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.