Lesson Plan - Electric Fields from Point Charges

This lesson is designed to address electric field and potential, and it is assumed the students have studied electric charge.

There are several misconceptions regarding the electric field and field lines.

1. Field lines are real.

   Field lines are a set of made up lines, like longitude lines and latitude lines on a globe, which help us describe the shape and strength of the electric field.

2. Field lines are the only places in a field where electric force is felt.

   Electric force is felt everywhere in the field. The field lines are meant only to give a feel for the shape and direction of the force. If field lines were placed everywhere that the electric field exists, all you would see would be black, because the electric force is everywhere.

3. Field lines show the direction that charges move.

   The field lines show the direction of force that a charge would feel. This is not the direction the charge would move in, but the direction the charge would accelerate in, assuming no other forces exist.

Answers to Questions

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.

   The field around a single positive point charge points away from the charge, and the field strength is greater close to the charge.

2. Repeat the above question with one negative charge.

   The field around a single negative point charge is similar to a positive point charge, except the field points towards the charge.

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?

   very close to each charge, the field looks similar to a point charge. Very far away from the charges, the field also looks similar to a point charge. The total field strength is the strength that you would get from a single charge with twice the charge value.

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?

   The fields from the two charges cancel each other out at the halfway point between the two charges, so the field strength would be high near the first charge, drop to zero as you move towards the second charge, and at the halfway point between the two charges start to rise again.

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?

   Very close to each charge, the field looks like a point charge, but the fields add in the same direction between the two charges. The field as you move further away seems to bend, as if the field was emerging from the positive charge and entering into the negative charge. Very far away, the field strength is much smaller, as if there was no charger there at all.

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 highest field strength would still be close to the charges, but there would still be a strong field between the two. The field would drop to a finite value at the halfway point, and increase again as you move towards the second charge.

7. Describe the potential around a positive charge.

   The potential around a positive charge is positive and gets larger as you get closer to the charge.

8. Describe the potential around a negative charge.

   The potential around a negative charge is negative and gets greater in magnitude as you get closer to the charge.

9. Suppose you were to plot the potential along a line between two positive charges, what would it look like?

   It would start high, drop to a finite positive value, and return to a very high value.

10. Suppose you were to plot the potential along a line between two opposite charges, what would it look like?

    It would start high, drop to zero, and drop to a negative value with a high magnitude.

The applet meets the following National Standards: Science Content Standards: 9-12

• CONTENT STANDARD A:
• Use technology and mathematics to improve investigations and communications.
• Formulate and revise scientific explanations and models using logic and evidence.
• Recognize and analyze alternative explanations and models.
• Communicate and defend a scientific argument.
• CONTENT STANDARD B:
• Structure and Property of Matter