Solve Electrostatic Problem with Y.K.Lim Problem & Solutions

[dingo_d]

Homework Statement

I'm looking at Y.K.Lim's Problems and solutions in Electromagnetism and there's one thing that's confusing me:

[PLAIN]http://img545.imageshack.us/img545/2218/capturegb.png

Homework Equations

Where does the square in r comes from? When in the beginning it says that the electric field is:

$$E=A\frac{e^{-br}}{r}e_r$$ not$$E=A\frac{e^{-br}}{r^2}e_r$$

What am I missing?

And what should $$e_r$$ stand for? Charge?

 

[hikaru1221]

This E-field is not due to a single charge. It is due to a distribution of charges, so it needs not be proportional to r^2. For example, a uniformly charged plane produces uniform E-field on each side of the plane, obviously not proportional to distance^2, correct?
$$e_r$$ is the unit radius vector.

 

[dingo_d]

Ok it needs not be proportional to r^2, but why did he put it then? When taking the divergence of the field?

 

[hikaru1221]

Oh I see. Then it seems to me that it's a typo

 

[dingo_d]

Oh, I thought it's something new I didn't know XD