# Solid sphere inside a hollow sphere.

• budder
In summary: Since the total charge on the hollow sphere--counting both inner and outer surfaces--happened to be zero, the charge on the outer surface must also be zero.
budder
Question Details:
The figure shows a solid metal sphere at the center of a hollow metal sphere.

1- What is the total charge on the exterior of the inner sphere?

2- What is the total charge on the inside surface of the hollow sphere?

3- What is the total charge on the exterior surface of the hollow sphere?

These problems seem easy, but I just can't figure them out. No similar examples are found in my book. Any help is much appreciated!

This is what I've done:
Create an imaginary Gaussian sphere of radius 8cm so that it passes through the point where E is 15000N/C.
The flux should equal to (-15000)(4pi0.08^2) = q/(8.85x10^-12) the answer i get is wrong!

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budder said:
This is what I've done:
Create an imaginary Gaussian sphere of radius 8cm so that it passes through the point where E is 15000N/C.
The flux should equal to (-15000)(4pi0.08^2) = q/(8.85x10^-12) the answer i get is wrong!
Looks good to me!

If this is a web-based problem, you may have used the incorrect significant figures. Since the radius is only 1 significant figure of accuracy, your answer should be only 1 sig. fig. when typed in. Give that a try So... -1x10^-8C I think is what you get.

Steve

Maybe you DEVIDED instead of MULTIPLYING.. make sure to multiply the FLUX on the left side of the equation X Epsilon constant...

I have the exact same problem and I am having trouble with parts b and c. Wouldn't the inside surface of the hollow sphere just be the same charge as the solid inner sphere but positive?

Thanks,
KEØM

KEØM said:
Wouldn't the inside surface of the hollow sphere just be the same charge as the solid inner sphere but positive?
That's right.

Thank you for your reply. For part c, I can't say now that the charge on the outer surface of the hollow sphere is exactly negative of the charge on the inside surface of the hollow sphere can I? I think I have to use Gauss's Law and choose my gaussian surface to be a sphere with a radius 17cm long measured from the center of the solid inner sphere. Would this be right?

KEØM

KEØM said:
For part c, I can't say now that the charge on the outer surface of the hollow sphere is exactly negative of the charge on the inside surface of the hollow sphere can I?
No, that's not true in general. (That would be true if the total charge on the hollow sphere--counting both inner and outer surfaces--happened to be zero.)
I think I have to use Gauss's Law and choose my gaussian surface to be a sphere with a radius 17cm long measured from the center of the solid inner sphere. Would this be right?
Exactly right. Gauss's law will give you the total charge within the gaussian surface. That, along with the previous results, will tell you the charge on the outer surface.

Thanks again for all your help Doc Al.

KEØM

Question, I am doing the same problem, but i do not understand why a (-) sign is put for part A) but not B) & C).

ch2kb0x said:
Question, I am doing the same problem, but i do not understand why a (-) sign is put for part A) but not B) & C).
How do the answers for parts A) and B) relate to each other? How did you solve part B)?

Hey I'm just wondering where did you get this quesion from because this was on my midterm and if there's a database of these questions I'd like to study from them

Although I am not the original poster, I got mine from the textbook "Physics for Scientists and Engineers" by Randall D. Knight.

Sorry if that is not much help.

Why the inside surface of the hollow sphere has the same charge as the solid inner sphere but positive?

Junweingoh said:
Why the inside surface of the hollow sphere has the same charge as the solid inner sphere but positive?
Apply Gauss's law, realizing that the field anywhere within the conducting material must be zero.

## 1. What is a solid sphere inside a hollow sphere?

A solid sphere inside a hollow sphere is a physical model that consists of two concentric spheres, where the inner sphere is solid and the outer sphere is hollow.

## 2. What is the purpose of this model?

This model is used to demonstrate concepts such as density, mass, and volume in a tangible way.

## 3. How does the density of the inner sphere compare to the outer sphere?

The density of the inner sphere is greater than the density of the outer sphere. This is because the inner sphere is solid, while the outer sphere is hollow.

## 4. What happens to the mass of the model if the inner sphere is removed?

If the inner sphere is removed, the mass of the model will decrease. This is because the mass of the inner sphere is no longer present.

## 5. What factors affect the stability of this model?

The stability of this model can be affected by factors such as the size and weight of the spheres, the thickness of the walls, and the materials used to make the spheres.

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