# Vectors are confusing me. I'm not sure if I'm doing it right

1. Jul 17, 2016

### josh12399

1. The problem statement, all variables and given/known data
Charge q1 = +8.36 μC is fixed at the origin and charge q2 = -4.28μC is fixed on the +x-axis, 0.371m from the origin.

(a) Find the direction and magnitude of the electric field at a point P that has coordinates (0.466, 0.466) m.

(b) Find the direction and magnitude of the force on a -2.00 μC charge placed at the point P. (Please state the direction as an angle measured counterclockwise from the positive x-axis.)

2. Relevant equations
E = Kq/r^2

3. The attempt at a solution

Find θ1 = atan(0.466/0.466) = 45 degrees
θ2 = atan(0.466/0.095) = 78.48 degrees

I figured you just add the two vectors. For some reason I dont know how to... What is confusing me is that a can't use -4.28μC, for example, as the actual value. I think I need to use E=kq/r2 Here's what the diagram looks like

I really appreciate any help

EDIT:

I got the first part. It is attached
Would the second part just be F=Eq?

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Last edited: Jul 17, 2016
2. Jul 17, 2016

### Staff: Mentor

You should review this as its important.

Bascially convert the two vectors into components along the x and y axis and go from there.

3. Jul 17, 2016

### josh12399

But what values should I be using?

4. Jul 17, 2016

### SammyS

Staff Emeritus
Hello josh12399 . Welcome to PF .

The resultant you have for E1 + E2 looks to be correct providing that you used correct magnitudes for E1 and E2. (You have placed the vectors in a rather unusual position relative to point P.)

What did you get for the magnitudes of E1 and E2 ?

5. Jul 17, 2016

### josh12399

To be honest I have no idea what to use for the magnitude. I tried a bunch of different things and got totally different answers.

I thought of splitting them up into EX1, EY1, EX2 and EY2, but I don't know what the actual magnitude should be. Should it be (k*q*cosθ)/r2 for the x and (k*q*sinθ)/r2 for the y?

6. Jul 17, 2016

### SammyS

Staff Emeritus
Use the following:
for the electric field (magnitude) due to each charge.

7. Jul 17, 2016

### josh12399

For each component (x and y)? Or should I just use the pythagorean theorem to find the diagonal distance from the charge to P?

8. Jul 17, 2016

### SammyS

Staff Emeritus
... just use the Pythagorean theorem to find the diagonal distance from the charge to P

9. Jul 17, 2016

### josh12399

E1 = 1.730*105 N/C
E2 = -1.701*105 N/C

Now I take the X and Y components using sine and cosine?

10. Jul 17, 2016

### SammyS

Staff Emeritus
Yes.