# Magnitude - point charges

1. Sep 17, 2006

### duki

Problem:

A charge of +18 uC is on the y axis at y = +3.0m. A charge of -12 uC is at the origin. Last, a charge of +45 uC is on the x axis at x = +3.0m. Determine the magnitude and direction of the net electrostatic force on the charge at x = +3.0mm. Specify the direction relative to the -x axis.

So what I have drawn is:

O-> 18 x (10)^-6
|
|
| /\
| 3.0m
| \/
|
|
|
|
| Origin Charge: -12x10^-6
|O_ _ _ _ _ _ _ _ _ _ _ O -> 45 x (10)^-6
< 3.om >

So I did:

F1 = (8.99 x 10^9)(12 x 10^-6)(18 x 10^-6) / 9 = 1.942 N
F2 = (8.99 x 10^9)(12 x 10^-6)(45 x 10^-6) / 9 = 0.539 N

Can I use trig to determine the magnitude now? I tried it but I got this:

1.942cos90 = 0
1.942sin90 = 1

Surely I've messed up somewhere. Can someone point me in the right direction?

Thanks a bunch!!

2. Sep 17, 2006

### duki

or would I do:

.539 cos 45
.539 sin 45

3. Sep 17, 2006

### Tomsk

If you're finding the force on the charge at x=3m, you need 18x45 and -12x45 (with appropriate powers of 10, of course), and use pythagoras to find the distance between the 18uC and 45uC charges.

To find the magnitude, find the resultant force in the x and y directions, then use pythagoras.

4. Sep 17, 2006

### duki

Ok so I've done some of it right?

Then I do:

(3²)(3²) = 9² = 9??

i don't understand the second part

5. Sep 17, 2006

*bump*....