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 Flux of Two Hemispheres with different radii

  1. Jun 5, 2016 #1
    1. The problem statement, all variables and given/known data
    Two hemispherical surfaces, 1 and 2, of respective radii r1 and r2, are centered at a point charge and are facing each other so that their edges define an annular ring (surface 3), as shown.


    2. Relevant equations
    The field at position r⃗ due to the point charge is:

    E⃗ (r⃗ )=C/r^2 r_hat

    where C is a constant proportional to the charge, r=∣∣r⃗ ∣∣, and r_hat=r⃗ /r is the unit vector in the radial direction.

    3. The attempt at a solution
    The hints imply that because the electric field lines everywhere are parallel to the annular ring (surface 3), the angle between the area and electric field vector is 90, which implies that the flux through surface 3 is 0. I am confused as to why this is the case, as looking at the picture I only see one possibility:

    From the side view, the electric field lines seem to only go through surface 1, meaning no electric field passes the surface of surface 3, which leads to 0 electrical flux.

    Can anyone help explain why there are electrical field lines that form a 90 degree angle with the area vector?
  2. jcsd
  3. Jun 5, 2016 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Be sure you understand the convention for the direction of an area vector.

    In the side view picture, pick any point on surface 3 and draw the area vector for a small patch of area at that point. Also draw the direction of the field at that point.

    EDIT: I think the confusion is with the wording of the hint. It should have said the the angle between the area vector and the electric field vector is 90o.
    Last edited: Jun 5, 2016
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted