# Homework Help: General question abotu the raidus on an electric field due to line of charge.

1. Sep 13, 2009

### 1stepatatime

This is due to a line of charge that rests on an axis and evaluating the electric field at a point that is also on the same axis. Will the radius, no matter what always be positive?

So if one end of a uniform line of charge is placed at the origin that extends in the negative x-axis and there is some point at -b (that is not on the line of charge) that I am to evaluate the electric field, would my radius be r=(x-b) in the integrand?

edit: sorry for misspelling the title.

Last edited: Sep 13, 2009
2. Sep 13, 2009

### Kalvarin

Could you give a little bit more description? The electric field around an infinite line charge is radial and has symmetry, it doesn't matter where along the line you choose to evaluate the field, all that matters is the radial distance from the wire.

Sorry not bio-savart law lol, my mistake.

3. Sep 13, 2009

### 1stepatatime

Sorry about that, to clarify:

Given my example, if a rod with uniform charge density had one end placed at the origin of the x-axis, and extended some distance of x=-a in the negative x direction and there was some point P at x= -b which is further down the negative x axis than x=-a. Would the radius function in the integrand of the electric field at point P be r= (-b-x)? Since the x values along the rod are negative this would make r positive in value. Is this what my goal for r should be?

Last edited: Sep 13, 2009
4. Sep 13, 2009

### Kalvarin

Yes i believe that is right. The vector from a point on the line charge to the point in space is
-b-x like you said.

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