2022W 136-2 01 Coulomb

Lab 1: A Law of Electrostatic Force In this lab, you will use an online simulation to investigate how the charges of two objects and the distance between them effect the electrostatic force between them. After this lab, you should be able to describe the functional dependence of the force on charge and radius and draw simple conclusions from this understanding. You should also be able to linearize data and understand how to investigate functional dependence of a physical quantity by systematically changing related physical quantities. Introduction You are probably aware that like charges repel, while unlike charges attract. You may also have learned the quantitative formula for the attractive or repulsive force. These facts, like all physical laws, were established through rigorous experiments. In this lab we will use a simulation to experimentally derive the equations for electrostatic force. Electric charge is measured in units of Coulombs , abbreviated as C. In practice, a single Coulomb is a lot of charge, so in this lab we will mostly be dealing with µC (microcoulomb, = 1 × 10 − 6 C). Familiarization and Setup First, open the simulation 1 and click on the simulation. Play with the simulation for a little while to familiarize yourself with its controls. We will use the “Macro Scale” option. a) The vertical dotted lines through the centers of the two charges, q 1 and q 2 indicate the locations of the center of the charges. The ruler itself is also movable. b) The legends above the two charges give the force of interaction. The “force values” checkbox at the bottom right enables this option. c) The controls at the bottom allow you to change the value of the two charges. 1 https://phet.colorado.edu/sims/html/coulombs-law/latest/coulombs-law_en.html Page | 1

Force versus Charge First, determine how the electrostatic force depends on the electric charge. This simulation allows us to vary the charge on the two objects ( q 1 and q 2 ) as well as the x-positions of those charges ( x 1 and x 2 ). To do this, we will set x 1 , x 2 , and q 2 to arbitrary values and leave them alone while varying only q 1 . 1.1 (5pts) Record your values for x 1 , x 2 , r and q 2 below. ( r = x 2 − x 1 , the distance between the two charges.) A positive q 2 and x 1 = 0 will be most convenient, but any values will work. x 1 (cm) x 2 (cm) r = x 2 − x 1 (cm) q 2 (  C) 4.00 7.00 3.00 8  C 1.2 (5 pts) Now, vary q 1 as shown in Table 1 and complete the table. If the force is attractive (arrow on q 2 points toward q 1 ), enter a negative number. If it is repulsive, enter a positive number. Table 1: Electrostatic force versus electric charge data from PhET simulation. Charge 1 (  C) Force on Charge 2 (N) -9 -639.115 -6 -479.336 -3 -239.668 0 0.000 3 239.668 6 479.336 9 639.115 1.3 (20 pts) We will guess that force and q 1 have a linear relationship : that is, the force obeys an equation of the form F = k 1 q 1 (1) for some as-yet-unknown value of the constant k 1 . Using the LineFit.xslx file included in the Canvas assignment, plot the force (vertical axis) vs. the charge (horizontal axis) and fit the data to this straight-line equation (set the intercept to zero). Save the graph as an image file, and insert the image here. Do not just copy and paste the graph into this document – if you do that and submit your work as a .docx file, your graph may not show up in the submitted file! Record the slope k 1 (determined by the straight line through the data) here. Don’t forget to state the units of Page | 2