# Homework Help: Find the Electric Field at a midway point

1. Feb 25, 2012

### 1126

The problem is:
Find the electric field at a point midway between two charges of +30.0 x 10^-9C and +60.0 x 10^-9 C separated by a distance of 30 cm.

I used E=k (Q/r^2)
My attempt is:

E=(9x10^9)(30.0 x 10^-9C)/(.3m^2)
E=3000 N/C

E=(9x10^9)(60.0 x 10^-9 C)/(.3m^2)
E=6000 N/C

added they are 9000 N/C. I am fairly certain this is incorrect, but I do not understand the material. I really have no idea why or how to go about finding the correct answer. Please help!

2. Feb 25, 2012

### susan14

I think you might want to try subtracting them like 6000-3000=3000.
I think that electric fields go away from positive charges

3. Feb 26, 2012

### 1126

I still don't really understand. Is .3m really the right radius? It's the distance between the charges, but I'm not sure if it's the right radius to use. Any suggestions? :/

4. Feb 26, 2012

### Staff: Mentor

Always draw a diagram. If the two charges are separated by 30 cm, how far is the mid point from each charge? Pay attention to the directions of the fields that each charge produces at the given location.

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5. Feb 26, 2012

### 1126

Thank you so so so much!! that was so helpful! I think I solved it correctly now. :) thanks so much!