Coulomb's Law Pre-lab Assignment

Your name: _____________________

Print this page, record your answers on it, and show it to your lab TF at the start of your lab session. In this experiment you will investigate the force between charged objects by giving two balls equal charges and placing them at various distances from one another. It is difficult to measure the force one ball exerts on the other, so instead you will measure something that should be proportional to this force.

In the lab the force that one charge exerts on the other is balanced by a force associated with a twisted wire - the force provided by the wire should be proportional to the twist angle of the wire. We'll do something similar here, except that the balancing force will come from an ideal spring.

Click the button to run the simulation. For several positions of the movable charge, record the charge position (r) and the position of the blue rectangle (d, the length of the spring) after you have moved the blue rectangle to balance the forces. Then calculate x, the distance the spring is compressed or stretched compared to its equilibrium length of 10 units.

Distance between charges (r)______ ____________ ____________ ______
Position of blue rectangle (d)______ ____________ ____________ ______
x (Note that x = 10 - d)______ ____________ ____________ ______

How should the magnitude of the spring force depend on x? Is the force proportional to x, x2, or is there some other relationship?

Our goal now is to determine how the force that one charge exerts on the other depends on r, the distance between them. Since the spring force should be proportional to x (and the spring force should be equal to F, the force between the two charges, since you balanced the forces) we can use the measured values of x in place of F. We'll be off by some numerical factor, but we don't care about that - we're interested here in the r dependence.

Enter your values of x (which is essentially F) and r into Graphical Analysis and figure out how the force between the charges depends on r.