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# Electromagnetism Gauss' Law question Watch

1. Hi, so I'm really stuck on a past exam question, and my lecturer doesn't bother writing up any working (just the answers) so I have no clue how to get to these answers...the question is as follows:

A single point charge Q is placed in the centre of a circle of radius a, that forms one end of a long cylinder of length h.

a) How much electric flux from the charge passes through the cylinder? (The answer is apparently Q/2*epsilon0, I know I need to use Gauss' Law but how?!)

b) What fraction of this flux passes through each part of the cylinder (each end circle and the curved body of the cylinder)? You may assume h=30a (There is no flux through the face the charge lies on, I get that, but the ratios for the other two surfaces are 1/1800 to 1799/1800, and I'm not sure which value corresponds to which surface).

Any help would be greatly appreciated!
2. (Original post by torakrubik)
Hi, so I'm really stuck on a past exam question, and my lecturer doesn't bother writing up any working (just the answers) so I have no clue how to get to these answers...the question is as follows:

A single point charge Q is placed in the centre of a circle of radius a, that forms one end of a long cylinder of length h.

a) How much electric flux from the charge passes through the cylinder? (The answer is apparently Q/2*epsilon0, I know I need to use Gauss' Law but how?!)

b) What fraction of this flux passes through each part of the cylinder (each end circle and the curved body of the cylinder)? You may assume h=30a (There is no flux through the face the charge lies on, I get that, but the ratios for the other two surfaces are 1/1800 to 1799/1800, and I'm not sure which value corresponds to which surface).

Any help would be greatly appreciated!
Do you have a diagram of this? Gauss' law states that the total flux coming off a point charge is equal to . So I'd have said the answer to (a) is .
3. http://www.bath.ac.uk/library/exampa...PH/PH20014.pdf

Does this link work for you? It's question 2.

I have just realised that, I typed the wrong answer to begin with, that is the given answer yes. But why is it halved when the charge lies on one of the faces? That means the flux through that face is zero right?
4. (Original post by torakrubik)
Hi, so I'm really stuck on a past exam question, and my lecturer doesn't bother writing up any working (just the answers) so I have no clue how to get to these answers...the question is as follows:

A single point charge Q is placed in the centre of a circle of radius a, that forms one end of a long cylinder of length h.

a) How much electric flux from the charge passes through the cylinder? (The answer is apparently Q/2*epsilon0, I know I need to use Gauss' Law but how?!)

b) What fraction of this flux passes through each part of the cylinder (each end circle and the curved body of the cylinder)? You may assume h=30a (There is no flux through the face the charge lies on, I get that, but the ratios for the other two surfaces are 1/1800 to 1799/1800, and I'm not sure which value corresponds to which surface).

Any help would be greatly appreciated!
So only half the charge is enclosed in the cylinder => The flux through a closed surface depends only on the enclosed charge
5. (Original post by torakrubik)
http://www.bath.ac.uk/library/exampa...PH/PH20014.pdf

Does this link work for you? It's question 2.

I have just realised that, I typed the wrong answer to begin with, that is the given answer yes. But why is it halved when the charge lies on one of the faces? That means the flux through that face is zero right?

The question is basically just a geometry question - I haven't actually done it but it looks okay at a glance. You have an open cylinder, and a change at one end. What proportion of the field lines:

1) Do not enter the cylinder?
2) Hit the walls?
3) Go through the cylinder without hitting the walls?

The whole point of Gauss law is to formalize that field lines actually kinda mean something, and that all of the field lines =

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