Coulomb's law describes electrostatic forces. Electrostatic force is an important force because it is the force that keeps molecules together.
The first verification of the law of electrostatic forces was made by the French engineer Charles Augustus Coulomb (1736-1806). Using hairs and wires he constructed a torsion balance similar in design to Cavendish's experiment. The amount of torsion required to bring a charged pithball within various distances of another pithball allowed for the calculation of k, the electrostatic constant.
Materials: ring stand with wooden dowel, two pith balls, string,
fur, charging rod, protractor, electronic balance, ruler
1. Obtain an average mass for the two pith balls and measure the length of the string (L) to the center of the pith ball.
2. Suspend the two pith balls from the wooden dowel attached to the ring stand such that their strings have a common point of origin.
3. Using the fur, charge the rod negative and transfer the charge to the two pith balls.
4. Measure the angle (= 2 q in the equation below) that the two strings make with each other at the point of origin.
1. Sketch a free body diagram for each pith ball showing all forces acting on each while suspended by the electrostatic force.
2. Using the value of electrostatic constant (k) and the problem we solved in class, determine the charge (q) on each pith ball.
3. Using dimensional analysis, verify that the units in your equation condense
to a coulomb.
4. Calculate the number of excess electrons on each pith ball while each was suspended.
5. Why were equal sized pith balls used?
6. If an ampere is defined as the flow of electrons in Coulomb/second (C/s), how many amps would each pith ball generate if it flowed in a wire. Compare to the current consumed by a television set.
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