This simulation allows you to place electric charges by clicking inside the working area, and then drawing the electric field from the configuration by clicking Start. The simulation will keep drawing until all field lines are beyond the borders or by clicking Stop. The button labeled Positive means that the charges you place will be positive (will show up as blue circles). When you click it, it then say Negative and that means all charges will be negative (will show up as red circles). Restart will clear the drawing but keep the charge configuration, and Clear will restart everything from scratch. Checking Grid? draws a cartesian grid, and checking Snap snaps all charges you create to the grid point. If you click Q_test and then click on the canvas, it will place a test charge on the canvas heading to the right and then let it go, subject to the force from the charges you've placed there. You can then see what a test charge would do when subject to the forces from the placed charges.
Note: this simulation uses javascript and as such is not as accurate as it could be if one were to have double precision etc. (Drew Baden December 2013)

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Start Stop Clear Restart    Charge is: Positive    Grid? # Lines: Snap    # Field Lines    Q_test