ok, so I have a physics test monday, and I know the prof will make us calculate the e-fields, and voltages everywhere is space produced by various objects with uniform charge densities using Gauss's law. So, he wants us to derive a general expression that describes the e-field and voltage everywhere in space around a charged object, say, as a function of the radius....he also will want us to determine the capacitance due to a certain voltage between two oppositely charged objects. ok, so that isn't too bad, right?(adsbygoogle = window.adsbygoogle || []).push({});

Well, I know he is going to give us a problem where we have a solid sphere of uniform charge density (+) and this solid sphere is surrounded by a spherical shell with uniform surface charge density (-). He will want us to determine the e-field everywhere (meaning, inside the inner solid sphere, in between the inner solid sphere and outer shell, and outside the outer shell.

I have trouble seeing how the fields from the outer, middle, and inner regions cancel one another out or add to one another. Inside of the solid sphere, the fields cancel out. In between the outer shell and the inner solid sphere it seems like the fields would add, since they are pointing in the same direction, right? Finally, outside the outer shell, it would just be the field due to the shell, right? no charge would contribute to the outer shell's field, other than the outer shell charge, correct? I just want to ensure that my line of thinking is correct here.

Also, we have only talked about electric fields so far, but I am aware that magnetic fields are a different phenomenon...can you kinda explain the difference to me? From what I understand, electric fields are simply forces produced by stationary charges (where the efield is measured in force/coulomb), and these fields can facilitate work on other charged objects. As I understand it, magnetic fields are a different ype of field, but related to electric fields, that is due to charges in motion, right? Can someone elaborate on this for me?

Also, how do e-fields and m-fields tie in with electromagentic fields. I am just horribly confused by all of this stuff lately.....it is making me depressed. :(

Also, what exactly are electromagnetic waves? Are they just electromagentic fields?

bah...

Oh, and the prof said he is putting a quadrupole problem on the test, which consists of determining the field produced by an arrangement of equal charges in the following order: + - + -. The charges are on the same axis. The distance between the charges is 'd'. He wants us to find the e-field at a point a distance 'a' from the first charge, where a >> d. the e-field point lies on the same axis as the 4 charges. I would just reason that since a >> d then the fields due to the charges are about the same magnitude and cancel out, since a ~= a + d ~= a + 2d ~= a + 3d. However, they don't QUITE cancel out and a more accurate answer can be determined....HE (evil bastard) wants us to use the binomial theorem to determine the e-field. :grumpy: Can somone explain how this problem would be solved? This leaves me very confused.....he did it in class, but I missed part of the lecture (yes, I go to 99% of my lectures...). I went to him and asked for his help, and he was quite useless, and nobody else in the class really knows how to do it either...There's a 99% chance this problem will be on the test. I talked to a physics tutor the other day and he was having trouble with the problem too.

Any help with any of these problems would be greatly appreciated.

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# Homework Help: E-field questions

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