Why Is Gauss' Law Failing to Solve This Problem?

[danago]

http://imageupload.com/out.php/i120384_Untitled.gif

I havnt had any luck with this question.

The only possible approach i can think of is to use gauss' law for electric fields.

I know I am supposed to show my working, but this really had me stumped, and I've really gotten nowhere. I did try a few different gaussian surfaces, but with no luck.

Any hints are greatly appreciated.

Thanks,
Dan.

 

[michalll]

try to approximate it with infinite plane surface, the thickness is small enough to the other proportions

 

[danago]

How can i do that?

Would that mean that the electric field at any point will be given by $$E=\frac{\sigma}{2\epsilon_0}$$ where sigma is the charge per unit area?

 

[michalll]

Yes since the point is at the center this approximation exact enough, think of it as a point between two charged planes each with its own $$\sigma$$.

 

[danago]

How can i find the charge density sigma? I tried by assuming that the charge is evenly distributed over each of the two larger surfaces, but didnt manage to get the correct answer, which is supposed to be E

 

[alphysicist]

Hi danago,

I got that answer, but I can't tell what you did without you posting numbers. What numbers did you use to find the charge densities?

Once you had those, what did you do to find the total field E?

 

[michalll]

find the charge density rho over whole volume, then compute sigma1=0.001*rho sigma2=0.004*rho, compute the two fields and subtract them after that you should get the correct answer.