# Homework Help: Point charge problem

1. Sep 13, 2010

### Chandasouk

Two point charges are placed on the x-axis as follows: one positive charge, Q1= 3.96nC , is located to the right of the origin at x = 0.195m , and a second positive charge, Q2= 5.05nC , is located to the left of the origin at x = -0.295m .

What is the total force (magnitude and direction) exerted by these two charges on a negative point charge, Q3= -5.97nC , that is placed at the origin?

Use 8.85×10−12 for the permittivity of free space. Take positive forces to be along the positive x-axis. Do not use unit vectors.

For my answer, I put 0.000002575N on mastering physics but it said

Not quite. Check through your calculations; you may have made a rounding error or used the wrong number of significant figures.

Here is my work

F1 = (8.988 * 109)[(0.000000004C)(0.000000006)/(0.195m)2] = 0.000005673N

F2=(8.988 * 109)[(0.000000005C)(0.000000006)/(0.295m)2]=0.000003098N

Fx=F1-F2 = 0.000002575N

2. Sep 13, 2010

### Staff: Mentor

Well, you do appear to be rounding off the charge values. Why are you doing that? I'd suggest 1) use scientific notation in your equations above, and 2) do not round off the charge values. Do you get the right answer now? Everything else looks okay.

3. Sep 13, 2010

### Chandasouk

Thank you, my calculator automatically rounded it for some reason.

4. Sep 13, 2010

### Staff: Mentor

It looks like it's because it wasn't using Scientific Notation maybe, and ran out of room... Glad you figured it out.