```

WHAT IS IT -----------

The purpose of this model is to demostrate what 2-D electrostatic fields look
like and how charged particles move in the presence of such fields.

HOW TO USE IT --------------

1) Push the SETUP button; this will clear the graphics screen and initialize some
constants used in the computations.

2) Push on the forever button called SETUP-CHARGES; with this button depressed
you can mark the points where you want the charges to appear by clicking with the
mouse on the Graphics screen.  Each time you click you will see the corresponding
patch turning red.  You can position at most 10 charges.  If you click more than
10 times you will see old charges disappearing and reappearing in the places
where you click.

3) Once you have all the charges in the right places, press on the SETUP-FIELD
button.  You will see the 2D plot of the corresponding electrostatic field.

4) As soon as you have the field you can place a test-particle and see how it
moves.  To do that press PLACE-PARTICLE and click on the screen where you want
the test-particle(s) to appear.  This model assumes that the particle's charge
has the same sign (positive or negative) as the fixed charges.

5) To see how the particles move, click on the RUN button.

THINGS TO NOTICE --------------- What is the shape of the path taken by the test
particle when it approaches a fixed charge?

THINGS TO TRY -------------

You can place all ten charges symmetrically in the vertices of a 10-gon and then
put a test-particle into the center of the 10-gon.  The particle will then be
confined to a little spot around the center and will never leave the region.  How
many charges does it take to cage the particle?  Can one trap the particle in the
center of a square or an equilateral triangle?

EXTENDING THE MODEL ------------------- Try making charges with different
strengths, or with different signs.

Give the test particle an initial velocity.

Note that the particle and the field "wrap" the screen.  What if the particle
bounced off the walls?

What if the field were forced to be zero at the edge of the screen?

STARLOGOT FEATURES ------------------ Note the use of arrays to define the vector
field.
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