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Unformatted text preview: HB 02-22-06 Coulomb Balance Lab 2 1 Coulomb Balance Lab 2 Equipment Coulomb balance, 5x5 inch plastic plate, Vernier calipers, power supply, set of small weights (less than 500 mg) with tweezers, Resistance box set at 1 MΩ, 5 leads Reading Coulomb’s Law in your text book Important Information 1. HIGH VOLTAGE FROM A POWER SUPPLY IS APPLIED TO TWO CAPACITOR PLATES. DO NOT TOUCH THE WIRES PROVIDING THIS VOLTAGE. BE SURE THAT A 1 MΩ RESISTOR IS IN SERIES WITH THE CAPACITOR. 2. This experiment uses a laser beam as an optical lever arm. DO NOT LET THE LASER BEAM OR ITS REFLECTION ENTER YOUR EYE. SERIOUS DAMAGE TO YOUR EYE MAY RESULT. 3. A capacitor plate pivots on two knife edges. The knife edges rest on flat surfaces. Both the knife edges and flat surfaces are easily damaged. Please handle them with care, using the centering rod, described below, to center the knife edges and to gently lower the knife edges onto the flat surfaces. 1 Introduction In SI units the magnitude of the force F between two charges q 1 and q 2 in vacuum is given by F = q 1 q 2 4 π R 2 , (1) where R is the distance between the two charges and is a constant called the vacuum permittivity. The unit of charge in SI units is the coulomb. This equation shows that 1 / plays the role of a proportionality constant, which for any values of the charges gives the correct magnitude and dimensions for the expression so that the quantity on the left properly represents a force. The inverse of permittivity therefore expresses the scale for the strength of the electrostatic interaction between charges. In this sense the permittivity plays a role for electrical forces which is analogous to that played by the gravitational constant in Newton’s expression for the gravitational force between two masses. The experiment to be performed with a Coulomb Balance is to measure the force between two separated charged objects having a known voltage between them and having a known configuration. From these data the value of can be deduced. The experiment is the electrical analog of the Cavendish experiment for gravitational forces, which measures the gravitational force between two known masses. Maxwell showed in the mid-19th century that by combining his equations containing the electric and magnetic fields he could predict the existence of a propagating wave containing both types of fields. He predicted that the speed c of the wave in vacuum is given by c = 1 √ μ . (2) HB 02-22-06 Coulomb Balance Lab 2 2 The new constant μ is called the “vacuum permeability.” It appears in the expression for the force between two electrical currents separated by vacuum and is a magnetic effect. The predicted value for c , approximately 3 × 10 8 meters per second, has proved to be identical to the value obtained by Michelson for the speed of light, thereby supporting the theory that light is an example of electromagnetic radiation. Consequently, by measuring and μ we have an indirect way of determining c , the speed of light. This experiment is concerned with, the speed of light....
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