Electric Fields
Studio Physics I
This activity will be carried out using a simulation module on the computer. You will need to install and
run a “share-ware” program on your computer. (You can uninstall it later.) To get the program, find the
link at the bottom of the activity list on the Activities page of the Physics 1 web site. Run the installation
program and accept the default locations. After installation, start the program with Start – Program –
Electric Field – Electric Field. When the program starts, select File / New and accept the defaults. The
program will encourage you to register, but you should just exit the registration window. You will
probably want to maximize the program and plotting windows on your computer.
Part I – Electric Field Vectors and Electric Field Lines
1. Click on the Add Charge button (third to the right after the Printer button) and then click
approximately in the center of the plotting grid. Specify 10 units. (The default unit of charge is 10 nano
Coulombs.) You should see a red circle where you clicked with a “10” inside. On your write-up, put “1.
Done.”
2. Next, click on the Tester button (looks like a little volt meter). As you move the cursor around on the
graph, you will see an arrow representing the electric field vector. The length of the vector is roughly
proportional to the magnitude of the vector. You should see that the vector always points away from the
positive charge. On your write-up, put “2. Done.”
3. Click on the Field Lines button (first to the right after the Printer button). The program will draw
electric field lines. A field line is a continuous line that follows the direction of the field vector at each
point on the line. If you put the cursor back in the data area and move it around, you can see that the
electric field lines are in the same direction as the electric field vectors at every point. Your plot should
look something like the picture below:
Answer the following questions:
3A. If an electric field line or vector is not
drawn at a point, does that mean that the
electric field is zero there?
3B. Is there a fundamental law of physics that
gives the length for drawing an electric field
vector of a given magnitude, or is it just a
matter of scaling to make a nice visual
representation?
3C. Where the field lines are closer together, is
the magnitude of the electric field vector less
than, equal to, or greater than locations where
they are further apart. (Check your answer.)
3D. Do the field vectors and field lines point in
the direction you expect for a positive charge?
4. Click on the Add Charge button again and put a charge of –10 units to the right of the positive charge
Erase the field lines with the Cleaning Cloth button and then redraw the lines with the Field Lines button.
The field lines should curve from the positive charge to the negative charge. Click on the Tester button
again and check the directions of the field vectors at various points versus the directions of the field lines.
Also check the magnitude of the electric field to verify the relationship with the distance between field
Rev. 2003 Bedrosian
lines that you should have found for question 3C. Do your results make sense in terms of what you know
about electric field vectors and field lines? If not, check with your TA or instructor.
Part II – Connecting Electric Field to Electric Force
5. The direction of the electric field at a given point is defined to be the direction the electrostatic force
would be on a positive charge if one were placed at the point. The magnitude of the electric field at a
given point is the magnitude of the electrostatic force that would be present on a 1.0 C charge if it were
placed at the point. However, there does not actually have to be a charge present at that point.
Using this information, your knowledge of the electrostatic force and the simulation of the positive and
negative charges, describe at least two ways that what you observe with electric field vectors is consistent
with this definition of the direction and magnitude of the electric field.
Part III – More Complex Configurations
6. Add 4-6 additional charges between –10 and 10 units in random locations. Erase the old field lines
and draw new ones. Can you make a general statement about where electric field lines begin and end?
7. Erase all charges and start over. Create a horizontal chain of about eight +1 charges and below it a
chain of the same number of –1 charges. Draw the electric field lines (click the button). (This will look
something line the picture shown below.) How would you describe the electric field between the charges
(as compared to the field you saw in step 6) – approximately constant in magnitude and direction or
changing rapidly from spot to spot? If you had to calculate the force on a charged particle due to a
distribution of charges like those shown below, in the region where the electric field is nearly constant,
would it be easier to use the electric field approach or go back to Coulomb’s Law? Explain.
Rev. 2003 Bedrosian