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

  • Thread starter MitsuShai
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My professor didn't explain this well.

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


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


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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Answers and Replies

  • #2
tiny-tim
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Hi MitsuShai! :smile:

(try using the X2 and X2 icons just above the Reply box :wink:)
… 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:
 
  • #3
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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....
 
  • #4
tiny-tim
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Hi MitsuShai! :smile:

(have a square-root: √ and an epsilon: √ and a pi: π :wink:)
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:)
 
  • #5
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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)
 
  • #6
tiny-tim
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Hi MitsuShai! :smile:

(just got up :zzz: …)
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:
 
  • #7
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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..... :/
 
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  • #8
tiny-tim
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Hi MitsuShai! :smile:

(please use the X2 icon just above the Reply box :redface:)
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:
 
  • #9
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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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