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The positive and negative charges appear only on one side of each plate so I don't think there should be any field outside.

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- Thread starter zorro
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- #1

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The positive and negative charges appear only on one side of each plate so I don't think there should be any field outside.

- #2

LeonhardEuler

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In reality, there is a nonzero field outside the plates of a capacitor because the plates are not infinite. A charged particle near the plates would experience a stronger force from the closer plate that is not totally canceled out by the farther one.

And to answer your other question, edge effects are ignored when deriving the simple expression for the field between parallel plates of charge you are probably referring to, but they can also be taken into account to give a more complicated expression.

- #3

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and the fields of oppositely charged plates cancel outside, but reinforce each other between the plates.

I did not get this point.

Refer the figure:

At the point P (not far away from the positive plate), there is a net electric field towards left.

There is a net electric field towards right at the point Q.

(btw do we consider the charge on one side of negative plate to find the electric field at Q? Or is it isolated from Q?)

but they can also be taken into account to give a more complicated expression.

Wow...we can derive the field near the edges too? Never knew that.

- #4

LeonhardEuler

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If the plates were infinite, the two plates would completely cancel each other outside the region between the plates.

- #5

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In reality, there is a nonzero field outside the plates of a capacitor because the plates are not infinite. A charged particle near the plates would experience a stronger force from the closer plate that is not totally canceled out by the farther one.

Now I don't understand this point

Can't we apply this explanation of yours to the above statement? -

"at the point P, the field from the positive plate pushes to the left, and the negative one pushed to the right, so the fields tend to cancel in this region. In between the plates, the positive plate pushes to the right and the negative one pulls to the right, so the fields reinforce."

- #6

LeonhardEuler

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Can't we apply this explanation of yours to the above statement? -

"at the point P, the field from the positive plate pushes to the left, and the negative one pushed to the right, so the fields tend to cancel in this region. In between the plates, the positive plate pushes to the right and the negative one pulls to the right, so the fields reinforce."

Yes, but the key is "

- #7

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Are you saying that non-zero electric field is due to increased/decreased field due to edge effects ?

- #8

LeonhardEuler

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Are you saying that non-zero electric field is due to increased/decreased field due to edge effects ?

You have that backwards: The electric field is independent of distance

- #9

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How do you prove that the field at a nearby point outside the capacitor is not 0?

- #10

LeonhardEuler

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[tex]F_{tot} = \int_{0}^{\theta_{MAX}}g(\theta)d\theta[/tex]

We don't need to know what form g has. The only important thing is that each ring contributes something to the force, and the sign is the same. For a positive plate the field will be away from the plate.

Now look at a plate farther down from this one. The integral is exactly the same, but [itex]\theta_{MAX}[/itex] is less. So the force must be less.

- #11

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hmmm....I understood it clearly now.

Thank you very much!

Thank you very much!

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