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 triangle

  1. Oct 26, 2015 #1

    I'm trying to solve this exercize :

    I/3 charges Q, positives, are at the corner of an equilateral triangle of centre O, in (x,O,y) . We want to compute the electric field created by the 3 charges on the axis z, passing by O and perpendicular at (x,O,y), thus computing the electric field E(M) in each M point of (Oz), located at an height z above O. We have OA=OB=OC.


    II/ 1) Describe the symmetry plane passing by each point M, located on the axis z. Tell the sense of direction of E(M) in each point of z.

    As the point M is on axis z above A, I would say (Oz) is the only symmetry plane and the electric field is from A to the low triangle.

    2) By a similar way, deduce the value of the electric field at the centre O

    As the three charges are positives the vectorial sum of the 3 electric fields at point O would give an electric field whose value is O, but how could we deduce it by symmetry ?

    3) Why would we have the electric field norm on the axis z created by the three charges reached a maximum at a unique height above O ?

    I do not see how to answer this one.

    Thank you very much
  2. jcsd
  3. Oct 26, 2015 #2


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Hello Various (!?) , welcome to PF :smile: !

    Oz is a line, not a plane !

    If you have brought 1) to a good conclusion, this one will be easier. There is one more symmetry plane for O

    Again, this will be easier once we have 1) and 2).
  4. Oct 26, 2015 #3
    thanks for your answer,

    indeed the only symmetry plane would be (ux,uz) but how could we deduve the sense according to this plane ?
  5. Oct 26, 2015 #4


    User Avatar
    Science Advisor
    Homework Helper
    2017 Award

    Well, if, with (ux,uz) you mean the x-z plane, I don't agree !
    Applying this 'symmetry' would flip the picture upside down and then it isn't the same as the original !
  6. Oct 26, 2015 #5
    I mean the plane which splits in two the triangle from bottom to top vertically but I do not know how to write it, until now I was doing electric field in 3D with cylindric or spheric so it was also the same planes, here we have a triangle so I would say if we take cylindric coordinate (r; theta, z) the plane (r,z) splits vertically the triangle and it is the same
  7. Oct 26, 2015 #6


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Are you certain that you have a equilateral triangle?

    Maybe I'm having a brain cramp, but it doesn't seem to have three sides of equal length.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted