Capacitance concentric spherical shells

In summary, the problem involves finding the capacitance of two concentric spherical metal shells with radii a and b (a < b) and surface charge densities Sa and Sb, where Sa = -Sb. The attempt at a solution raises questions about the definition of capacitance in this case and the ambiguity of the problem, as it is unclear which shell the charge is applied to. Further clarification is needed to accurately solve the problem.
  • #1
Anoonumos
16
0

Homework Statement


Given two concentric spherical metal shells, with radii a and b (a < b), and surface charge densities Sa and Sb.
Find the capacitance if Sa = - Sb.


Homework Equations



C = Q/V

The Attempt at a Solution


I would know how to solve this if the absolute values of the charges were equal, but here the densities are equal and not the charges. I don't know how the capaticance is defined in this case. How do I solve this?
 
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  • #2
What is the electric field between the shells?

What is the relationship between electric field and electric potential?

[Edit: Oh, and I just realized that you'll also have to calculate the electric potential is outside both shells too, since the absolute values of the charges are not equal. So what is the electric field and/or the electric potential outside of the outer shell?]

[Second edit: Thinking about this problem a little more, the problem might be a little ambiguous. I think the answer depends on which shell the charge is applied. I.e. in the C = Q/V, is the charge Q applied to shell a with the charge on shell b following it's own charge accordingly, or is the charge applied to shell b with shell a following accordingly. In other words, I think it matters how Q is defined. How does the measured charge get to the system. By what mechanism/configuration is it that Q is being measured, and thus the capacitance. Is there a wire attached to shell a that can measure the current)and some hidden mechanism on shell b that ensures the σb = -σa relationship, or is it the other way around (swapping a and b)? Or, is Q defined as the net charge on both spheres?

By all that I mean it can be looked at the following way. When you measure the capacitance of something you measure it between two terminals. Say between two wires. But the problem statement didn't specify where these wires are. Is one wire attached to each shell? And if so, is the extra change (that keeps the σb = -σa relationship) flowing in/out of shell a or shell b? Or is one of the wires out at infinity, similar to a single-shell, spherical capacitor? ... I'm starting not to like this problem. :frown: ]
 
Last edited:
  • #3
Hi,

I'll ask if they could have meant equal charges instead of equal densities.
I'll let you know.
Thank you for your time.
 

1. What is capacitance concentric spherical shells?

Capacitance concentric spherical shells is a concept in electromagnetism that describes the capacitance between two spherical conductors with different radii and centered at the same point. It is also known as the capacitance of a spherical capacitor.

2. How is the capacitance of concentric spherical shells calculated?

The capacitance of concentric spherical shells can be calculated using the formula C = 4πε0εr(ab)/(b-a), where ε0 is the permittivity of free space, εr is the relative permittivity of the material between the shells, a and b are the radii of the inner and outer shells, respectively.

3. What factors affect the capacitance of concentric spherical shells?

The capacitance of concentric spherical shells is affected by the distance between the shells, the radii of the shells, and the types of materials used for the shells and the space between them. It also depends on the dielectric constant of the material between the shells.

4. How does the capacitance of concentric spherical shells change with the radius of the shells?

The capacitance of concentric spherical shells is directly proportional to the radius of the outer shell and inversely proportional to the radius of the inner shell. This means that as the outer shell's radius increases, the capacitance also increases, while the inner shell's radius decreases, the capacitance decreases.

5. What is the significance of capacitance of concentric spherical shells?

The capacitance of concentric spherical shells is an important concept in understanding and designing electrical circuits and devices. It helps in calculating the energy stored between the shells and the potential difference between them. It is also used in practical applications such as capacitors in electronic devices and power transmission systems.

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