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 12 e 2 qq 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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General Physics II Lab EM1 Coulomb’s Law Figure 1 above shows the setup of the Coulomb balance. A conductive sphere is mounted on a rod, counterbalanced and suspended from a thin torsion wire. Under an external force, the sphere can swing in the horizontal plane, causing the torsion wire to twist. The sphere reaches a mechanical balance when the external force is counterbalanced by the torsional force. If the torsion balance is calibrated against known rotational angles, one can use it to accurately measure the force acting on the sphere. Figure 2 Experimental setup To measure the electrostatic force between two charged spheres, an identical conductive sphere is mounted on a slide assembly so that it can be positioned at various distances from the suspended sphere. This is shown in Fig. 2. To perform the experiment, both spheres are charged, and the sphere on the slide assembly is placed at a fixed distance from the equilibrium position of the suspended sphere. The electrostatic force between the two spheres causes the torsion wire to twist. The angle through which the torsion wire is twisted back to its original balance position is directly proportional to the electrostatic force between the two spheres. All the variables in Eq. (1) can be varied and measured using the Coulomb balance.
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This note was uploaded on 03/31/2011 for the course PHYS 1 taught by Professor Nianlin during the Spring '11 term at HKUST.

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

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