PROCEDURE Think about the electroscope that we used in the first lab.S

Physics 360 Neumann Virtual Lab 3A – Capacitance Excel Exercise

PURPOSE To practice with simple Excel commands, including writing a function, graphing, and finding a best-fit line.

PROCEDURE Think about the electroscope that we used in the first lab. Hopefully, you noticed that the more charge was transferred to the electroscope, the higher the gold foil leaf was rising. Let’s see if we can use that to verify Coulomb’s law, while practicing working with various Excel commands and option.

Recall that the entire body of the electroscope (not including the casing) is metallic – and thus the charge that you transfer to the ball will be uniformly spread throughout the electroscope. Basically, that means that q1 = q2 = ½ q, where q1 is the charge on the metal plate, q2 is the charge on the gold foil, and q is the total charge transferred to the electroscope. Below is the data representing the charge transferred to the electroscope vs. the separation between the tip of the gold foil and the tip of the metallic plate. We will use this distance to represent r in Coulomb’s law, although, strictly speaking, it’s not quite correct as the charges we are using are not point charges.

q [pC] r [cm] q [pC] r [cm] q [pC] r [cm]

0.80 0.050 1.20 0.090 1.80 0.130

0.90 0.060 1.50 0.100 1.85 0.140

1.00 0.075 1.60 0.120 1.95 0.150

1.10 0.080 1.70 0.130 2.00 0.150

1. Transfer this data into Excel or Google sheets (make columns 1 and 2). Label the columns!

2. Convert cm to m, and pC to C (1 pC = 10-12 C) – do this using formulas in Excel, not by hand (columns 3 and 4).  Calculate q1 (column 5). Again, don’t forget to label the columns (with units).

3. Use Excel formulas to calculate Coulomb force for each case (column 6).

4. Plot F vs. r (Chart 1, leave in Sheet 1). Remember to label the graph, and both axes with units.

5. What shape did you expect for your plot in #4? What shape did you get?

6. Calculate r2 (column 5) and 1/r2 (column 7).

7. Plot F vs. (1/r2) (Chart 2, leave in Sheet 1). Remember to label the graph, and both axes with units.

8. What shape did you expect for your plot in #7? What shape did you get?

9. Plot q2 vs. r2 (Chart 3 – put in separate sheet). Remember to label the graph, and both axes with units. Your graph should be linear. Create a best-fit line, and display the equation and the R2 value on the graph.

10. Make a second q2 vs. r2 (Chart 4 – put in separate sheet), this time make the line go through the origin. Again,  create a best-fit line, and display the equation and the R2 value on the graph.

11. Think about the slope of the two lines in #9 and #10. What does the slope represent? Think about the physics of this problem. Which of the two graphs is more appropriate, and why?

12. Based on your answer to #11, calculate the Coulomb force. Show your work.

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