Coulomb's Law Pre Lab Answers

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Last revised December 15, 2014

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Experiment 6
Coulomb’s Law - PRELAB
0. Pre-lab Homework (2 points)
The pre-lab homework must be handed to the lab TA at the start of the lab.
1. Why is it important to recharge the spheres before each measurement?

2. Consider the function y = xn (for n = -2). Take the natural logarithm of both sides and plot ln(y) vs. ln(x) on the graph below. Explain or show how to obtain “n” from the graph. (Hint:
Can you fit the graph below to a straight line?) (use x=1,
2.2 Theory
Take one gram of protons and place them one meter away from one gram of electrons. The resulting force is equal to 1.5 x 1023 Newtons-roughly the force it would take to lift an object from the surface of the Earth that had a mass about 1/5 that of the moon-not a small force. So, if such small amounts of charge produce such enormous forces, why does it take a very delicate torsion balance to measure the force between charged objects in the laboratory? In a way, the very magnitude of the forces is half the problem. The other half is that the carriers of the electrical force-the tiny proton and the even tinier electron-are so small, and the electrons are so mobile. Once you separate them, how do you keep them separated?
The negatively charged electrons are not only drawn toward the positively charged protons; they also repel each other. Moreover, if there are any free electrons or ions between the separated charges, these free charges will move very quickly to reduce the field caused by the charge separation.
So, since electrons and protons stick together with such tenacity, only relatively
3. Laboratory Work
3.1 Procedural Outline
The verification of Coulomb’s Law proceeds as follows. A diagram can be found in Figure
6.1.
1. Charges are placed on the spherical conductors with a high voltage power supply.
2. Due to the presence of the charge, a force is induced between the two spheres.
3. The force (F) between the spheres will produce a deflection () of the sphere attached to the torsion fiber. These two quantities are related in the following way,
F = k
Equation 6.2
Where k is the torsion constant of the fiber, it is not the coulomb constant (which we call k1 )

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4. The charge dependence is measured by altering the charge of one sphere, keeping the spheres’ separation constant, and measuring the resulting change in  (Thus inferring