# AP Physics C E&M

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1. May 12, 2013

### sarvesh0303

1. The problem statement, all variables and given/known data
Could somebody help me on the problems given in the attachments? I know E&M well but these problems stumped me(In the RC Circuit I need help only on part (d) of the question). Tomorrow is my AP C E&M exam. I am too nervous about it. Based on how these problems stumped me, do you think I should write tomorrow or should I write it next year or next semester.

(Note: Since the course is not offered in my school or neighborhood, I learnt(and still am learning) E&M through a course on edX(I joined the course before planning to write the exam))

Do I need to know how to solve second order ODEs to solve the RC Circuit?

2. Relevant equations
Gauss' Law
Kirchoff's Law
Mathematical Definition of Potential Difference

3. The attempt at a solution

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2. May 12, 2013

### rude man

No.
Since the Forum says my answer is too short, I repeat - no.

3. May 12, 2013

### ehild

You do not need to solve ODE-s to solve the RC circuit. You might need to calculate an integral
in the other two problems. These are not difficult problems. If you are totally stumped, better not to try that exam tomorrow. Do you really have no idea how to solve the problems?

ehild

4. May 12, 2013

### sarvesh0303

I think I can solve the RC Circuit. But help with the other 2 would be really helpful(especially the first one). (I'm sorry if I sound like an idiot.)

5. May 12, 2013

### ehild

How would you find the potential at the point shown in the second figure?

ehild

6. May 12, 2013

### sarvesh0303

I just got an idea for the first problem. Could we use Gauss's Law with the point lying on the Gaussian Sphere? Or is it ineffective since the charge would not be uniformly distributed on the Gaussian surface?

I got the answer to the problem in the second thumbnail.

Last edited: May 12, 2013
7. May 12, 2013

### ehild

You can use Coulomb's Law to get contribution of the charge on a small piece on the arc to the potential at the given points. The potentials add up, so you get the potential at the given points by integration along the charged arc. Are the position of the points exactly given? Do you need to calculate the potential, or only guess where is it the highest? Note the distances between points of the arc and the given points.

ehild