Illustration 23.2: Electric Fields from Point Charges

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This Illustration allows you to add charges by clicking on the appropriate link. All charges are added at the center of the animation; you must drag charges from the origin to see the effect of subsequent charges. Restart.

First, examine the field around one charge. What does the field look like for a 1-C charge? Clear the charge and add a 2-C charge. How is that field configuration different? Clear the charges and then add a -2-C charge. What is the difference? Notice that the strength of the field is represented by the color of the field vectors. White is the smallest magnitude (zero) and black is the greatest, with blue, green, and red in between. When the charge is negative, the electric field vectors change direction and now point in the opposite direction. Positive charges have field vectors that point radially outward, and negative charges have field vectors that point radially inward.

Clear the charges and add two positive charges of the same magnitude. Notice that since the charges are added at the center, 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 farther 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 simply the principle of superposition. You have seen this in the previous chapter: The force on a charge is due to the sum of the Coulomb forces from the surrounding charges. Notice that the force vector of an individual charge points in the direction of the electric field due to the other charge. It does not, however, point in the direction of the electric field due to both charges. The field configuration shown would be the field experienced by a third particle (not the force experienced by either particle).

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? The field vectors point in the opposite direction.

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 be described in terms of the vector sum of the field from the two particles.

Try two charges of different magnitude. What does the field look like? Notice that there is a point at which 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? Try it. Notice that the force on the third charge is simply due to the electric field from the other two charges (multiplied by the charge of the third charge).

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 labels on the charges), which ones are positive and which ones are negative? How can you tell which ones have more charge?

Illustration authored by Anne J. Cox and Melissa Dancy.

Physlets were developed at Davidson College and converted from Java to JavaScript using the SwingJS system developed at St. Olaf College.

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