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: Couple of electromagnetics questions

  1. Mar 5, 2005 #1
    E = 15 i - 8 k at a point on a conductor surface, what is the surface charge density at that point? assume Eo.

    surface charge density is just -Eo*E correct? but i take the magnitude of the electric field and than multiply it by Eo.
    am i right?
     
  2. jcsd
  3. Mar 5, 2005 #2

    dextercioby

    User Avatar
    Science Advisor
    Homework Helper

    How come??What equation did you use...?

    Daniel.
     
  4. Mar 5, 2005 #3
    Gauss' Law

    another quick question.

    Region y is great than or equal to 2 is occupied by a conductor. If the surface charge on the conductor is -20 nC/M^2, find D just outside the conductor.
    Since the surface is y = , than the normal would be in the y direction. and D = Eo*E. but E doesnt have a y component (its the same E from above), so Dn would be 0. Dt is also 0 correct? so overall, D would be 0.
     
  5. Mar 6, 2005 #4
    if somebody can confirm my answers that would be great. i also have another one i need confirmation on.

    A conducting sphere of radius a is half-embedded in a liquid dielectric medium of permittivity E1. The region above the liquid is a gas of permittivity E2. If the total free charge on the sphere is Q, determine the electric field intensity everywhere.

    E inside the conductor is zero.

    E at boundary with region 1 = (Q/surface area)/E1

    E at boundary with region 2 = (Q/surface area)/E2

    surface area = 4*pi*a^2

    is there sometihng at the boundary between the two materials?
    the only E is going to be normal to the sphere, so it would seem to me that the electric field would not be passing the boundary.
     
  6. Mar 6, 2005 #5
    question 1: Seems correct to me, although the minus-sign is arbitrary (it depends on whether the field points toward or away from the conductor).

    question 2: Unreadible

    question 3: why do you think the surface charge (Q/surface area) is uniform on the whole sphere?
     
  7. Mar 6, 2005 #6
    it doesnt not say the surface charge is not uniform. if its not uniform, than i dont see any way to solve it with the information ive been given.

    Fixed Question 2

    Region y is greater than or equal to 2 is occupied by a conductor. If the surface charge on the conductor is -20 nC/M^2, find D just outside the conductor.
    Since the surface is y = 2, than the normal would be purely in the y direction. D = Eo*E. E doesnt have a y component (its the same E from above), so Dnormal would be 0. Dtangential is also 0 correct? so overall, D would be 0.
     
    Last edited: Mar 6, 2005
  8. Mar 6, 2005 #7
    if the surface charge was not uniform, would the electric field be proportional to the permittivity of the medium?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook