Electric Force and Electric Field
Prelab
Note: It will be helpful to read the appropriate chapter in the text to complete the prelab AND as
a resource for the lab.
1. Like charges _______________ and unlike charges _____________
2. What is the definition of the electric field?
3. In Part 2, what is the name of the PhET electric field simulation?
_______________________________
4. What are the steps to add a charge to the PhET simulation?
5. In Part 3, Electric Field I, one must identify the animation with the “true” test charge. What
are the 2 properties of a test charge that will allow you to do that.
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Electric Force and Electric Field
Physlets and PhET Simulations
Part 1: Electric Force Physlets
Procedure:
Use the Internet browser of your choice to go to Physlet Simulation page:
http://faculty.sdmiramar.edu/gbochicchio/eforce.htm (Look for the link in Blackboard).
Click on the links to the following physlets and answer the following questions. Click on the back button
of the browser to return to the Physlet Simulation page
Electric Force I
How many charges have like signs?___________________
Explain what you did to determine your answer.
Electric Force II
Determine the signs of the unknown charges. You may consider neutral as a possible answer.
1.(blue)______________
2._________________
3._________________
4._________________
Explain how you determined your answers.
You will return to the Physics Simulation page for Part 3.
Part 2: PhET Electric Field Simulation – Electric Field of Dreams
Procedure: Use the Internet browser of your choice to go to PhET Simulation page:
http://phet.colorado.edu/en/simulation/efield (Google phet physics to get started and then look for the
simulation called Electric Field of Dreams). Click on the “run now” option.
To add charges to the simulation, use the following steps:
a. Click on Properties. A box will appear in the upper left corner of your screen. Adjust charge value as
desired, and click Done.
b. Click on Add. The blue charge will appear in the box. The arrows indicate the direction of the electric
field produced by the charge.
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Add a 1 coulomb negative charge to the simulation. You may move the charge around by dragging it
with your curser.
1) Where is the field produced by the charge the largest. How is this shown?
2) What do the direction of the field vectors indicate about the sign of the charge?
3) In the results section, below, draw the charge and the field around it showing how the length of the
vectors change with distance.
Clear the simulation’s field by clicking the reset all buttons and choosing yes to the reset all question.
Click the properties button and change the -1 charge to a +1 charge. Click the add button on the
simulation. This will add a 1 coulomb positive charge to the simulation.
4). How are the directions of the field vectors different than for the negative charge?
5) In the results section below, draw the charge and the field around it.
Add another positive charge by just clicking the add button. The two charges should start to move around
the screen with the field lines (arrows) indicating the direction and magnitude of the electric force at each
point in space. Hit the pause button to stop the charge movement and drag the charges to the center of
the box on opposite sides. Carefully move them toward each other until you can get the vector arrow
between them to shrink into a blue dot).
6) Where is the electric field the weakest in this configuration? Why is that?
7) In the results section below, draw a picture of the charges and the field between them as well as just
outside.
Remove one of the positive charges and add a negative charge. Don’t forget to change the sign in the
properties menu first! The two charges should move around the screen somewhat attracting one another.
Hit the pause button and use your curser to drag the positive charge to the right side of the screen and the
negative charge to the left side of the screen. This is the electric dipole configuration. Move the two
together until there are 2 vector arrows between. Draw the charges and the field between them and just
outside.
8). Is the strongest field between the charges of the dipole, or outside the charges? Why is that? How
does that compare with the two positive charge?
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Results Section:
Draw Single Negative Charge
Draw Single Positive Charge
Draw 2 Like Charges
Draw Electric Dipole
Part 3: Electric Field Physlets
Procedure: Return to the Physlet Simulation page for the next set of physlets.
Electric Field I
In which animation is the red charge a true "test" charge? ___________
Explain why you chose that animation.
Electric Field II
Based on the direction of the vector arrows in the simulation, is the unknown, black charge is positive or
negative? Explain your answer.
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Determine the magnitude of the charge on the unknown particle in Coulombs. Show your work. A good
strategy for this problem is to place the test charge at a location where either x or y is equal to zero. Note
that the unknown charge is at position (0,0).
If the unknown charge is positive, determine the number of “missing” electrons. If the unknown charge is
negative, determine the number of excess electrons that are on the sphere. The charge on an electron is
equal to -1.60 x 10-19 C. Show your work.
Electric Field Lines I
Which numbered charges, if any, are positive? ________________________
Which, if any, are negative? ________________________________
How did you determine this?
Rank the magnitudes of the charges from smallest to largest, including ties. How did you determine this?
Electric Field Lines II
1. Are the charges like or unlike? How did you determine this?
2. Which charge has the larger magnitude? How did you determine this?
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