Re: Electric fields and points of stability

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

OK--think I understand now.  In fact, I remember when writing the code, I
ran into these situations, lines heading straight for each other.  The
algorithms went into spastics--namely the test charge would go so far, be
repelled, then head back to the mother charge, then be repelled, etc. but
with computational round-offs eventually diverge from the line and left a
mess on the screen.   I put some fudge factor in to prevent the head on
lines--I'd have to try and read my code to see exactly what I did, but at
least the original code was true point charge code, that I modified not to
show the head-ons, so, believing John, I guess the code now approximates
point charges but is fine for line charges.

Rick

----- Original Message -----
From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
To: <PHYS-L@lists.nau.edu>
Sent: Monday, October 14, 2002 2:57 PM
Subject: Re: Electric fields and points of stability


> I didn't make my concerns clear.  It seemed to me before (and it does
> even more so now that I've had a chance to look more carefully) that
> the drawing does not properly represent the field of four *point*
> charges.  If it did, many of the field lines that head *toward* the
> center would actually *go* to the center rather than curving back
> outwards as they do in your diagram.  The field of four infinite line
> charges, on the other hand, *does* look like your drawing which makes
> me think that your program may actually be calculating the fields of
> line charges rather than point charges.  This is commonly done in
> two-d representations so that the resulting field obeys Gauss' law in
> two dimensions since the field due to point charges does not!
>
> > The way the generator works is how we explain field lines.  I place a
> > positive test charge at equally spaced (angularly) positions around each
> > charge and then using all the charges in place, calculate the path that
the
> > test charge would move.  That is what is traced.  The number of initial
> > charge positions is fixed and may not be properly chosen in this example
to
> > show all the details.  This is really designed just to introduce the idea
of
> > field lines to the class.  The way the lines are drawn, one can actually
> > watch them being generated.  With today's really fast machines, it
happens
> > pretty fast, but not too fast (yet) to see.
> >
> > Rick
>
> --
> John Mallinckrodt        mailto:ajm@csupomona.edu
> Cal Poly Pomona          http://www.csupomona.edu/~ajm
>
> This posting is the position of the writer, not that of SUNY-BSC, NAU or
the AAPT.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.