# Lovely physics XD - A conducting rod inside a conducting shell - electricity/gauss

1. Aug 3, 2009

### vorcil

http://img197.imageshack.us/img197/2942/assgn3q1.jpg [Broken]

I took a good crack at this question, could somebody please check?

a)
I thought the shell could be modeled as a conductor with a hole in it,
as there is no electric flux inside a hole of a conductor and the charge of the shell is on the outside of the surface, there is no electric field, so the shell does not contribute to the electric field inside the shell

- this leaves the infinitely long rod!
I'm assuming the rod can be modeled as an infinite line of charge
and the formula for the electric field it creates is

(1/(4pi(epislon nought) ) ) * (2 lavender)/r
-n.b i would go through the proof of this formula but it's so long! and we all know it lol.

- the 2 cancels out 2pi
so the end formula is

lavender / 2pi * episolon nought * r

b) the inner surface charge density? LOL Inner and surface made me automatically think the surface charge density of the inside of the rod is 0

but i know you'll probably want my proof
i know that it is a conducting rod, so all the charges get sent to the surface of the rod and there is no electric field/and flux inside SO my awnser is 0?
(can someone tell me why the charges get sent to the 0? I thought that it would be uniformly charged in a conductor, but at electrostatic equillibrium)

c)(will postsoon, i just need some time to do them)

Last edited by a moderator: May 4, 2017
2. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

I got it wrong, It is non-zero apparently?

this is mastering physic's response:

3. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

c)
I know the charge on the shell is -2λ
the formula for length charge density is λ = Q/L
that means, Q(the charge over a length) = -2λ * L

we want the surface charge density so i'm lookin for Q/A

the the surface area of a rod = pi*r² * L

we want q/a,
(-2λ * L) / pi*r² * L
leaving me with
-2λ / pi*r²

4. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

still lookin for help with B) please

5. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

HOW DID I GET THIS WRONG??????????????????????????????????????
mastering physics is bloody stupid

6. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

They are asking what the surface charge density is on the inner surface of the shell, NOT the rod. In fact the rod doesn't even have an inner surface.

7. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

so it's the same as the charge on the outside of the shell?

- is C) right?

8. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

I think i may have gone wrong for finding the surface charge density on the outside of the shell

it's 2*pi*r * L

so
Q / 2pi*r * L is what i'm looking for

-

Q = λ*L

-2λ*L = Q

9. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

You're correct that you calculated the area of the shell incorrectly. However how does the calculation you're attempting now differentiate between the inner surface and the outer surface? Can you provide any arguments why exactly this pertains to the outer surface?

10. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

I can't provide any arguments,

I know the surface charge density on the inside of the shell is exactly the same as the surface charge density on the outside

i don't know how to differentiate between the field created outside the shell and inside the shell,

except that i know the field inside the shell should cancel out because of it being orientated in a circle

-

11. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

Since the shell is a conductor the electric field inside the shell is indeed zero, as hinted by MP.

What are your arguments for thinking that the surface charge density is the same on both the inner an outer surface?

12. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

well it's a conducting shell

and also it dosen't state a thickness, only a length and radius from centre of the shell

13. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

This is not an argument. If the conducting shell would have had no charge on it what would the surface charge density be?

14. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

zero,

I ment that the cylindrical shell is made of a material that allows charge to be conducted through it?

15. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

It would not be zero. Could you explain to me what a conductor is?

16. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

something/a medium that allows electrons to flow freely?
things like metals are conductors
things like rubber are insulators

i know conductors allow electrons to flow freely because they have valence shell electrons (or the outer atoms aren't held in very well)

17. Aug 3, 2009

### vorcil

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

i see what you mean, i should've said electrons instead of charge
because positive charges (from protons) aren't able to flow through a conductor, but charge from electrons are (what about positrons?)

18. Aug 3, 2009

### Cyosis

Re: Lovely physics XD - A conducting rod inside a conducting shell - electricity/gaus

More precisely something that allows charges to flow freely. So if the conductor in your problem would have a zero line charge density, would you still think that the surface charge density would be zero as well? Explain your answer.

Hint: What happens to the charges if you put a conductor in an electric field.