# Calculating electric field due to charge distribution

1. Apr 4, 2015

### henry3369

1. The problem statement, all variables and given/known data
http://imgur.com/EiCFIDe [Broken]

2. Relevant equations
E = kq/r2
λ = dq/dx
Q = λa

3. The attempt at a solution
Here is the book solution:
http://imgur.com/aKSeKo0 [Broken]

I need help with part (a). I understand everything in the solution except for the distance they used for between (r2 in the equation above). It seems they used (a+r-x) and they say that is the distance between a segment at x and the charge q. What does q have to do with the problem at all? Part (a) asks for the electric field due to the charge distribution Q where x > a. So shouldn't the distance be between dx(a segment on the charge distribution) and x?

Last edited by a moderator: May 7, 2017
2. Apr 4, 2015

### TSny

You are right, it should not have mentioned q in the solution for (a). But the derivation is OK if you think of r as the distance from the right end of the rod to some arbitrary point P located to the right of the rod.