[Phys-L] Re: Electric Field?

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Draw a free body diagram for the test charge.  The force vector from the
positive charge points straight away, from the negative charge the vector is
smaller and pointed to the right and slightly down.  The existence of that
second vector means that the path of the test charge will be deflected
slightly to the right as opposed to following the path directly away from
the positive charge.  So--draw the test charge a little higher and slightly
to the right of the projected path of the original force vector due to the
positive charge.  Repeat.  It is easy then to see how the test charge keeps
getting deflected to the right of the positive charge's force vector--until
the test charge is equi-distant from both charges.  Then the force vector
due to the negative charge become greater which continues the rightwards
deflection but also starts to produce a downwards deflection.

So--to answer your question more simply, just do a series of free body
diagrams picking the next position for the test charge by looking and the
relative strength of the horizontal and vertical force components.

Rick

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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----- Original Message -----
From: "David Abineri" <dabineri@FUSE.NET>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Tuesday, February 07, 2006 8:27 PM
Subject: Electric Field?


> In looking at the electric field about a pair of equal but opposite
> fixed point charges situated on the x axis in my high school class, we
> mapped part of the field by looking at the forces exerted on a test
> charge (proton) by the two fixed charges.  A student then asked me to
> show that if one places the test charge  to the left of the left-most
> charge (the positive charge) but just slightly above the x axis that the
> field line that it would follow would in fact 'circle' around in a large
> path until it eventually approaches the other charge from the upper
> right.  It seems to me that the force of repulsion  would always tend to
> push the charge leftward making it difficult to see how it would ever
> 'circle' to the right.
>
> Is there a way to see that the test proton will eventually move
> rightward?  Any help appreciated.   Thanks.
>
>    David Abineri
>
>
> --
> dabineri@fuse.net
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