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The key is that it completely encloses a volume I think. Disks are right out, as are any open shapes.
by Colman (colman at eurotrib.com) on Tue Jan 24th, 2012 at 03:37:39 PM EST
[ Parent ]
For added fun, we can now go into how you determine whether a curve "encloses an area in the plane", and how to tell whether a point is in its interior. This is the Jordan curve problem and it's a bitch. The only real solution is to throw one's hands up and consider points for which the Cauchy integral of 1/z is zero. But this just defines the points around which the curve has a winding number of zero. One can then define "exterior points" as the connected component of the locus of those points which contains the point at infinity.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker
by Migeru (migeru at eurotrib dot com) on Tue Jan 24th, 2012 at 03:43:19 PM EST
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Or, one can think about water balloons...

Frankly, I have never understood why this is such a roadblock. You don't want edges because the integrals don't behave well there. You prefer not to have complicated shapes because you have to worry about insides and outsides. So why make it complicated?

It's just a surface that surrounds a point, which is intuitively obvious enough for physics. Take further questions over to the math department.

by asdf on Tue Jan 24th, 2012 at 03:47:38 PM EST
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Taking it to the math department makes it complicated.

It's okay to say "the flux of the electric field across a closed surface depends only on the total charge contained within it". The problem is when you try to make a mathematical statement out of that.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker

by Migeru (migeru at eurotrib dot com) on Tue Jan 24th, 2012 at 03:49:50 PM EST
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Yep, because mathematicians want to know what you mean by "inside." Anybody else already knows...

The thing that always tripped me up was that nobody ever explained that the curl is just the direction a little submerged ball would spin...did not find that out until a couple of decades after taking E&M...

by asdf on Tue Jan 24th, 2012 at 06:01:49 PM EST
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How fun that a rigidly rotating field has a nonzero curl and that a cylindrically symmetric irrotational field diverges on the axis.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker
by Migeru (migeru at eurotrib dot com) on Tue Jan 24th, 2012 at 06:07:34 PM EST
[ Parent ]
Anybody can observe a water balloon, but the issue here is to calculate its properties from first principles.

tens of millions of people stand to see their lives ruined because the bureaucrats at the ECB don't understand introductory economics -- Dean Baker
by Migeru (migeru at eurotrib dot com) on Tue Jan 24th, 2012 at 03:52:16 PM EST
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I have faint memories of that stuff. I believe we spent damn near a whole term of a second year course developing the machinery to do that sort of thing.
by Colman (colman at eurotrib.com) on Tue Jan 24th, 2012 at 04:36:29 PM EST
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In most cases you learn it first in physics, then the math department comes along a couple of semesters later and muddies the waters in trying to clean up the mess...

Or, you take it in engineering, then a couple of semesters later the physics department comes along and muddies the waters, then the math department later.

And I suppose the philosophy department would want to be in the discussion eventually, as well...

by asdf on Tue Jan 24th, 2012 at 06:03:30 PM EST
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Fortunately, nobody takes the Philosophy department seriously on these things, the chance of getting those waters unmudied in 2000 years is slim

Any idiot can face a crisis - it's day to day living that wears you out.
by ceebs (ceebs (at) eurotrib (dot) com) on Tue Jan 24th, 2012 at 07:27:48 PM EST
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