Electric Charge

1. Jul 4, 2006

sophzilla

Hello -

I worked out this problem but I got a wrong answer.

First, I used the Pythagorean theorem to find the radial distance between A and each charges. So 1.2m divided by 2 (= 0.6), then the square root of .6 squared + .6 squared = .849, which is the radius.

Then I used Coulomb's Law to calculate the net force:

kqAq1/R2 + kqAq2/R2 + ... and so forth.

I took out the kqA/R2, which is the same for all, and came up with:

kqA/R2 (q1 + q2 + q3 + q4).

But it so happens that the numbers inside the parenthesis turns out to be 0.

What did I do wrong?

2. Jul 4, 2006

neutrino

I think you're forgetting the vector nature of force. Simply adding up the numbers won't do any good. You have to add them vectorially.

3. Jul 4, 2006

Kurdt

Staff Emeritus
First your pythagorean theorem is a bit off. it should be:

$$a^2=b^2+c^2$$

EDIT: You got it right I misinterpreted what you had done originally.

Then you have neglected to take any directions when working out the force so try setting up a reference freame and adding the directions into your equation.

4. Jul 4, 2006

nazzard

Hello sophzilla,

I think you don't take into account that the forces are vectors.

You can start applying Coulomb's law for one diagonal at a time. For instance $$Q_4, q, Q_2$$. Do you agree that the 2 forces will add up to a net force pointing from $$q$$ towards $$Q_2$$?

Regards,

nazzard

5. Jul 4, 2006

sophzilla

I would appreciate any help. Thanks.

6. Jul 4, 2006

Kurdt

Staff Emeritus
Those are not vectors. That is why it is not working. Consider the unit vectors i and j and how they would add up to pointin the directions you require to the charges from the centre. The magnitudes are then as you have calculated.