# Workout: Coulomb's law

Read the web page: Coulomb's law and the Follow-on pages.

## Launch

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

## Set up

Choose to the menu icon (the three parallel horizontal lines) at the lower right and select Options. In the box that appears, turn on "Projector Mode". Then, in the selections box at the upper right, turn off everything except "Grid". To add an element to the page, grad a positive charge (red +), negative charge (blue -), or sensor (yellow dot), and put them anywhere on the page. To get rid of them, move them back to the place where you got them.

## Answer these questions

1. From the palette of tools at the bottom of the page in the middle, grab a + charge and put it in the middle of the grid on a node (where two of the stronger lines cross). Take a sensor and put it one horizontal space away. The program shows a representation of the force the sensor feels. Explore how this depends on the position near the + charge. What kind of charge is the sensor? Do you think the program is correctly using Coulomb's law to represent the force on the sensor? Do a couple of quick and easy tests and explain why you think so.
2. Put a positive and a negative charge on the grid two boxes apart as shown in the figure. Place a sensor one box to the left of the positive charge and measure the force it feels. If you now put a positive charge one box to the right of the negative charge along the same line with the positive charge, how do you expect the force to change? (Get bigger? Smaller? Stay the same?) Explain why you think so and then test it. Did the result agree with your prediction?
3. Now move the sensor to the vertical line between the two charges and one box up from the line joining them. In what direction does the force on the sensor point? Why does it do that? Examine what's going on by removing each of the two charges one at a time to see the force they would create on the sensor by themselves and explain why the combined result looks like it does.

Joe Redish, Fall 2017

Article 532