Fixed charge inside a metallic sphere

[student85]

There is a question someone asked me and I'm not sure about the answer:
Suppose you have two identical positive, fixed in position, charged particles. One of them is inside a metallic sphere with air inside it, so it's more like an empty shell. What will be the force acting on the particle inside the sphere in comparison to the one outside it? Are there any differences? Does the sphere affect anything?

 

[ranger]

Gauss' law tells us that the electric field inside a sphere is zero. The electric field outside sphere is the same as the field of the [net] charge of Q.

Do you know why the force is zero inside a sphere? Mayb of you work out an example yourself, it may become clear. Just choose any point in the sphere and call it Q, if you apply some geometry to the situation, I'm sure you can figure out why forces cancel out.

 

[student85]

Even if there is a charge inside the sphere, you're telling me there is no charge inside the sphere?
I really don't think that. Take your gaussian surface as a sphere smaller than the actual metallic sphere and clearly there is a charge inside it.
So could someone please answer my question, please read it carefully.

 

[Hootenanny]

Gauss' law tells us that the electric field inside a sphere is zero.

This is true for a charged conducting spherical shell, but is not true for a point charge placed inside a spherical shell.

There is a question someone asked me and I'm not sure about the answer:
Suppose you have two identical positive, fixed in position, charged particles. One of them is inside a metallic sphere with air inside it, so it's more like an empty shell. What will be the force acting on the particle inside the sphere in comparison to the one outside it? Are there any differences? Does the sphere affect anything?

That depends. If your spherical shell in uncharged then the electric field of the point charge will not be perturbed; i.e. the electric field of either point charge will be unaffected. However, if your spherical shell is charged, then you can find the resultant electric field by finding the vector sum of the electric field from the spherical shell and the point charge(s).