1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electric field between two plates

  1. Jul 9, 2010 #1

    1. Positive and negative charges are distributed evenly in two parallel plates with width "w". The density of the positive and negative charge is +[tex]\rho[/tex] and -[tex]\rho[/tex]. Assume that the length of the plates at y direction is [tex]\infty[/tex]

    (a) Find the electric field in the plates
    (b) Let the electric potential at x=-w equal to 0, find the electric potential in the plates.

    2. Relevant equations
    [tex]\Delta[/tex]E=k*Delta-Q*r/r2 where r is the unit vector -- (1)

    3. My approach
    I sliced an infinitesimal portion of the plate horizontally denoted as Delta-y and represent the charge Delta-Q = rho *Delta-y*2*w. Is my expression for Delta-Q correct?

    r is the observation point which is at opposite to the portion Delta-y on the negative plate of which magnitude is "w".

    Substitute my expression for Delta-Q into (1), I obtain Delta-E = k*rho/w.

    Do I have to integrate Delta-E from -infinity to +infinity to obtain the electric field in the plates? If so, I will get E=0. Does this make sense?

    Thank you.
    Last edited: Jul 9, 2010
  2. jcsd
  3. Jul 9, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is a Gauss's Law problem. Use the standard "pillbox" with its faces perpendicular to the x-axis.
  4. Jul 9, 2010 #3
    I obtained [tex]\rho[/tex]*2w/vacuum permittivity. I also did a sanity check that the unit is correct.

    Thanks for the guidance. Now I learnt that for highly symmetric problem, Gauss's Law really makes the problem much easier than using Biot-Savart Law.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook