PHYSICS 127 EXPERIMENT NO. 1

ELECTROSTATICS

Introduction

Electrostatics is the study and analysis of static, electrically charged objects, the charges, and the forces between them. In this experiment you will observe some of the phenomena associated with electrically charged objects, and you will measure the amount of charge on a small conducting sphere.

Experimentation

  1. Charge the black rod by rubbing it briskly with a cloth, and then touch the top plate of the electroscope with the rod. Describe and explain the behavior of the electroscope by drawing a diagram and showing the location of the charges.
    The electric charge on the rod was arbitrarily defined as negative (by Benjamin Franklin), and thus you give a negative charge to the electroscope when you touch it with the rod.
    Next, remove the rod, and touch the plate of the electroscope with your finger. Describe and explain the behavior.
  2. Charge the black rod again and bring it close to, without actually touching, the electroscope plate. Describe and explain the behavior.
    Touch the electroscope plate momentarily with your finger and then remove the black rod away. Describe and explain the behavior.
    With this process, you have charged the electroscope by induction. Does it have a negative or positive charge? Describe how you can answer this question experimentally.
  3. Repeat the above procedures using each of the plastic strips instead of the black rod. The plastic strips can be distinguished from each other by noting that one has a small piece cut off at one of its corners. Determine whether the charge produced by rubbing is positive or negative for each case. Of the three objects (rod and strips) charged by rubbing, which received the largest charge?
  4. Charge the Faraday cup negatively. Using the small metal dish with the insulated handle to transfer the charge, check the charge on the outside of the cup by transferring it to the electroscope. Do the same for the charge on the inside of the cup. Explain what you observe.
  5. For this part, use the two small conducting balls suspended from a common point by threads. Put as much charge on the balls as you can. The balls will repel each other and will eventually come to equilibrium with the threads making an angle with each other. Estimate the angle between the threads and the length of the threads. Measure the mass of the balls, and assuming that they have equal charges, draw a force diagram and determine the charge in Coulombs.

Revised: January 7, 1998