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EM1_Manual

# EM1_Manual - General Physics II Lab EM1 Coulombs Law...

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General Physics II Lab EM1 Coulomb’s Law General Physics II Lab EM1 Coulomb’s Law Purpose In this experiment, you will learn the basic operation of the Coulomb balance and use it to study Coulomb’s law. Equipment and components Coulomb balance, slide assembly, high voltage power supply, charging probe and grounding probe with holders, acrylic transparent box to cover the whole setup, hair-blower, and stop watch. Background Coulomb’s law states that the electric force between two point charges q 1 and q 2 at separation r is 1 2 e 2 q q k F r = , (1) where k e = 8.9875 × 10 9 N m 2 /C 2 is the Coulomb constant. In Equation (1), the repulsive electric force has a positive sign and the attractive one has a negative sign. How large is this Coulomb force? Take one gram of protons and place them one meter away from one gram of electrons. The resulting force is equal to F 1.5 × 10 23 Newtons -- roughly the force it would take to “lift” an object that had a mass about 1/5 of that of the moon from the surface of the Earth -- not a small force. Because of the strong attraction between the positive and negative charges, only a small amount of charge differentials can be sustained in the laboratory. As a result, one needs a sensitive device to measure the electric force. In this experiment, you will use a delicate torsion balance, named Coulomb balance, to investigate the force between two identical charged spheres. Figure 1 Setup of the Coulomb balance Revised: 31 July 2009 1/8

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