# Current density

Discussion in 'Homework Help' started by thexy, Jan 18, 2016.

1. ### thexy Thread Starter Member

Dec 13, 2015
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Plot the surface current density in the shell as a function of the measured from the apex of axial coordinate z.

So here is photo and result. I don't understand why are they using perimeter? Why is there tan?
Can someone explain me on easy way because I don't understand anything.

2. ### WBahn Moderator

Mar 31, 2012
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4,917
This is a shell, right (something like a very thin funnel)? So, at any given height, all of the current is passing through a ring. How long is that ring?

3. ### thexy Thread Starter Member

Dec 13, 2015
126
0
This is not easy explanation. I don't how much is long. I would say it's long like the time that currents flows through it.

4. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
This makes no sense whatsoever.

What is the surface current density (which would be expressed in amperes per inch) if the current is flowing lengthwise through a cylinder that has a diameter of two inches?

5. ### thexy Thread Starter Member

Dec 13, 2015
126
0
It will be I/Area

6. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
No, because it is not asking for current density, it is asking for surface current density. So it is I/Length.

7. ### thexy Thread Starter Member

Dec 13, 2015
126
0
But I don't understand why is there tan?

8. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
What is the circumference of the shell a distance z above the apex? Isn't that a function of the angle of the cone?