Coulomb’s Law – PHY 202 1. Introduction The electrostatic force between two-point charges (two charged point particles) is inversely proportional to the square of the distance between the charges and directly proportional to the product of the charges (or to each charge). This relationship is stated mathematically in Coulomb’s Law: F = (k q1 q2)/R2 where F is the electrostatic force, q1 and q2 are the charges, R is the distance between the charges and k is a proportionality constant whose numerical value depends on the system of units used. The magnitude of the force is the same on both particles and its direction is along the line that crosses the centers of the two particles. If the two charges have the same polarity the forces are repulsive (point away from the other particle) and if the two charges have different polarities the forces are attractive (point towards the other particle). In this virtual laboratory we are going to calculate the value of the proportionality constant k using a simulation to determine the force between two charges as a function of distance and as a function of the charge of one of the particles. We will then plot the data using the Capstone software and finally we will fit the data, using the appropriate line type, to determine the value of k. 2. Data Generation Using the PhET Simulation Open the PhET Coulomb’s Law simulation at: https://phet.colorado.edu/sims/html/coulombs-law/latest/coulombs-law_en.html Select the atomic scale mode. a) Force as a function of distance. Assign to each particle a charge of +2e. Position charge 1 at 0 pm and charge 2 at 10 pm, record the force (magnitude and sign). Repeat the measurement increasing the separation by 10 pm each time until you reach 100 pm. Create a table with two columns, one for separation and one for force. b) Force as a function of charge. Assign to particle 1 a charge of +2e and to particle two a charge of -10e. Position charge 1 at 0 pm and charge 2 at 40 pm, record the force (magnitude and sign). Repeat the measurement increasing the charge of particle 2 by +2e increments each time until you reach +10e. Create a table with two columns, one for the charge of particle 2 and one for the force. Notes: • • • e is the electron charge = 1.6022×10-19 Coulomb 1 pm = 1 picometer = 1×10-12 m Use SI units 3. Data Analysis a) Use manual entry to create a table for the force as a function of distance data (one column is force the other is distance). Fit the data with the fitting function inverse square no offset (y = A/x2 +B) and determine the value of k from the fitting value of A. b) Use manual entry to create a similar table for the force as function of charge data (one column is force and the other is charge). Fit the data with a linear function (y = m*x + b) and determine the value of k from the fitting value of m. c) Compare the values of k obtained in a) and b) with the accepted value k = 8.988×109 Nm2/C2, by calculating the % Difference Error between the fitting value and the accepted value: % Difference Error = 100*([Fitting Value]-[Accepted Value]) / [Accepted Value]. Explain the source of possible discrepancies. d) For your report include the tables, graphs, curve fittings, values of k, % Difference Errors and explanation of discrepancies.
