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This java applet demonstrates electrostatics and static current distributions in two dimensions. There is also a three dimensional applet which is more accurate but less flexible.

When the applet starts up you will see the electric field of a single positive charge. The yellow circle represents the charge; the white circles around it are equipotential lines (lines of constant potential). The green arrows indicate the electric field. The charge can be dragged around the screen with the mouse.

In general, yellow represents positive charge, and blue represents negative charge.

Conductors, dielectrics, and media with bound charges will show up as gray. There aren't enough colors or shades of gray to make the different kinds of media look different. Use the Show Material Type menu option to tell them apart.

The Setup popup can be used to view some interesting pre-defined experiments. Once an experiment is selected, you may modify it all you want. The choices are:

The Mouse popup controls what happens when the mouse is clicked. The following settings are possible:

The Show popup determines which fields or other quantities to display, and how to display them.

The Accuracy popup controls the accuracy used to calculate the fields. Lower accuracy is faster but may display misleading or incorrect results.

The Clear All button clears the screen.

The Stop Calculation checkbox stops the calculation of the fields. You can check this temporarily if you want to make a bunch of changes without having the calculation slow you down. Then when you're done, uncheck it to have the fields calculated.

The Enable Current checkbox allows current to flow. When this is checked, the top of the screen is fixed at positive potential and the bottom of the screen is fixed at negative potential. If there is a conducting path from the top to the bottom of the screen, then current will flow. Any conductors not connected to the top or bottom of the screen will be at ground.

The Draw Equipotentials checkbox draws equipotential lines, which are lines of constant potential.

The Resolution slider allows you to speed up or slow down the applet by adjusting the resolution; a higher resolution is slower but looks better.

The Brightness slider controls the brightness, just like on a TV set. Also when the brightness is higher it causes more equipotentials to be drawn.

The Equipotential Count slider allows you to control the number of equipotentials independently from the brightness.

Warning! With low accuracy settings, the applet may give bad results for current flow in circuits with dead ends, like this one:

This is obviously not correct; it's not possible to have steady-state current flow into a dead end. The problem is that the branch has a very thin separation between it and a conductor at higher potential, and that confuses the applet.

To fix this, either use a higher accuracy setting (which will be slower) or make the separation wider, like so:

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