# Current density

#### thexy

Joined Dec 13, 2015
130
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.

#### WBahn

Joined Mar 31, 2012
26,398
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?

#### thexy

Joined Dec 13, 2015
130
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?
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.

#### WBahn

Joined Mar 31, 2012
26,398
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.
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?

#### thexy

Joined Dec 13, 2015
130
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?
It will be I/Area

#### WBahn

Joined Mar 31, 2012
26,398
No, because it is not asking for current density, it is asking for surface current density. So it is I/Length.

#### thexy

Joined Dec 13, 2015
130
No, because it is not asking for current density, it is asking for surface current density. So it is I/Length.
But I don't understand why is there tan?

#### WBahn

Joined Mar 31, 2012
26,398
What is the circumference of the shell a distance z above the apex? Isn't that a function of the angle of the cone?