18.2

The Electric Field Lines of Point Charges

Student Notes

With this simulation, you can investigate several topics dealing with electric fields created by point charges.
Here are some things to consider. The default settings, with one charge set to zero and the field vectors off, allow you to focus on the electric field from a single point charge. Observe the field line pattern as you change the magnitude and sign of charge 1. How does the pattern change as you increase the magnitude of the charge? How does it change when you keep the magnitude the same and reverse the sign? Note how the field lines get further apart the further away from the charge they are. What does this tell you about how the magnitude of the electric field changes with distance from a charge? Consider the point exactly halfway between the charges. Can you adjust the charges so that, with non-zero charges, the net electric field is zero there? With one charge set to +4 microcoulombs and the other to +1 microcoulombs, where on the line joining the two charges is the net field equal to zero? What about when one charge is +4 and the other is -1 microcoulombs? For these two situations calculate where the net field is zero, given that the charges are 1 meter apart. Do your calculations agree with what the simulation shows?

Self-test

Do electric field lines start or end at a positive charge? What about a negative charge? When there are two charges, the charges exert forces on one another. Under what conditions do the charges attract one another? Under what conditions do the charges repel? How does the force on a charge depend on the magnitude of the charge? With two charges, if one charge has a larger magnitude than the other, which charge experiences more force? There are many parallels between gravitation and electrostatics. What are the similarities between the two? What are the differences? What, for gravity, plays the role that E, the electric field, plays for electrostatics?

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