[Phys-L] Re: Electric Field?

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

I'm not too sure of what John writes below.  I have written a program (part
of the E&M Animated Chalkboard package) to map electric field lines in 2
dimensions.  The way it works is that I place a test charge near one of the
charges that makes up the charge distribution.  I then calculate the force
on the test charge due to all charges in the distribution, the acceleration,
the velocity, and the new position taking a very small time interval.  I
move the test charge to that position plotting the path to get there and
then repeat until the charge either gets far off the visible screen or ends
up at a second charge.  This method produces the field line patterns we all
know.  Of course, to do this one 'nails down' the original charges.  Another
animation allows up to 6 charges to be 'released' and now they all scramble
around.

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
********************************************************
Free Physics Educational Software (Win & Mac)
      NEW:  Updated MOTION animations.
      COMING: Spanish language versions.
www.saintmarys.edu/~rtarara/software.html
Energy 2100--class project
www.saintmarys.edu/~rtarara/ENERGY_PROJECT/ENERGY2100.htm
********************************************************

----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
> It won't.  Your intuition is correct.  The field line that passes
> through the test charge's initial position clearly does wrap around
> to the right and eventually approaches the negative charge from the
> upper right.  But the test charge doesn't follow that field line or
> any other field line.  Instead it receives a relatively big initial
> push to the left and slightly upward that gives it a relatively large
> amount of momentum in that direction.  After a short time it finds
> itself in a region where no significant force acts on it and it
> coasts to the left and slightly upward forever.
>
> Exploring the behavior of a charge released from rest in an
> electrostatic field is a good way to develop one's appreciation of
> inertia and also of the difference between velocity and acceleration.
>
> A positive charge released from rest always STARTS moving in the
> direction of the field line, but if the field line curves, the charge
> CAN'T follow it because of its inertia.  It is the acceleration that
> is always in the direction of the field, NOT the velocity.
>
> All of that said, it is interesting to note that in a world dominated
> by viscosity, Newton's Second Law essentially becomes
>
>               sum of all forces EXCEPT viscosity
>   velocity = -----------------------------------
>                   viscous drag coefficient
>
> In such a world, objects subject only to the electric force DO follow
> electric field lines.
>
> --
> John "Slo" Mallinckrodt
>
> Professor of Physics, Cal Poly Pomona
> <http://www.csupomona.edu/~ajm>
>
> and
>
> Lead Guitarist, Out-Laws of Physics
> <http://www.csupomona.edu/~hsleff/OoPs.html>
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