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Re: [Phys-L] electric field above a circular charged disk



Yup, that's what I got in Mathematica. The increase in E_tot between r=0
and r=8.7 is only about 5% however. And presumably (I didn't try it
however) if we kept at a larger constant height then E_tot would
monotonically decrease as r increases, which is what I was expecting.

For small z, the increase in the r-component is large enough to compensate
the drop in the z-component. I'm visualizing it as a series of rings of
charge. Being directly over the circumference of the ring gives a larger E
than being on the axis of the ring, for the same small z (compared to the
radius of the ring).

On Sat, Jul 11, 2015 at 5:40 PM, Bill Nettles <bnettles@uu.edu> wrote:

I'm doing the problem using Python and the scipy.integrate.dblquad()
function. I get an E-field magnitude which increases as I move radially
outward, peaking about r=8.71 after about a 6% increase over the r=0 value,
then decreasing. The z-component is decreasing slowly and non-linearly,
while the r component rises until r=10.
________________________________________
From: Phys-l [phys-l-bounces@www.phys-l.org] on behalf of Carl Mungan [
mungan@usna.edu]
Sent: Friday, July 10, 2015 5:02 PM
To: PHYS-L
Subject: [Phys-L] electric field above a circular charged disk

The following surprised me a little. I’m curious what you all think. I
asked Mathematica to solve this problem numerically.

Consider a circular, uniformly charged metal disk of radius 10 (in
whatever units you like).

Start at a point directly above the center of the disk, say at height 1.
Now move radially outward at the same height.

Since we started close to the disk (relative to the radius) we expect the
net electric field to stay approximately constant as we move radially. But
what does it do exactly: decrease a bit, stay exactly constant, increase a
bit, or some combination (such as first decrease and then increase) as we
move to a position at height 1 and radius 9?

ps: I just switched my PHYS-L subscription to non-digest mode, because I
haven’t received any digests from the list in weeks even though the
archives tell me there have been postings. I’ll hope to see this message
echo back to me after I click send.

-----
Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/

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--
Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu http://usna.edu/Users/physics/mungan/