First computer exercise for 202.

EM Fields tutorial

This exercise is designed to help you visualize electric fields and electric potentials by representing them as vectors and lines on the screen. Please follow the directions and answer the questions as you go along. Your report should consist of answers to each of the questions and a series of sketches of the field and potential lines. Instructions for finding and starting EM fields is found below.

Part one

Start the EM program. You may read the instructions now or later. Use the mouse to pull down the Select menu and choose 3D point charges. You will see a row of spots along the bottom of the screen. The light colored spots represent negative charges, the dark spots represent positive charges. Pull one positive and one negative charge of the same magnitude onto the screen and place them a few inches apart.

1) Pull the Field &Potential menu down and select Directional Arrows. Now click on the screen with the mouse. At each point that you click the program draws a vector whose direction shows you the direction of the electric field and whose length indicates the magnitude of the electric field at that point. Include a sketch of this in your report.

2) After you have made many arrows (say 50 or so) at varying distances from each charge, write a careful description (this will require several sentences) of the electric field around the charges.

3) Now choose the Field lines option from the Field & Potential menu. Now click in several places on the picture. What is the relationship between the lines drawn by the computer now and the arrows you saw in step 2?

4) From the Display menu choose Clean up screen. The two charges should remain on the screen.

5) Now choose Potential from the Field & Potential menu. Again click on the screen at several locations. The numbers represent the electrical potential at each point. Sketch and describe the picture and tell why there are no arrows in this picture.

6) Choose Equipotential from the Field & Potential menu and click on the screen in several places. What is the relationship between the numbers in step 5 and the equipotentials now on the screen (you may want to go back and choose Potential from the Field & Potential menu and place some numbers directly on the equipotential lines to answer this question).

7) Choose Clean up screen again. The two charges should remain on the screen.

8) Now select Equipotentials with numbers and put 5 or 6 equipotential lines on the screen. Then choose Potential Difference from the Field & Potential menu. Click on one of the equipotential lines and while holding the mouse down drag it to another equipotential line. Electrical potential difference is the number a voltmeter registers between its two probes. How do the numbers that appear on the screen (which represent the electrical potential difference) relate to the numbers on the equipotential lines? Explain how you would get the numbers for potential differences from the equipotential numbers?

9) Find the potential difference between two equipotential lines by dragging the mouse. Now drag the mouse back the other way (from the second line back to the first) to find the potential for the reverse trip. How are they related?

10) Clear the screen again and put 5 or 6 equipotential lines on the screen as you did in step 6. Now choose the Field lines option and put several electric field lines on the page. What is the relationship between electrical equipotential lines and electric field lines? How are they related mathematically (according to your book)?

Part two

Repeat steps 1 through 10 above but using a different charge distribution. This time make a horizontal row of positive charges, all the same size and nearly touching. Then make a parallel row of negative charges located about and 1 1/2 inches away (they should look like the cross section of a parallel plate capacitor). Be sure to draw several sketches of the field lines and equipotentials.

Part three

Repeat step 10 above but using a different charge distribution. Use any charge distribution you think might be interesting. Be sure to draw a sketch of the field lines and equipotentials and label which are which.

Where is the EM Field program?

EM Field is a cT program now located on the Macintosh computers in PS100. You may also request a copy of the program from the instructor. Currently it is only available for Macintosh computers.

Return to the IUS Physics Top Page.

Contact Kyle Forinash, kforinas@ius.edu, for comments/suggestions/corrections.