# Determine the magnitude of the force on each charge

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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BvU
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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 ?

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

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

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

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

BvU
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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 ?

• 1 person
okay, I agree.

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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BvU
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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...

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

BvU
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Yes and yes, IMHO.

• 1 person
SammyS
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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?
Do like charges attract, or do they repel?

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

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

Mastering Physics is still counting the angles wrong :/

SammyS
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Mastering Physics is still counting the angles wrong :/

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

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

Here is how it looks

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SammyS
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Maybe they want

θ1, θ2, θ3, θ4 =

315, 225, 135, 45

but I see you have only 2 tries remaining.

• 1 person
I got it! ..... thanx guys!

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SammyS
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I got it! ..... thanx guys!
That's good to hear.

M P really should have taken your answers with the negative angles. They give exactly the same orientation for the vectors.

BvU