# Electric fields, Superposition, and Interneing Matter

1. Sep 24, 2011

### johns120

Edit: This is my first post. This question isn't related to any homework problems but was inspired by my physics book. I apologize if this is the wrong forum but It seemed the right place based on the other posts.

So my physics textbook briefly mentions

What I don't understand is: If the paper is put between the charges, those charges would induce a dipole, right? Since there are charges on both sides and "like-charged" which way would the dipole point. The book says the tape is attracted to the paper (which your left to infer creates a force pulling each piece of tape in towards the paper) I just can't visualize how this is possible and I certainly don't understand what the superposition principle has to do with it.

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Last edited: Sep 24, 2011
2. Sep 26, 2011

### Naty1

Hi johns....since 178 views have occurred without an answer, I'll give it a try.

I agree that example raises some questions....I'd ask the physics teacher or another student what they make of the "dipole".

In physics, Maxwell's equations imply that the distributions of charges and currents are related to the electric and magnetic fields by a linear transformation....so the superposition principle CAN be used to analyize fields which arise from given charge or current distribution.

I take it the objects on each side of the paper are "like charged pieces of tape".

In a conductor, which paper is not, any induced charges would reside on the surface of the (paper) plane,and the interior of the conductor would generally have a zero electromagnetic field for static conditions. I don't necessairly see a "dipole" condition.

With paper, an insulator, there is likely some interior charge and surface charge caused by attraction to the adjacent pieces of tape. I think your text is making the point that such an intermediate charge will weaken the fields of the two adjacent liked charged tapes assuming some charge builds up. With an isolated piece of paper, I don't think that would happen, but we assume that it does...a strong field could ionize air, for example. That charge would be opposite to the charges of the two adjacent tapes, and I would not expect a dipole configuration in this case either.

So I'd expect the field of the induced charges to "superimpose" it's field on that of the tapes, and I'd expect it to weaken the overall field, but I don't see that a dipole in induced on the paper whether a conductor or an insulator. Maybe the text is assuming that if, say, negative charge builds directly between the tapes, then positive charge will be left at the distant sides of the paper??