Field lines are an abstract concept (in that they don't really exist and can be difficult to visualize.) We recommend that you use this applet to get a feel for electric field lines and electric potential lines for a number of different point charges with different magnitudes, signs, and values.
This applet should be fairly straight forward and easy to use. For a more in-depth description of what each of the buttons in this applet does read on.
Add a charge: this radio button lets the user add a charge, using the mouse, at some location in the plane. Click the "Add a charge" button. Position the mouse pointer over the plane where the charge is to be placed and click. A dialog box will open asking for further information on the charge's x and y coordinates and the magnitude of the charge. The field above is a Cartesian plane with the lower left hand corner being [0,0] and the upper right hand corner being [10,10]. A charge may be placed anywhere within this region. The magnitude of a charge can be expressed in either elementary charges or Coulombs. An elementary charge is a charge with the magnitude of and electron. Remember, elementary charge are much smaller a Coulomb in magnitude. If charges on the order of magnitude of elementary charge and charges on the order of magnitude of Coulombs are mixed in the same plane, the charges on the order of magnitude of Coulombs will vastly out power the smaller charges. To make a charge negative simply place a negative "-" sign in front of the number. Once the fields contain the desired values press the "Okay" button and the new charge will be draw. Positive charges are red, negative charges are blue.
Delete a charge: this radio button lets the user delete a charge using the mouse. Choose the "Delete a charge" radio button and then place the mouse pointer over the charge to be deleted. Click once. The charge should now disappear. Note: if more than one charge exists near same location it may take multiple mouse clicks to remove all of the charges.
Draw a field line: this radio button lets the user draw an electric field line on the plane. Click the "Draw a field line" radio button. Then position the mouse over a point through which the field line should pass. Click once. An electric field line should appear though this point.
Draw an equipotential line: this radio button lets the user draw an electric equipotential line. Click the "Draw an equipotential line" radio button. Position the mouse over a point on the plane through which the equipotential line should pass. Click once. An electric equal potential line should appear though this point. Equipotential lines should always be closed loops (unless they run off the screen). Note: The way in which the program calculates where to plot points on an equal potential line is not exact. Sometimes the line will appear to jump from the ending point to the starting point in order to complete the loop. At other times the line will continue drawing even after the loop has been completed (in this case the line will appear to spiral.) The program will only draw a certain number of line segments for each loop (in order to avoid infinite loops), so sometimes the equipotential line will stop drawing before the loop has been completed. This usually only occurs if there are many charges on the plane. Warning: In order to reduce inaccuracies many calculations using very small numbers are made each time an equipotential line is drawn. DO NOT get impatient and try to draw many lines at once. The computer will keep track of all the lines to be drawn and try to draw them all. This takes an enormous amount of time and can hang your system. Just click once and wait for the line to complete before trying to draw another line.
Auto draw field lines: this button will cause the program to automatically draw field lines. Simply click the button. The auto draw function works by stepping through a matrix of coordinates and drawing a field line at each point in the matrix. Note: because the matrix is Cartesian in form it is possible no field lines will be drawn through certain sections. This does not mean that the field doesn't exist in these areas, simply that auto draw function has missed them. Indeed it is possible to fill the entire plane with field lines. At points where the field does not exist a field line cannot and will not be drawn. Clicking on this button more than once with the same configuration of charges will not cause different lines to appear. The program will draw of the top of the old ones.
Auto draw equipotential lines: this button will cause the program to automatically draw equipotential lines. Simply click the button. The auto draw button works by stepping through a matrix of coordinates and drawing an equal potential line at each point in the matrix. Note: because the matrix is Cartesian in form it is possible that no equipotential lines will be drawn through certain sections. This does not mean that the field doesn't exist in these areas, simply that auto draw function has missed them. Indeed it is possible to fill the entire plane with equipotential lines. At points where the field does not exist an equipotential line cannot be drawn. Warning: Clicking on this button more than once with the same configuration of charges will not cause different lines to appear. The program will simply draw of the top of the old ones. Since the calculations of the equipotential lines are intensive, DO NOT push the button more than once for each configuration of charges. Doing so could cause your computer to hang.
Clear all lines: this button will clear all of the field and equipotential lines from the plane. It will leave all of the charges.
Clear all: this button will clear all of the field and equipotential lines from the plane and remove all of the charges. In essence restarting the program.
Bron: http://www.slcc.edu/schools/hum_sci/physics/tutor/2220/index.html