# Electric field at a distance from a charged disk

by kb1408
Tags: charged, disk, distance, electric, field
 P: 6 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*
Mentor
P: 41,439
 Quote by kb1408 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.
 P: 1,020
 P: 6 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?