Electric Fields
Studio Physics I
This experiment will be carried out using a simulation module on the computer. You will need a program
named “electric fields.html”. To get the program, find the link at the bottom of the activity list under the
activity icon on the webpage. Once the program opens, the box on the left of the screen will contain a
table of contents for this module. If you lack information about necessary formulas or relationships, you
can likely find what you need in one of the sections shown in that box. We will start off this exercise by
investigating the concept of electric field lines.
Part I-Electric Field Lines
Click on "More Examples" in the table of contents. Investigate some basic situations by using the
simulation examples that are available. When the cursor is placed in one of the rectangular box, it gives
some information about the electric field due to the charge(s). An arrow appears with its tail on the
cursor. Think of the arrow as a vector that represents the electric field at the point of the cursor. The
direction of the arrow gives the direction of the electric field, and its length tells the strength of the
electric field. The tail of the vector is always on the cursor, and the head at the other end.
The following commands can be used with this simulation:
A) Right click on the mouse button draws a field line.
B) Left click on the mouse button while holding down both alt and control will drop a permanent
arrow representing the field.
C) Type "e" to erase a field line
1. Do electric field lines point from or to a positive charge? Do they point from or to a negative charge?
2. Choose one of the example simulations and click the right mouse button several times somewhere
within the box. Several electric field lines should become visible. Consider a region in the box in which
no field line is visible. Is the electric field necessary zero in that region? Explain your reasoning.
Be sure to look at the more complicated examples at the bottom of the page.
3. Suppose that you have two charges, one of these charges has a charge of +3 C and the other charge
has a charge of +2 C. Sketch a graph of the electric field associated with the two charges.
Go to the very bottom of the "More Examples" page. Click on the phrase "animated example".
View the simulation if desired, but be sure to go to the interactive version with is linked to at the
bottom of this page.
4. Suppose that you have a pair of charges as shown below. Sketch this configuration on your paper and
mark the approximate location of all points (other than at infinity) along the line connecting the two
charges where the field is equal to zero. If it is not zero anywhere along this line (expect at infinity) state
that.
1Q
2Q
5. Suppose that you have a pair of charges as shown below. Sketch this configuration on your paper and
mark the approximate location of all points along the line connecting the two charges where the field is
equal to zero. If it is not zero anywhere along this line (expect at infinity) state that.
1Q
4Q
6. Suppose that you have a pair of charges as shown below. Sketch this configuration on your paper and
mark the approximate location of all points along the line connecting the two charges where the field is
equal to zero. If it is not zero anywhere along this line (expect at infinity) state that.
1Q
-2 Q
Part II-Hidden charges
Click on "Hidden Charges" in the table of contents. The program displays a rectangular area that
contains some hidden charges. Use the cursor to explore the area containing the charges, and then answer
the following questions:
The following command can be used with this simulation:
A) Left click on the mouse button while holding down both alt and control will drop an arrow
representing the field.
7. How many charges are in the rectangular area?
8. Give the coordinates and sign of each charge. (The coordinates of the point where the cursor is
located appear at the lower left of the rectangular area. If you manage to put the cursor directly on a
charge, the line will disappear, and a small circle will appear at the site.)
9. Based on the number and sign of charges you found on the screen, predict at how many points in the
area shown the electric field is equal to zero. CAREFULLY DESCRIBE HOW YOU CAME UP WITH
YOUR PREDICTION. DRAW A PICTURE IF YOU WISH, BUT BE CLEAR ABOUT YOUR
EXPLANATION.
10. Now actually determine at how many points in the area on the screen the electric field is equal to
zero. Was your prediction above correct? If not, explain now how you would predict the number of zero
electric field points on the screen.
11. Give the coordinates of those points where E = 0.
12. Determine how far the zero point(s) that you found is (are) from each charge.
13. Rank the magnitude of the charges least to greatest. HOW DID YOU COME UP WITH
YOUR ANSWER? Please explain clearly using complete sentences.