# Electrostatic Force Direction

1. Feb 8, 2009

### keemosabi

1. The problem statement, all variables and given/known data
A charge of -3.00 µC is fixed at the center of a compass. Two additional charges are fixed on the circle of the compass (radius = 0.125 m). The charges on the circle are -4.00 µC at the position due north and +5.00 µC at the position due east. What is the magnitude and direction of the net electrostatic force acting on the charge at the center? Specify the direction relative to due east (0°).

I used Couloumb's Law to get the above picture, but what do I do now?

Last edited by a moderator: May 4, 2017
2. Feb 8, 2009

### AEM

Well, you seem to have found the resultant vector. That's the right idea. Don't you have a theorem from geometry that will let you figure its length from the other two? (Hint c^2 = a^2 + b^2)
The angle involves the arctangent.

3. Feb 8, 2009

### keemosabi

How do I know the side length of the dotted line? It looks shorter than the 6.912 across from it.

4. Feb 9, 2009

### AEM

Well, it seems to me that you have been too sloppy with your diagram. You will have a vector to the right representing the force of the charge to the East on the charge in the center and a vector down representing the force due to the charge to the north. You've computed these values. Make a scale drawing of the vectors and then use the parallelogram law for addition of the vectors. The diagonal of the parallelogram is the resultant. Here your parallelogram will be a rectangle and you can easily figure out its length.