Workout: Electric field


Read the web page: Electric field.


To give you some practice on working with electric fields, go to the PhET simulation Charges and Fields to perform a few tasks.

Set up

Select the menu icon in the lower right (three parallel horizontal lines) and choose "Options." Select "Projector Mode" in the popup box. Turn on "Grid" in the gray controls box at the upper right and make sure all other boxes are unchecked. Your screen should look like the figure below.

Answer these questions

1. Place a positive 1 nC charge at a grid-line crossing. Place a sensor somewhere near it and describe what you see. Does the length of the arrow on the sensor show that it is measuring something that falls off like the square of the distance?

2. Now put the positive charge back and replace it with a negative charge. Does what happens give you enough information to answer the two questions:

  1.  Is the sensor measuring the electric force felt by the sensor or the electric field at the sensor's position?
  2.  Is the sensor a positive charge or a negative charge?

If you have enough information to answer, what are the answers? If you don't what additional information would you need to decide?

3. Now put the sensor back and check the boxes "Electric field" and "Direction only" in the controls box. Put a source charge (either positive or negative) somewhere on the board. Describe what you see. What do the arrows represent? What do the little white dots in the middle of the arrows mean?

4. Uncheck the box "Direction only". What happens? What is representing the strength of the electric field?

Joe Redish 2/11/18

Article 391
Last Modified: February 6, 2019