# Electric field at a distance from a charged disk

1. Jan 20, 2013

### 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*

2. Jan 20, 2013

### Staff: Mentor

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.
That's the one you want, but you need to replace Q with σ.

3. Jan 20, 2013

### andrien

4. Jan 20, 2013

### 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?

5. Jan 21, 2013

### haruspex

6. Jan 21, 2013

### 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

7. Jan 21, 2013

### haruspex

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

8. Jan 21, 2013

### kb1408

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

9. Jan 21, 2013

### haruspex

Yes, as in the link andrien posted.

10. Jan 21, 2013

### kb1408

cheers!

11. Jan 21, 2013

### kb1408

And thank you andrien for the link!

12. Jan 21, 2013

### Staff: Mentor

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.