Physics-Electric Fields: Can someone please explain this to me?

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Homework Help Overview

The discussion revolves around understanding the forces acting on point charges in an electric field, specifically in a configuration involving multiple charges positioned at the corners of a square. The original poster expresses confusion regarding the calculations and the application of coordinate systems in determining the resultant forces.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants explore the relationships between the forces exerted by the charges, questioning the coordinate system used and how it affects the calculations. There is a focus on the components of the forces along the diagonal and the implications of these components on the total force calculation.

Discussion Status

Participants are actively engaging with each other's reasoning, attempting to clarify misunderstandings about the force components and their contributions to the total force. Some guidance has been offered regarding the need to consider components of forces rather than the forces in their entirety.

Contextual Notes

There appears to be some confusion regarding the coordinate system and the assumptions made about the positions of the charges, which are under discussion. The original poster also notes a discrepancy between their calculations and the expected result, indicating a need for further exploration of the mathematical relationships involved.

MitsuShai
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My professor didn't explain this well.

Question: http://i324.photobucket.com/albums/k327/ProtoGirlEXE/q1.jpg


Answer: (part 1) http://i324.photobucket.com/albums/k327/ProtoGirlEXE/q2.jpg
(part 2) http://i324.photobucket.com/albums/k327/ProtoGirlEXE/q3.jpg


I'm completely lost on this one. I don't understand how this problem was solved.

So I'm guessing q4 is the point where you measure the forces from the other 3 points. But I thought that q3 won't have a y value and q1 won't have an x value.
So q2 is only measured by the diagonal right? So it would just be F= k Q^2/(2L^2)---I understand this

why wouldn't q1 and q3 just be F=k Q^2/L^2?

I would like it if someone would explain this whole problem because I feel like I'm completely lost on it.
 
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Hi MitsuShai! :smile:

(try using the X2 and X2 icons just above the Reply box :wink:)
MitsuShai said:
… But I thought that q3 won't have a y value and q1 won't have an x value.
So q2 is only measured by the diagonal right? So it would just be F= k Q^2/(2L^2)---I understand this

why wouldn't q1 and q3 just be F=k Q^2/L^2?

They are. :smile:

I think you're confused about what the x and y coordinates are.

Your professor has looked at the diagram, and decided that it's obvious that the total force will be along the diagonal …

and so he's decided to make his x coordinate in that direction (instead of along the bottom of the square, as you'd expect).

Does his work make sense now? :smile:​
 
tiny-tim said:
Hi MitsuShai! :smile:

(try using the X2 and X2 icons just above the Reply box :wink:)


They are. :smile:

I think you're confused about what the x and y coordinates are.

Your professor has looked at the diagram, and decided that it's obvious that the total force will be along the diagonal …

and so he's decided to make his x coordinate in that direction (instead of along the bottom of the square, as you'd expect).

Does his work make sense now? :smile:​



Oh ok I think I know what you mean. The forces are the diagonal [F= k Q[SUP]2[/SUP]/(2L2)] and the x and y components of the diagonal, which are F=k Q2/L2 each.
And to get the total forces, you have to add up these forces, but the answer is suppose to be [Q2/(8pi*epsilon_0*L2)] (1+2sqrt(2)) and you don't get that with these forces...
 
Hi MitsuShai! :smile:

(have a square-root: √ and an epsilon: √ and a pi: π :wink:)
MitsuShai said:
And to get the total forces, you have to add up these forces, but the answer is suppose to be [Q2/(8pi*epsilon_0*L2)] (1+2sqrt(2)) and you don't get that with these forces...

Show us what you get. :smile:

(btw, I've just noticed i should have said "y" not "x" in my last post :rolleyes:)
 
tiny-tim said:
Hi MitsuShai! :smile:

(have a square-root: √ and an epsilon: √ and a pi: π :wink:)Show us what you get. :smile:

(btw, I've just noticed i should have said "y" not "x" in my last post :rolleyes:)
F(total)= [ k Q2/(2L2)] + [ k Q2/(L2)] + [ k Q2/(L2)] = [ k Q2/(2L2)] + [2k Q2/(L2)]= [ k Q2/(2L2)] + [4k Q2/(2L2)]= 5k Q2/(2L2)= 3k Q2/(L2)
 
Hi MitsuShai! :smile:

(just got up :zzz: …)
MitsuShai said:
F(total)= [ k Q2/(2L2)] + [ k Q2/(L2)] + [ k Q2/(L2)] = [ k Q2/(2L2)] + [2k Q2/(L2)]= [ k Q2/(2L2)] + [4k Q2/(2L2)]= 5k Q2/(2L2)= 3k Q2/(L2)

ah I see …

no, you need to use the component of F1 and F3 along the diagonal, not the whole of F1 and F3 …

try again! :smile:​
 
tiny-tim said:
Hi MitsuShai! :smile:

(just got up :zzz: …)ah I see …

no, you need to use the component of F1 and F3 along the diagonal, not the whole of F1 and F3 …

try again! :smile:​
Oh right, I forgot about that part, so
F(total)= [ k Q^2/(2L^2)] + [ k Q^2/(L^2)]sin(45) + [ k Q^2/(L^2)]cos(45)= 2[ k Q^2/(L^2)](1/sqrt(2)) + [ k Q^2/(2L^2)]= [ k Q^2/(2L^2)](1/sqrt(2)) + 4k Q^2/(2L^2)][1/sqrt(2)/(1/sqrt(2)] I'm doing this wrong, somehow... :/
 
Last edited:
Hi MitsuShai! :smile:

(please use the X2 icon just above the Reply box :redface:)
MitsuShai said:
2[ k Q^2/(L^2)](1/sqrt(2)) + [ k Q^2/(2L^2)]

that's correct …

i can't see where you've gone wrong after that :confused:
 
tiny-tim said:
Hi MitsuShai! :smile:

(please use the X2 icon just above the Reply box :redface:)


that's correct …

i can't see where you've gone wrong after that :confused:



Where do I go from there? I was thinking of adding them and to do that I would need to have common denominators, so I would have to get common denominators and I just noticed that I typed that in wrong... ._.

2[ k Q^2/(L^2)](1/sqrt(2)) + [ k Q^2/(2L^2)]= [ k Q^2/(2L^2)](1/sqrt(2)) + 4k Q^2/(2L^2)](1/sqrt(2))= 5k Q^2/(4L^2), which isn't right
 

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