A pair of point charges with opposite sign create a dipole field.

Electric Field Exploration

Use the drop down menu to select the Custom Defined configuration to create and position up to ten charges.

First, examine the field around a single 1 unit charge.  What does the field look like?  Increase the charge to 2 units.  How is that field configuration different?  Change to charge to -2.  What is the difference?  Notice that the strength of the field can be represented by either arrow length or brightness (grayscale).  When the charge is switched from positive to negative, the electric field vectors change direction.  A positive charges has field vectors that point radially outward and a negative charge has field vectors that point radially inward.

Drag the test charge toward and away from a charge at the origin.  How does the field magnitude change if you change the radial distance from 1 to 2?  Does your relation hold if you start at a different distance and double that distance?

Clear the charges and add two positive charges of the same magnitude.  Note that if you added the charges at the same position, you must drag a charge away to see the one underneath.  How is the field different with two charges compared with one?  Move one of the charges closer and further away from the other one.  When the charges are sitting on top of each other, what does the field look like?  When you move them far apart, what does it look like?  Notice that the fields add together (it is nothing more than vector addition).  The fact that the electric field at any point is the vector sum of the electric fields due to the surrounding charges is a consequence of the principle of force superposition. The force on a test charge is due to the sum of the Coulomb forces from the surrounding charges and the field vectors points in the direction of the electric field due to all the charges.

What do you predict the field will look like with two negative charges (of equal magnitude)?  Try it.  What are the similarities and differences between the two positive and two negative charge distributions?

What about a dipole, one positive and one negative charge?  How is it the same or different from two charges of the same sign?  What is the direction of the field at the midpoint between the charges?  The vector field can again be described in terms of the vector sum of the field from the two particles.  What happens to the electric field if you place a two dipole charges at the same location?

Try two charges of different magnitude.  What does the field look like?  Notice that there is a point where the electric field is zero directly in between the two charges.  If you added a third charge at that spot, what do you predict the force on it would be?

Add three or four charges and look at the field.  Pick one point of the electric field and explain why it points in the direction it does.  How can you tell, simply by looking at the field (and not the color on the charges), which ones are positive and which ones are negative?  How can you tell which ones have more charge?