Electric field at a distance from a charged disk

[kb1408]

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]

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.

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]

http://www.physics.udel.edu/~watson/phys208/exercises/kevan/efield1.html

 

[kb1408]

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]

How about you post all your working and the target answer?

 

[kb1408]

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]

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

 

[kb1408]

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

 

[haruspex]

Yes, as in the link andrien posted.

 

[kb1408]

There was an advertisement above that link earlier so I ignored it thinking it was tied to that ad.

Alright, thanks for your help!

cheers!

 

[kb1408]

And thank you andrien for the link!

 

[jtbell]

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.