# Finding cooridinates of a charge from vector quantities

1. Oct 4, 2006

### mkienbau

Well I'm stumped at how to do this problem, its probably ridiculously simple, but I just can't get it. I've answered the first part correctly, but the last 4 parts is where I'm having problems. I've attached a picture of the problem itself to clarify this as much as possible:

I'm simply using the equation 1/(4pi Epsilon naugt) * q/(r^2) and setting it equal to the quantity I'm given. For example the first one that I got right I did the following:

-225000= (-12*10^-9)/[(4pi(8.85*10^-12)*(r^2)]

Moved around I got:

Sqrt[((-12*10^-9)/-225000)/(4pi(8.85*10^-12))]=r

r=.02189m

Did the conversion to centimeters to get 2.19cm, added it to the original 2.0cm to get the 4.19cm. I've tried repeating this process with the 161000 and -80500 to get the wrong answer everytime according to webassign.

I get the sqrt(-6.702e-4) as my answer for the 161000. If you ignore the negative and take the square root its 2.6 cm. But then add the negative back in, its -2.6cm from 2 should be a -.4cm. There must be a flaw in my math or my work, but I'm just not sure where.

Last edited: Oct 4, 2006
2. Oct 4, 2006

### Noesis

Well, you've got to remember a couple of key points.

Coulomb's law will only give you the magnitude of the electric force/field, unless you do it vectorially.

So you have both the x and y components of the force, that should be enough to give you a magnitude.

Once you have this magnitude also remember, that the field will have the same magnitude in this particular case in a symmetrical spherical distribution. So you will have to take into account the direction of the force as well.

Think about that and give it a try.