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Unformatted text preview: Lab 1: Coulomb’s Law 1 Introduction Coulomb’s law describes the electrostatic force be- tween two point charges. It is encountered whenever there is a charge build up (as in a lightning storm), or when charges are stripped off by frictional forces (static during winter months). When two point charges, q 1 and q 2 , are separated by a distance r , the force between them is described by Coulomb’s law F = kq 1 q 2 r 2 (1) The Coulomb constant k is defined as k = 1 4 πε (2) where, ε = 8 . 85 × 10- 12 C 2 /Nm 2 is the permittivity of free space. This constant specifies the strength of the electrostatic force, and is related to the speed of light in vacuum. The strength of the Coulomb force falls off with the inverse square of the distance between the charges. It is interesting to note that the gravitational interaction between two masses ( F = Gm 1 m 2 r 2 ) is also characterized by an inverse square law. Although the inverse square law is encountered in other areas of Physics (for example, the variation of light intensity as a function of distance from a point source), it is often presented as a fundamental rule, and seldom verified in a laboratory! During this lab, you will have the opportunity to test the prediction of Coulomb’s law by measuring the force between two charged spheres as a function of their distance of separation. You will also be able to test the dependence of the electrostatic force on the charges on the spheres. Finally, you will learn about how to analyze your results by rescaling your data in appropriate ways. EXERCISE 1 PERTAINS TO THE BACKGROUND MATERIAL AND EXERCISES 2-11 PERTAIN TO THE EXPERIMENTAL SECTIONS. 2 Background The experiment involves the use of a torsion balance to measure the electrostatic force between two charged spheres. The balance consists of a thin torsion wire that suspends a conducting sphere on a rod with a counter weight on the other side. There is a dial on the top of the balance that allows you to twist the wire through a given angle. Two sighting lines, one on the counter weight, and the other on the stand, allow you to adjust the balance to a known position. The wire can also be twisted using the screw on the base, which can be used to zero the balance. Figure 1 is a sketch of the apparatus. The figure la- beled top view shows the setup observed looking down from beneath the dial. For a photo of the apparatus, refer to Appendix E. Caution: This balance is very fragile. The torsion wire should be handled with care. When you need to adjust the dial or the screw at the base of the balance, turn them slowly. There are two other screws, one under the dial and the other below the counter weight, that shouldn’t be touched....
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