[E&M] Concentrical metalic spheres

[Xsnac]

Homework Statement

I two concentric metalical spheres r1 =/= r2.
What to do to have:
a) potential energy = 0 and electric charge =/= 0
b) potential energy =/= 0 and electric charge =0.

In eighter one of the spheres.

Relevant equations
Gauss law.

The Attempt at a Solution

a)
If I charge the outside sphere, I will have potential energy = 0 on it and electric charge =/= 0.

b)
I tought If I charge the one inside with Q, I will have a potential =/= 0 on the bigger sphere and 0 charge.

Is this correct? The problem does not give correct answer.

 

[Physicist97]

Do any of the spheres have an initial charge on them? And are you at a point in between the two spheres?

 

[Xsnac]

I'm outside and I can charge eighter the bigger or the smaller or both of them. I have to have the specs from eighter a) or b) on 1 of them.

 

[Physicist97]

Well, when you have a net positive or negative charge the potential will approach zero only at infinity (unless otherwise specified such as the ground on a circuit). If you charge the outside sphere, why would potential energy be zero on it? May you show us what equation you are using for electric potential energy?

 

[SammyS]

Homework Statement

I two concentric metallic spheres r1 =/= r2.
What to do to have:
a) potential energy = 0 and electric charge =/= 0
b) potential energy =/= 0 and electric charge =0.

In either one of the spheres.

Relevant equations
Gauss law.

The Attempt at a Solution

a)
If I charge the outside sphere, I will have potential energy = 0 on it and electric charge =/= 0.

b)
I thought If I charge the one inside with Q, I will have a potential =/= 0 on the bigger sphere and 0 charge.

Is this correct? The problem does not give correct answer.

Electric potential

or

Potential energy stored in the system?

 

[Xsnac]

Electric potential

or

Potential energy stored in the system?

Electric potential

 

[haruspex]

Electric potential

That makes more sense. Electric potential is different from electrical potential energy.
So, what equation do you have for finding the electric potential at a given distance from a uniformly charged sphere?

 

[Xsnac]

That makes more sense. Electric potential is different from electrical potential energy.
So, what equation do you have for finding the electric potential at a given distance from a uniformly charged sphere?

k * q1 / r ( where K is 1/4 pi epsilon )

 

[SammyS]

k * q1 / r ( where K is 1/4 pi epsilon )

What does "r" represent ?

 

[Xsnac]

What does "r" represent ?

the distance from where the electric potential is 0 to where you want to measure the electric potential.

 

[haruspex]

the distance from where the electric potential is 0 to where you want to measure the electric potential.

No. Check your course notes.

 

[Xsnac]

No. Check your course notes.

that's what I have written in the manual. Electric potential V = Q/(4*pi*epsilon) * 1/r. Should I upload a picture of the manual?

 

[haruspex]

that's what I have written in the manual. Electric potential V = Q/(4*pi*epsilon) * 1/r. Should I upload a picture of the manual?

I'm not querying your equation. It's your definition of r that is wrong.

 

[Xsnac]

then I'm lost. what's the definition? I always tought it is the distance between the place where electric potential is 0 to where you want to measure the electric potential..

 

[haruspex]

then I'm lost. what's the definition? I always tought it is the distance between the place where electric potential is 0 to where you want to measure the electric potential..

The equation you quoted can be applied in two contexts.
In the most basic one it gives the potential at distance r caused by a point charge Q, if taking the potential at infinity to be zero.
It also works if the charge is spread uniformly over a spherical shell or through a solid sphere. In these cases, r is the distance from the centre of the sphere, and the equation only works if r is greater than the radius of the sphere.