# Coulomb's law in 3-D nightmare

1. Nov 14, 2013

1. The problem statement, all variables and given/known data

Charge q1 = 5 μC is at position ( 1 m; 2 m; -1 m) and a second charge q2 = -3 μC is in position ( -2 m; 1 m; 3 m). Find:

a) The electrical force in vectorial form exerted on q1;
b) The electrical field in vectorial form at the origin of the system (0; 0; 0);
c) The electric potential at the origin of the system (0; 0; 0).

2. Relevant equations

Fe=k q1q2/r2

3. The attempt at a solution

I'm still at a)
I've tried calculating Fe and found 5,19*10-3
but i don't know how to put it in vectorial form
I think adding the 2 vectors should give me this (-1, 3, 2)

2. Nov 14, 2013

### ehild

3. Nov 14, 2013

Hey! Thanks for anwsering :)

I've seen this formula online while looking for help, but i don't understand how to apply it.

4. Nov 14, 2013

Do you know where I could find an example to be able to picture it? Because at this point I'm ready to give up entirely :(

5. Nov 15, 2013

### ehild

See picture attached. The force Q2 exerts on Q1 (F12) acts in the line that connects them, that is, F12 has the same direction as r12 = r1-r2.

$$\vec F_{12}= k\frac {Q_1Q_2}{|\vec r_{12}|^3} \vec r_{12}$$

ehild

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Last edited: Nov 15, 2013
6. Nov 15, 2013

I still don't get it but thanks for trying. Have a good day!

7. Nov 15, 2013

### Staff: Mentor

In part (a), what is the distance between the two charges? Can you express a relative position vector from point 1 to point 2 in component form? If so, what is the relative position vector? Do you know how to determine the components of a unit vector in the same direction as the relative position vector? If so, what are the components of this unit vector? What is the relationship of the direction of this unit vector to the direction of the force between points 1 and 2?