# 3 concentric shells - potentials

1. Oct 27, 2008

### alexmahone

Consider 3 charged spherical shells A, B, C with charges $q_A, q_B, q_C$ and radii $r_A<r_B<r_C$

$V_A=\frac{1}{4\pi\epsilon}[\frac{q_A}{r_A}+\frac{q_B}{r_B}+\frac{q_C}{r_C}]$
$V_B=\frac{1}{4\pi\epsilon}[\frac{q_A}{r_B}+\frac{q_B}{r_B}+\frac{q_C}{r_C}]$
$V_A=\frac{1}{4\pi\epsilon}[\frac{q_A}{r_C}+\frac{q_B}{r_C}+\frac{q_C}{r_C}]$

Can anyone derive these? Thanks.

2. Oct 27, 2008

### borgwal

Apparently I wasted my time. You seem to want someone to solve the whole problem for you by splitting it into several threads. That's not the policy here. In any case, you wrote down the wrong equations.

3. Oct 27, 2008

### alexmahone

Actually you wasted my time on the previous thread, getting us nowhere. Anyway, I solved the problem myself so consider this thread closed.

4. Oct 27, 2008

### Hootenanny

Staff Emeritus
Actually, borgwal acted by the book in your other thread.

Perhaps if you paused for a moment in your other thread and actually thought about the question and the information borgwal was giving you, you might have been able to solve it sooner and with less help.

5. Oct 27, 2008

### alexmahone

In which case, I disagree with the forum rules. In some cases, it is more helpful to post the full solution than to beat around the bush for 30 posts.

6. Oct 27, 2008

### Hootenanny

Staff Emeritus
You can disagree all you like, but they won't change.
I disagree, particularly in the case of a forum. If we were to provide students with a complete solution then they could simply copy the solution and hand it in as homework without actually understanding the concepts involved. The student's teacher would then mark the homework and assume that since the student gave a correct solution, they totally understand the material and hence there is no need to offer the student additional help. However, when the student comes to sit the exam there is no-one there to give the student a complete solution and since the student never understood the material in the first case, they fail the exam. Not only does the student fail the exam, but the teacher also looks like a poor teacher for not picking up on their student's lack of knowledge.

That said, I see no problem with students being given complete solutions by their teacher once they have handed in an attempted solution. This way the student gets to see how the question should be tackled and the teacher knowns that the student doesn't understand the material.

I know that you're revising for a physics Olympiad and that your posts aren't homework, but they are exam questions and are therefore classed as homework.

In addition to the previous points, I personally feel that one learns more if one has to think hard about a problem oneself, rather than simply being given the solution. Solving problems independently, particularly in physics, is the only way to really understand the concepts rather than simply knowing how to answer certain types of questions.

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