Lesson - Electric Fields from Point Charges

Coulomb's law describes the force felt by one electric charge due to another. However, in a real object, we do not have just one or two charges. Matter is made up of atoms with protons and electrons, and these are constantly moving.

The electric field and the electric potential are ways that scientists have used to describe electric forces in a more general way.

           k q q'
F=  --------------------
    (x*x + y*y + z*z) 1/2

Expressing the force in terms of x, y, and z helps us to understand a little about what the force would be anywhere in space due to the first charge, but we also want to be able to express the effect of the central charge without having to know how much charge our test charge is.

The strength of the electric field is defined such that if we wanted to find out the strength of the force on our test charge, we would just multiply the field strength with the magnitude of our charge

E = q' F

1. Describe the electric field around a single positive charge? What way is it pointing? Is it larger or smaller near the charge? Try it out on the Electric Field Viewer, press the mouse somewhere in the screen. Hold the mouse button down and drag to see what happens at different points.
2. Repeat the above question with one negative charge.

Just as the forces for multiple charges add as vectors, the field strength due to multiple charges also adds as vectors.

3. Set the Electric Field Viewer to two like charges. What happens to the field close to each point? What happens far away? Is the field at the edges of the screen greater or less than what it was with one point?
4. Suppose you were going to make a plot of field strength along a line from one point to the other, what would it look like?
5. Set the Electric Field Viewer for two opposite charges. What is the field like close to each point? What happens far away? Is the field at the edges of the screen greater or less than what it was with one point?
6. Suppose you were going to make a plot of field strength along a line from one point to the other, what would it look like?

The electric potential V is another way of describing the effect of many charges. Potential is a weird thing. If we think of a ball rolling down a hill, then the gravitational potential would be the shape of the hill. The steeper the slope, the greater the force, and the direction of the force is in the direction of greatest change.

For electric potential, the potential is defined such that the strength of the field is given by how steeply the potential changes from one point to the next, and the direction of the field is in the direction of greatest change of the potential.

7. Describe the potential around a positive charge.
8. Describe the potential around a negative charge.
9. Suppose you were to plot the potential along a line between two positive charges, what would it look like?
10. Suppose you were to plot the potential along a line between two opposite charges, what would it look like?