# Coulomb's Law and point charges

1. Apr 30, 2014

### meme177

1. The problem

Two negative and two positive point charges (magnitude Q = 3.16mC ) are placed on opposite corners of a square (side lengths a = 0.105m )

#1. Determine the magnitude of the force on each charge.

I GOT..... F1,F2,F3,F4 =7.44x10^6N (which was marked correct)

I am having trouble with this question.....

#2. Determine the direction of the force on each charge.
I know (Fy/Fx)=1
so tan-1(1)=45

Its asking for θ1,θ2,θ3,θ4 and I have no idea how to get the angles :(

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2. Apr 30, 2014

### BvU

Hello meme, and welcome to PF.

If you know the tangent, you know the angle (plus or minus pi), isn't it ?
However, the "I know (Fy/Fx)=1" applies to which of the charges, exactly ? How about the other three ?

And: have you altready defined what theta is ? Or was it given ?

3. Apr 30, 2014

### meme177

θ1,θ2,θ3,θ4
I have tried these answers (in order)
225,225,225,225=WRONG (but then I was like NO this is impossible)
Then I tried
135,45,315,225=WRONG

I don't know what to do anymore

4. Apr 30, 2014

### BvU

the "I know (Fy/Fx)=1" applies to which of the charges, exactly ?
It can surely not apply to all four of them !

5. Apr 30, 2014

### meme177

Charge #2 where I calculated the y and x component of the force for question #1

6. Apr 30, 2014

### meme177

I think I see it! charge #2 is 45 .......Charge#1 (45+90=135)......charge#4 (180+45=225).....charge#3 (45+90=135) ??

7. Apr 30, 2014

### BvU

Still a small matter of signs: my hunch for e.g. #2 is that two attracting forces from charges closer by overcome one repelling force from #4. From there, with symmetry considerations, all forces are towards the center of the square. Agree ?

8. Apr 30, 2014

### meme177

okay, I agree.

9. May 1, 2014

### meme177

with the forces all pointing to the middle, they cancel (no movement)
Now I have θ1= -45 θ2= -135 θ3= 45 θ4=135
Is this correct, now?

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10. May 1, 2014

### BvU

So theta is the angle wrt the positive x-axis.
I think you swapped 3 and 4 (in your post not in the drawing)
Movement doesn't apply here. The charges are positioned in fixed given places,
and there is no mention of how they are kept there...

11. May 1, 2014

### meme177

Yes, so θ1= -45, θ2= -135, θ3= 135, θ4=45 ? Am I doing the angles wrt positive x-axis right?

12. May 1, 2014

### BvU

Yes and yes, IMHO.

13. May 1, 2014

### SammyS

Staff Emeritus
Do like charges attract, or do they repel?

14. May 1, 2014

### BvU

@Sam: Meme is talking about the net forces, I should think. The magnitude was already calculated in part #1 of post #1.

15. May 1, 2014

### SammyS

Staff Emeritus
Okey-Dokey .

16. May 1, 2014

### meme177

Mastering Physics is still counting the angles wrong :/

17. May 1, 2014

### SammyS

Staff Emeritus

Otherwise M P may want you to specifically say, degrees, or ° .

18. May 1, 2014

### meme177

no, the units are already there (degrees) counterclockwise from the +x axis

19. May 1, 2014

### meme177

Here is how it looks

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20. May 1, 2014

### SammyS

Staff Emeritus
Maybe they want

θ1, θ2, θ3, θ4 =

315, 225, 135, 45

but I see you have only 2 tries remaining.