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 the electric field .
Part I-Electric Field Vectors and 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. Drop about 50 permanent field vectors on the example of the single positive charge (see B in the box
above). Drop some of the arrows so that they are (radially) in a line with the charge. Copy some
approximation of the figure shown below onto your activity sheet. Draw the electric field vectors at all
the points marked with an x. Be sure to show both the appropriate direction and length of the arrow
representing the vector.
x
x
x
x
x
x
x
x
x
x
x
2. Return to the simulation program and draw some field lines through the field vectors (see A in the box
above) . Draw field lines through the field vectors on your sketch in #1 above. Draw the field lines as
DASHED lines so that you can still see the vectors.
3. What is the relationship between electric field vectors (the arrows) and electric field lines? Do
electric field vectors point from or to a positive charge? Do field lines point from or to a positive charge?
Do field vectors and field lines point from or to a negative charge?
Part II -Connecting Electric Field to Electric Force
4. 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 place 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 charge if it were placed
at the point.
Using this information, your knowledge of the electrostatic force and the simulations of the single
positive charge and single negative charge, describe at least two ways that what you observe with electric
field vectors is consistant with this definition of the direction and magnitude of the electric field.
5. 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.
6. 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 the electric field vectors associated with the two charges in the region
surrounding the charges. Sketch the electric field lines.
Part III-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? Explain how you came up with your answer.
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 (in pixels) 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.