1. Not finding help here? Sign up for a free 30min 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!

EM field between 2 superimpose spheres

  1. Aug 24, 2009 #1
    1. The problem statement, all variables and given/known data

    Problem:

    2 spheres each one of them with a radius R and uniformly charged with ro+ and ro-, are situated in a way they superimpose partially ( see figure). Let be "d" the vector from the positive center to the negative center.

    http://img220.imageshack.us/img220/9633/diagrame.png" [Broken]

    2. Relevant equations

    Find the electric field in the hollow section.




    3. The attempt at a solution

    ?????????????????????????????????????'
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Aug 24, 2009 #2

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    Hi Luchopas, welcome to PF. Pretend that there is only one sphere. Can you find the E field in the region r < R for the other sphere?
     
  4. Aug 24, 2009 #3

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    Hi Luchopas, welcome to PF!:smile:

    Hint: what is the electric field [itex]\textbf{E}(\textbf{r}_{\pm})[/itex] inside a uniformly charged sphere of radius [itex]R[/itex] and charge density [itex]\pm\rho[/itex], where [itex]\textbf{r}_{\pm}[/itex] is the vector from the center of the positively/negatively charged sphere to the field point?

    P.S. This forum supports [itex]\LaTeX[/itex]. For an introduction to using it on these forums, click here. You can also click on ay of the above [itex]\LaTeX[/itex] images to see the code that generated them.
     
  5. Aug 24, 2009 #4
    sorry , dind't catch your question , i can find the electric filed of one sphere and the other... but not the electric field , in the hollow section...

    for a solid sphere the elctric field inside and outside is

    E= (1/ 4*pi*epsilon0) * ro/r-squared ( in r direction)

    for the other sphere is the same but with negative ro

    but after this is where i fail...
     
  6. Aug 24, 2009 #5

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    No,

    [tex]\textbf{E}=\frac{1}{4\pi\epsilon_0}\frac{\frac{4}{3}\pi R^3\rho}{r^2}\hat{r}[/tex]

    is the field outside (I'm assuming that typing ro instead of ro*volume was a typo on your part?), but not the field inside. What is the field inside?
     
    Last edited: Aug 24, 2009
  7. Aug 24, 2009 #6
    mm how do you get t this calculation???
    in the inside would be the same right?
    do you have any msn messenger to talk faster?
    thanks

    p.D: sorry im new in this physics subject
     
  8. Aug 24, 2009 #7

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    No, the field inside is different.

    The calculation is usually done as an example in most introductory EM texts...Which text are you using?
     
  9. Aug 24, 2009 #8
    Try using Gauss's Law? =)
     
  10. Aug 24, 2009 #9
    no text book just some cliff notes i founded,
    here i uploaded my argument for the electrix field outside the sphere

    sorry this one it is:

    http://img24.imageshack.us/img24/8111/newpuw.png [Broken]

    for r>R

    for r<R = ???
     
    Last edited by a moderator: May 4, 2017
  11. Aug 24, 2009 #10
    As stated, you will need Gauss's Law. Go wiki it. You ought to get a text though, cliff notes are horrible for learning.
     
  12. Aug 24, 2009 #11
    ok but how do i proceed with this problem...
    i just wnat to know the steps..
     
  13. Aug 24, 2009 #12

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    for r<R, the only thing that changes in your calculations is that the charge enclosed by your gaussian surface will no longer be [itex]\frac{4}{3}\pi R^3\rho[/itex], but just some fraction of it....do you know how to calculate that charge?
     
    Last edited by a moderator: May 4, 2017
  14. Aug 24, 2009 #13
    yes integrating the charge Q form 0 to R right?
    and replacing this result in Q in the gauss equation.
     
    Last edited: Aug 24, 2009
  15. Aug 24, 2009 #14

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    No, integrate the charge density, [itex]\rho[/itex] over the volume enclosed by your gaussian surface.

    [tex]Q_{\text{enclosed}}=\int \rho dV[/tex]

    In this case, [itex]\rho[/itex] is uniform/constant so the integration should be very easy....what do you get?
     
  16. Aug 24, 2009 #15
    THIS:

    http://img269.imageshack.us/img269/1200/bew222.png" [Broken]

    HOPE IS RIGHT
     
    Last edited by a moderator: May 4, 2017
  17. Aug 24, 2009 #16

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    Why is there an [itex]R[/itex] in your expression? Isn't [itex]R[/itex] the radius of the entire sphere of charge? When r<R, don't you want to use the radius of your Gaussian surface,[itex]r[/itex] instead?
     
    Last edited by a moderator: May 4, 2017
  18. Aug 24, 2009 #17
    sorry you're right then:

    http://img145.imageshack.us/img145/9430/wwwwiy.png" [Broken]
     
    Last edited by a moderator: May 4, 2017
  19. Aug 24, 2009 #18

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    Okay, so what does that make [itex]\textbf{E}[/itex] for r<R?
     
  20. Aug 24, 2009 #19
    this:

    http://img339.imageshack.us/img339/6308/53230410.png" [Broken]

    so what is the next step?
     
    Last edited by a moderator: May 4, 2017
  21. Aug 24, 2009 #20

    gabbagabbahey

    User Avatar
    Homework Helper
    Gold Member

    Right, so if the vector from the center of a sphere of charge density [itex]\rho[/itex] is [itex]\textbf{r}_{+}[/itex] the field inside the sphere will be

    [tex]\frac{\rho\textbf{r}_{+}}{3\epsilon_0}[/tex]

    And the field inside a sphere of charge density [itex]-\rho[/itex] will be

    [tex]\frac{-\rho\textbf{r}_{-}}{3\epsilon_0}[/tex]

    if the vector from the center of the sphere to the point inside the sphere is [itex]\textbf{r}_{-}[/itex]

    Right?

    So pick a point inside the cavity in your drawing in post#1, and label the vector from the center of the positively charged sphere to that point [itex]\textbf{r}_{+}[/itex], and label the vector from the center of the negatively charged sphere to that point [itex]\textbf{r}_{-}[/itex]....what is the field at that point due to just the negatively charged sphere? How about the field due to just the positively charged sphere? So the total field at that point is ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook