PHY 212 Laboratory for 1/20/04

Electrostatics

In this lab you will investigate the nature of static electricity through a series of experiments. Static charge on any object will quickly leak away in the presence of moist warm air. Consequently, the lab will be kept as cold and dry as possible. You have been warned. Wear a warm jacket to lab! When you enter the lab, do so by opening the door only slightly, quickly slip in, and quickly close the door behind you.

All matter is composed of atoms. In the classical picture, an atom consists of a massive positive nucleus with light negative electrons in orbit about the nucleus. In conducting substances, the outer electrons move freely from one nucleus to the next. (This does not mean they can freely move out of the substance!) In insulating substances, they are held tightly to the nucleus. Nevertheless, because of differences in electron affinity, it is often possible to rub two insulators together and transfer electrons from one to the other. The combinations we will be using in lab are Bakelite, Lucite, and glass with wool, cat's fur, and silk.

  1. Repulsion Between Like Charges

    1. Rub the Bakelite rod with wool and bring it near, but not touching, the pith balls. Note what happens. Make a sketch showing how charges are distributed. (See important note below!)
    2. Touch both pith balls with the rubber rod and sketch the results. (You may need to drag the rod across the pith balls to achieve the desired effect.)

    IMPORTANT NOTE:

    When indicating the location of charge in sketches, show only the excess positive or negative charge in the area. Never show "+" and "-" signs next to one another. Neutral matter is composed of positive and negative next to each other and should be represented without signs. The number of "+" or "-" signs is arbitrary, but be sure that the numbers are qualitatively correct. For example, assume an object has an induced charge separation, but is neutral overall. This would be the case for the pith balls before they are touched by the bakelite rod. The number of positive and negative signs indicated in the diagram must be equal.

  2. Determining Signs

    1. Charge the electroscope with the charged bakelite rod by rubbing the rod on the knob of the electroscope. This charge is labeled negative and will serve as the reference. It may be necessary to recharge it occasionally.
    2. Test the sign of the charge on each of the objects listed below by bringing them near (but not touching) the electroscope knob. Draw a picture of the electroscope and object showing how the charges are distributed. (It is only necessary to show this for one negatively and one positively charged object.)
      1. The wool used to charge the bakelite
      2. The cat's fur used to charge the bakelite
      3. Both the silk and glass rod after rubbing them together
      4. Both the wool and lucite rod after rubbing them together
      5. The Leyden jar after it has been charged by the Van der Graaff generator
        Important: Do NOT touch the Leyden jar to the electroscope!

  3. Charging by Induction

    Follow the four steps below and provide a sketch of each step showing how charges are distributed.

    What charge do you think now resides on the electroscope? Test your hypothesis using the method from part 2. (There is a slightly different procedure that might make it easier to see what has happened. Place your free finger on the electroscope first, then bring the bakelite rod near, remove your finger, then remove the rod.)

  4. Charge on a Spherical Conducting Shell

    In this experiment, you will determine exactly where charge resides on a conductor. When we study Guass's Law, we will see why this must be. Charge the sphere using the proof plane. The proof plane will be charged by touching it to a charged Leyden jar or directly to the Van der Graaff generator. Describe your results for each case.

    1. Charge the proof plane and touch it to the outside of the neutral shell. Use the electroscope to determine if a charge has been transferred to the shell.
    2. Repeat the last procedure, but touch the proof plane to the inside of the shell. Be sure not to touch the edge of the hole in the sphere.
    3. Charge the sphere by touching it to the Leyden jar or generator. Touch a neutral proof plane to the outside of the shell and check to see if charge has been transferred to the proof plane.
    4. Repeat the last procedure but touch the proof plane to the inside of the shell. Again, be sure not to touch the edge of the hole in the sphere.