Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] amusing electrostatics exercise



At points inside the surface of the original wire, the two skinny wires model gives you a magnetic field but it is not the same as the actual magnetic field of the original wire with the tunnel extending along its length so it is not appropriate to use it for points in space inside the surface of the original wire.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of Bruce
Sherwood
Sent: Thursday, February 28, 2013 11:51 AM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] amusing electrostatics exercise

Ampere's law for axial symmetry lets you get the magnetic field inside a wire
as well as outside, so you could if you wish calculate the field at all locations.

Bruce


On Thu, Feb 28, 2013 at 8:26 AM, Jeffrey Schnick
<JSchnick@anselm.edu>wrote:

In the two skinny wire model, find the magnetic field due to each wire
as if it were the only wire. For one wire of infinite length there is
enough symmetry to get the magnitude and direction of the magnetic
field at all points in space not on the skinny wire. Do the
coordinate transformation needed to get both results in the same
coordinate system. Add the results.
The result is only appropriate for points in space outside the
surface of the original wire.

-----Original Message-----
From: Phys-l [mailto:phys-l-bounces@phys-l.org] On Behalf Of John
Denker
Sent: Thursday, February 28, 2013 12:25 AM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] amusing electrostatics exercise

On 02/27/2013 02:45 PM, Bruce Sherwood wrote:
At the same
location as before, use Ampere's law to calculate the vector
magnetic field at that location.

How do you do that?

The only Ampère's law of which I am aware allows us to calculate the
/average/ field, averaged over some specified loop. I do not see
how to
use
it to calculate the "vector magnetic field" at any "location" ...
especially given
that the problem expressly said that the hole was "non co-axial".
That
rules
out the the sort of symmetry that might allow us to infer a local
value
from
the average value. The problem did not suggest any other symmetry,
so
the
only reasonable interpretation I can imagine is that the situation
is not symmetrical.

Also, the problem explicitly asked for "the mag. field" not some
average
over
the field. I say again, there cannot possibly be any simple solution.
Counterexamples abound. A hole on the left side is not equivalent
to a
hole
on the right side. The current knows the difference. The field
knows
the
difference.

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l