# Homework Help: Does it work this way (intro electrodynamics)?

1. Apr 22, 2010

### monkeykoder

1. The problem statement, all variables and given/known data
Have a permanently (uniformly not radially) polarized sphere surrounded by a charged conductor
How is the charge on the conductor distributed (the added charge)?

2. Relevant equations

3. The attempt at a solution

Since the conductor cancels the field generated by the polarized sphere I'm thinking the added charge should distribute uniformly over the surface of the conductor is there any reason it shouldn't?

2. Apr 22, 2010

### gabbagabbahey

Is the electric field produced by the polarized sphere going to be uniform over the spherical surface of the conductor? If not, why would you think that the charge distribution would be?

3. Apr 22, 2010

### monkeykoder

My reasoning (probably flawed) is that the field due to the polarized sphere is taken care of through polarization of the conductor then the added charge should be free of the field.

4. Apr 22, 2010

### gabbagabbahey

Definitely flawed. The polarized sphere produces some non-radial electric field. That electric field will push the free charges in the conductor around until they reach an arrangement that cancels the field. You should know that a uniform distribution of charge on a spherical surface produces an electric field that is radial...how can a radial electric field cancel a non-radial one?

Instead of guessing at the answer, try applying some of the methods and equations you've covered in your course.

5. Apr 22, 2010

### monkeykoder

I've calculated the charge distribution due to the polarized sphere I can't figure out how to work in the charge that has been placed on the conducting shell.

So assuming no charge on the conducting sphere $$\sigma = \frac{3Q_{b}cos(\theta)}_{\pi b^{3}}$$

I can't find a way to include the added charge other than assuming that it distributes free of the field.

Last edited: Apr 22, 2010
6. Apr 22, 2010

### gabbagabbahey

What are $b$ and $Q_b$ supposed to represent? What is the entire original problem?

7. Apr 22, 2010

### monkeykoder

b is the radius of the outer sphere $$Q_b$$ is the bound charge on the inner polarized sphere.

8. Apr 22, 2010

### gabbagabbahey

Again, what is the entire original problem statement?

9. Apr 22, 2010

### monkeykoder

Specifically a non-neutral conductor charged to a value Q is submitted to an electric field how do I calculate how the additional charge is distributed.

10. Apr 22, 2010

### gabbagabbahey

That really does not sound like a complete problem statement to me. There is a reason why the homework template tells you to include "all variables and given/known data". Are you given the polarization? Are you told the net charge on the conductor? Are the spheres concentric?

11. Apr 22, 2010

### monkeykoder

Problem I'm having is I'm trying to ask a question that helps me with a test problem I have without getting help on the test problem itself.

12. Apr 22, 2010

### gabbagabbahey

Well, the general method is to solve Laplace's equation (Poisson's equation in regions where the charge density is non-zero) subject to the boundary conditions of your problem. And then find the surface charge density from the potential.

That being said, the idea of a take home exam (which I gather this is for) is to test your knowledge of the material and your mastery of the problem solving methods you have learned. You really should not be seeking help for an exam when you are not permitted to do so.

13. Apr 22, 2010

### monkeykoder

That's why I'm having trouble asking a question in such a way as to not have the test question answered for me but to get a better grasp on the conceptual idea. I have the idea figured out now I'm pretty sure so I'll get on with it now. I'm sorry for posting this in the homework help section but I couldn't figure out how to get the conceptual part without a problem to work through.