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

MitsuShai
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

Homework Helper
Hi MitsuShai! (try using the X2 and X2 icons just above the Reply box )
… 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. 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? MitsuShai
Hi MitsuShai! (try using the X2 and X2 icons just above the Reply box )

They are. 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? 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....

Homework Helper
Hi MitsuShai! (have a square-root: √ and an epsilon: √ and a pi: π )
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. (btw, i've just noticed i should have said "y" not "x" in my last post )

MitsuShai
Hi MitsuShai! (have a square-root: √ and an epsilon: √ and a pi: π )

Show us what you get. (btw, i've just noticed i should have said "y" not "x" in my last post )

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)

Homework Helper
Hi MitsuShai! (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! MitsuShai
Hi MitsuShai! (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! 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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Homework Helper
Hi MitsuShai! (please use the X2 icon just above the Reply box )
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 MitsuShai
Hi MitsuShai! (please use the X2 icon just above the Reply box )

that's correct …

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