Activity 11: Electric Fields

In this activity you will use a Project Links module to examine the properties of an electric field.

Before You Start:
Answer the following questions to the best of your ability before doing the experiment.
What do you think the field from a positive point charge will look like?
How will the field change if the charge is negative?
How will the field lines change if the magnitude of the charge is increased?
What do you think the field lines for a pair of oppositely charged point charges (one positive, one negative) look like?
What do you think the field lines for a pair of oppositely charged parallel plates look like?
Give reasons for each of your predictions.
After you have thought about your answers, compare notes with your group members. Does everyone have the same predictions, or are there differing opinions?

Point Charges

Go to http://links.math.rpi.edu/devmodules/electromagnetism/ElectricField/index.html and then click on "More Examples" on the left menu bar. Work through that page, and answer the following questions.

1. Sketch the electric field lines of the positive point charge.

2. Sketch the electric field lines of the negative point charge. How are they different from those of the positive test charge?

3. Sketch the electric field lines of the oppositely charged point charges. How does this compare with what you predicted?

4. Can you find a point in the window where the electric field is zero? If so, give its coordinates (found in the lower left-hand corner of the window).

At the bottom of the page is a link. Click on "animated example of field lines". When that page comes up, click on "an interactive version" at the bottom of the page. This will open a new Navigator window. Make sure the radio button next to the minus sign is selected, then change the magnitude of the charge with the slider bar.

5. What about the field changes when you change the magnitude of the right-hand charge?

6. Set the magnitude of the negative charge to -8mC. Sketch the resulting electric field.

7. Compare the field lines around the larger charge on the left to those around the smaller charge on the right. How do they differ?

STOP. The class will discuss these questions before continuing.

Electric Field due to Parallel Plates

Consider the pair of plates shown below. The plates are each made of conducting material. The plate on the left has +8.0 mC of charge; the plate on the right has -8.0 mC of charge.

8.	Copy just the positive plate onto your paper, near the left margin. Using one "+" sign for each mC of charge, show how the charge will be distributed on the plate.
9.	With faint dashed lines, sketch the electric field lines for each of the charges on the positive plate. They will intersect and cross, which is ok for now.
10.	With darker solid lines, indicate the NET electric field (i.e., are there components which cancel?) due to the charges on the positive plate. Be sure your lines correctly indicate both direction and magnitude of the field.
11.	Sketch the negative plate onto your paper, beside the positive plate but with a few inches between them. Using one "+" sign for each mC of charge, show how the charge will be distributed on the plate. Using a different color of pen/pencil if you can, sketch the NET electric field lines due to the negative plate. Be sure your lines correctly indicate both direction and magnitude of the field.
12.	Examine your drawing. How does the field due to the positive plate compare with the field due to the negative plate between the plates? How do they compare outside of the plates?
13.	Sketch the combined electric field due to the plates. You may want to make another sketch of the plates to avoid mass confusion on your original drawing.

More Complicated Charge Configurations

Go back to Project Links, and close the window with the animation. Click on "More Examples" on the Project Links menu bar to go back to the page with the different configurations.

14. Sketch the electric field lines for the similarly charged point charges.

15. How do these lines differ from the lines for the oppositely charged point charges?

16. Can you find a place in the window where the electric field is zero? If so, give its coordinates (found in the lower left-hand corner of the window).

17. Sketch the electric field lines for the complicated case (four charges).

18. Do all four charges in the complicated case have the same magnitude? How can you tell?

19. If I place a negative charge at coordinates (200,200) in the “complicated case”, in which direction will it move? (up, down, left, right, up and to the right, . . .)

You will be asked to complete an evaluation of today's activity and lecture before the end of class. This evaluation counts as a free 5% of each activity grade. It will generally be done on WebCT in the last 5-10 minutes of class, but time constraints may lead to the occasional evaluation done on paper.

1.	Sketch the electric field lines of the positive point charge.
2.	Sketch the electric field lines of the negative point charge. How are they different from those of the positive test charge?
3.	Sketch the electric field lines of the oppositely charged point charges. How does this compare with what you predicted?
4.	Can you find a point in the window where the electric field is zero? If so, give its coordinates (found in the lower left-hand corner of the window).

5.	What about the field changes when you change the magnitude of the right-hand charge?
6.	Set the magnitude of the negative charge to -8mC. Sketch the resulting electric field.
7.	Compare the field lines around the larger charge on the left to those around the smaller charge on the right. How do they differ?

14.	Sketch the electric field lines for the similarly charged point charges.
15.	How do these lines differ from the lines for the oppositely charged point charges?
16.	Can you find a place in the window where the electric field is zero? If so, give its coordinates (found in the lower left-hand corner of the window).
17.	Sketch the electric field lines for the complicated case (four charges).
18.	Do all four charges in the complicated case have the same magnitude? How can you tell?
19.	If I place a negative charge at coordinates (200,200) in the “complicated case”, in which direction will it move? (up, down, left, right, up and to the right, . . .)