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 kb1408 Jan20-13 02:05 AM

Electric field at a distance from a charged disk

A disk of radius 2.4 cm carries a uniform surface charge density of 3.1 μ C/m2. Using reasonable approximations, find the electric field on the axis at the following distances.

I have used the equation E=(Q/ε0)(1/(4*pi*r2))
I also tried the equation E=(Q/2(ε0))(1-(z/(√(z2)+(r2)))

Thanks in advance for the help. Both equations have not led me to the correct answer.

*note, there is not a figure provided for this question*

 Doc Al Jan20-13 07:43 AM

Re: Electric field at a distance from a charged disk

Quote:
 Quote by kb1408 (Post 4235461) I have used the equation E=(Q/ε0)(1/(4*pi*r2))
This looks like the field from an infinite sheet of charge. You should write it as σ/2ε, where σ is the surface charge density. Not what you want.
Quote:
 I also tried the equation E=(Q/2(ε0))(1-(z/(√(z2)+(r2)))
That's the one you want, but you need to replace Q with σ.

 andrien Jan20-13 07:58 AM

Re: Electric field at a distance from a charged disk

http://www.physics.udel.edu/~watson/...n/efield1.html

 kb1408 Jan20-13 11:37 PM

Re: Electric field at a distance from a charged disk

Doc Al, thanks for your quick reply. Unfortunately I am still doing something wrong. I am using 3.1E-6 C/m2 for σ. Is that wrong?

 haruspex Jan21-13 12:05 AM

Re: Electric field at a distance from a charged disk

 kb1408 Jan21-13 12:14 AM

Re: Electric field at a distance from a charged disk

E= (3.1E-6/(2*8.85E-12))(1-((.0001/(√(.00012)+(.0242))
so E= 1.79E5 N/C
where:
σ=3.1E-6 C/m2
ε0=8.85E-12 C2/N*m2
z=.01E-2 m
r= 2.4E-2 m

 haruspex Jan21-13 12:34 AM

Re: Electric field at a distance from a charged disk

Did you simply replace Q by σ? What about the disc area?

 kb1408 Jan21-13 12:45 AM

Re: Electric field at a distance from a charged disk

Yes, that's what I did. Is it σ=Q/A then?

 haruspex Jan21-13 12:58 AM

Re: Electric field at a distance from a charged disk

Yes, as in the link andrien posted.

 kb1408 Jan21-13 01:09 AM

Re: Electric field at a distance from a charged disk

cheers!

 kb1408 Jan21-13 01:13 AM

Re: Electric field at a distance from a charged disk

And thank you andrien for the link!

 jtbell Jan21-13 01:25 AM

Re: Electric field at a distance from a charged disk

Note: This thread had already developed quite a bit before I noticed that it really should have been in one of the homework help forums. Therefore I've simply moved it instead of deleting it and asking the original poster to start over, which is the normal practice.

In the future, please post requests for help on specific exercises like this in one of the homework help forums, even if they're not actually assignments for a class. The "normal" forums are more for conceptual questions and general discussion of their topics.

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