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] non-conservative --> non-grady ???



Isn't the changing electric flux introducing a non-conservative magnetic interaction that the charged particle interacts with once it starts to move? Perhaps I'm not visualizing this in the sense that you meant, but it doesn't seem to be purely conservative.

Bob at PC

________________________________

From: phys-l-bounces@carnot.physics.buffalo.edu on behalf of John Denker
Sent: Tue 2/19/2008 9:40 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] non-conservative --> non-grady ???



Example 1: Grady force:
*) Electrostatic force is a grady force.
*) Ignore non-electric fields and potentials. Gravity is ignorable if the
particle is constrained to move horizontally.
*) Define "the system" to be a charged particle, initially at rest, but
free to move.
*) Apply an electric field, initially zero, then nonzero, then zero again.
*) The particle picks up a nonzero kinetic energy. BTW it also
picks up nonzero momentum. Energy and momentum were transferred
across the boundary of "the system".
*) Potential is the same before and after, so the *sum* of KE and PE
is changed, not just KE at the expense of PE.
*) If you want to get fancy, argue based on symmetry and/or timescales
that we can arrange that the electrostatic approximation is valid, i.e.
magnetic effects can be neglected (subject to mild restrictions).