Homework Help: Electron and Uniformly Charged Disk

1. Jul 8, 2010

thebert010

1. The problem statement, all variables and given/known data

An electron (e) is to be released from rest on the central axis of a uniformly charged disk of radius R. The surface charge density on the disk is +3.50 µC/m2.
(a) What is the magnitude of the electron's initial acceleration if it is released at a distance R from the center of the disk?
(b) What is the magnitude if it is released at a distance R/100 from the center?
(c) What is the magnitude if it is released at a distance R/1000 from the center?

2. Relevant equations

$$E(z,R)=\frac{\sigma}{2\epsilon_{0}}\left(1-\frac{z}{\sqrt{z^{2}+R^{2}}}\right)$$

3. The attempt at a solution
So I understand that the acceleration will increase slightly as I get closer to the disk. What I cannot figure out is exactly how R and Fractions of R affect the magnitude of the Electric field. I tried to assume that z=R which allowed me to say that for:
a) I showed that the R term turns into (1-(1/sqrt(2)))
Beyond that, I cannot figure out how to go about b) and c) and my online prof just keeps telling me to look at examples in the book....

Thank You!

2. Jul 8, 2010

pgardn

Well it asks for acceleration. So firstly you know Fe= E*Q What other Force equation helps to find the acceleration of an electron (that has mass) so you can make an equality and find a, acceleration?

Secondly R relates the radius of the disc to how far away the electron is (Z) from the center of the charged disc. So if you made them both = 1 m, would it then be easier to calculate (or "see") the strength of the electric field at a distance Z, that is 1/100 th the radius of disc with a radius of 1 meter? or 100 x closer than the radius of the disc?

3. Jul 8, 2010

thebert010

pgardn,
You are a Godsend

Thank You So Much!

4. Jul 8, 2010

pgardn

Please inform my wife of this revelation.