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PHY112Lab_3.docxOnline PHY112, PHY172, PHY262 Lab #3 Coulomb’s Law
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You will be using an interactive web-based program to complete this lab.
If you have questions or problems with the interactive website, be sure to ask your TA for assistance.
Instructions: Download and save this document to your computer. Answer the questions directly on this document. When you are done, SAVE the file and return it to your TA via BB Learn. Please contact your TA with any questions or other issues.
1) Given the mathematical representation of Coulomb’s Law,
, where k = 9.0×109Nm2/C2
describe in words what this equation tells us about the relationship among electric force, charges, and distance between the charges.
2) By how much does the electric force between a pair of charged bodies diminish when their separation is doubled? tripled?
3) A negative charge of 2.0 x 10-4 C and a positive charge of 8.0 x 10-4 C are separated by 0.30m. What is the magnitude of the force between the charges? Is this force repulsive or attractive?
4) Now let’s do a simulated experiment to help us better visualize the relationships among electric force, charges, and distance between the charges.
Open the following link in your internet browser:
https://phet.colorado.edu/en/simulation/coulombs-law
Click on the icon to open the PhET simulation for Coulomb’s Law.
Once open, click on the Macro Scale icon:
Don’t worry though, you won’t need to calculate the conversion factor for the change in units. The simulation does that for you, and gives the resulting forces in units of Newtons, [N].
Use the simulation to fill out the table below:
5) Now using the data that you collected in the table above, answer the following questions:
A) How does a change in separation between charges effect the force on the charges? Be specific.
B) If the charges on q1 and q2 differ in magnitude, is there a different magnitude of “felt” force for each charge, or is it the same for each?
C) The result for B) above, may or may not seem intuitive at first, but look at the Coulomb’s Law equation again and see if you can briefly explain in words why this is the mathematically required result.
D) Briefly explain how the force varies as the individual charges vary.
E) Assume that you didn’t know that opposite charges attract, and that like charges repel. Looking once again at the Coulomb’s Law equation, explain in words why you would mathematically expect the results you found in the last column of the table above.
6) How Coulombs law was discovered
Coulomb thought of a simple way to define the equation of force between two charges. Here is his basic process:
If there is q charge on a small metallic sphere and we touch it with another similar but uncharged sphere, then by the concept of symmetry, the charges on each of the spheres become q/2 due to redistribution. In other words, half of the charge on the first sphere is redistributed to the second sphere and now each have half of the original charge.
Continuing the above process, he could obtain charges of q/4, q/8 etc.
He also varied the distance between a fixed pair of charges and measured the force between them.
In another step, he varied the charges in pairs and then kept them at fixed separations.
To measure the force the charges had on each other, he used a device called a torsion balance.
Finally, he arrived at the law that we call the Coulomb's law. He did all this without knowing the explicit magnitude of the charges on the spheres.
Watch the following YouTube video. It is a simple representation of how Coulomb’s torsion balance worked.
Coulomb published the paper describing the Law which now bears his name in the year 1785.
There is another very important physical law which was published almost 100 years earlier in 1687. That was Newton’s universal law of gravity.
Look up Newtons gravitational law on the internet or in your physics text. In your own words describe the similarities between Coulomb’s law and Newton’s gravitational law.
In 1797–1798 British scientist Henry Cavendish preformed what became known as the Cavendish experiment. It was the first experiment to measure the force of gravity between masses in the laboratory. His experiment bears quite a resemblance to Coulomb’s earlier experiment involving the force between charges.
Look up information on the Cavendish experiment, and BREIFELY describe the similarities between it and Coulomb’s experiment.
Return this lab to your TA via BB Learn by the deadline.
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