# Homework Help: A Gauss sphere problem

1. Dec 3, 2011

### Jimmy84

1. The problem statement, all variables and given/known data
A solid, insulating sphere of radius a has a uniform
charge density 1 and a total charge Q. Concentric with this
sphere is an uncharged, conducting hollow sphere whose
inner and outer radii are b and c, Find the magnitude of the electric field in the
regions r=b and r= c?

2. Relevant equations

3. The attempt at a solutionIm stuck finding the charge there my guess is that since there is a charge one in the insulating sphere and none on the conducting one then the charge on b and on c must be 1 as well?

Im somewhat confusing, please any comment?

2. Dec 3, 2011

### Staff: Mentor

What's the electric field within the material of a conductor?

3. Dec 3, 2011

### Jimmy84

Also I dont understand polarization at all for isntance problem 24,56 from serway

Consider two identical conducting spheres whose surfaces
are separated by a small distance. One sphere is given a
large net positive charge while the other is given a small
net positive charge. It is found that the force between
them is attractive even though both spheres have net
charges of the same sign. Explain how this is possible.

From the solutions

P24.56 The sphere with large charge creates a strong field to polarize the other sphere. That means it pushes the excess charge over to the far side, leaving charge of the opposite sign on the near side.This patch of opposite charge is smaller in amount but located in a stronger external field, so it can feel a force of attraction that is larger than the repelling force felt by the larger charge in the weaker field on the other side.

In sumary what is it that is going on in tis process can anyone please explain? Thnaks so much.

4. Dec 3, 2011

### Jimmy84

zero but as far as I understand cause the charge must be zero inside a conductor. but im concerned about the outer sides of the conductor if it is a none charged conductor then the electric field there should have the same charge of the insulating sphere isnt it?

5. Dec 3, 2011

### Staff: Mentor

Right.
The charge on the inner surface of the shell will be equal and opposite to the charge of the inner sphere, which is Q. But that's not needed to answer the question about the field between b and c.

6. Dec 3, 2011

### Jimmy84

Thanks a lot, the questions of the first problem was about r=b and r=c ,not about b being less than r that is less than c.

About problem 2 how can two spehre of positive sign create an atracting force? what are the charges for the sphere that has a lot of charge, and for the one that has less charge ?

Say if the first sphere has a charge of +100 C , and the second sphere has a charge of +1 C , then what is the charge of each sphere after the polarization?

7. Dec 3, 2011

### Staff: Mentor

Polarization doesn't change the charge, it just rearranges them.

The like charges can be moved further away, leaving the opposite charges closer together. The resulting attraction will be greater than the repulsion, giving a net attractive force.