as the force experienced by a unit positive charge at that point. Hence, if a negative charge experiences a force to the left, a positive charge would experience a force to the right. Hence, the direction of the Electric Field is to the right.
  7. May 25, 2008 #6
    Ha, I can't believe I never thought of that. Thanks!
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