# Homework Help: Electric field strengths

1. Jul 12, 2008

### starrynight24

1. The problem statement, all variables and given/known data
A 10-cm-long thin glass rod uniformly charged to 9.00 and a 10-cm-long thin plastic rod uniformly charged to - 9.00 are placed side by side, 3.90 apart. What are the electric field strengths to at distances 1.0 cm, 2.0 cm, and 3.0 cm from the glass rod along the line connecting the midpoints of the two rods?

2. Relevant equations

E=kq/r^2

3. The attempt at a solution

I figured out E1. From getting E of left and right and then adding them together. But how do I find E2? or E3?

2. Jul 12, 2008

### G01

So you were able to find the field for a distance of 1cm away from the glass rod?

Finding the field at the other two points should be the same process, just with different distance values.

3. Jul 12, 2008

### dynamicsolo

Would it be safe to say that the charges are + or - 9.00 microCoulombs? nanoCoulombs? You don't say in your statement. You also don't give the units for the distance separating the rods, but I presume that it's 3.90 centimeters...

This is the electric field for a point charge, while you are dealing with two extended line segments of charge. Have you already worked out the field of a uniformly charged rod on the perpendicular line extending from the midpoint of the rod? If you don't have the result handy, you will need to set up an integration to solve this problem. (It's not too difficult.)