# Homework Help: Linear charge

1. Sep 17, 2012

### Mdhiggenz

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

Positive charge Q is distributed uniformly along the x-axis from x=0 to x=a. A positive point

charge q is located on the positive x-axis at x=a+r , a distance to the right of the end of Q.

Observe the figure below

My goal is to try to explain the problem to make sure I understand it since a similar one will be on my exam, and also get some questions that I don't understand addressed. (:

1. Calculate the x-component of the electric field produced by the charge distribution Q at points on the positive x-axis where x>a.

$\lambda$= $\stackrel{Q}{a}$

dQ=$\-lambda$ dx (I dont know why lambda is negative)

dq=-$\stackrel{Q}{A}$

First things First I know that the distance of Q is a and the distance from Q to a test point q+ is = a+r which the problem statement sets to x for simplicity.

-$\aintx$ $\stackrel{Q}{a}$dx

Here is where I get confused I dont understand why the insert $\stackrel{k}{x^2}$
not including the point charge q?

after that it is just simple integrals and algebra.

Ultimately getting an answer of $\stackrel{Qk}{a}$($\stackrel{1}{a}$-$\stackrel{1}{a+r}$)

2. Calculate the magnitude of the force that the charge distribution that Q exerts on q.

Here i'm also confused isn't the answer just a simplified version of part 1?

$\stackrel{kQa}{r(a+r)}$

Thanks for the help!