Purpose:

To investigate Coulomb's Law of electrostatic attraction and repulsion.

Required Accessories:

• Base with grid
• Chamber with mirror, ruler, and glass window
• Acetate strips and Vinyl strips
• Hardboard top, Guide blocks, Cotton squares, Wool squares, Graphite coated spheres
• Polyethyelene insulators Monofilament

Discussion:

The electrical interaction between two charge particles is described in terms of the forces exerted between them. Augustin de Coulomb conducted the first quantitative investigation of these forces in 1784. Coulomb used a very sensitive torsion balance to measure the force between two "point charges", that is, charged bodies whose dimensions are small compared to the distance between them. Coulomb found that the force grows weaker as the distance between the charges increase, and that it also depends on the amount of charge on each body. More specifically, Coulomb's force law states that:

The force of attraction or repulsion between two point charges is directly proportional to the charges and inversely proportional to the square of the distance between them.

Below in this web page an experiment is presented which verifies Coulombs laws by carefully counterbalancing mechanical and electrical forces through the use of a very simple apparatus. Read on to find out more:through

Experiment Description:

The apparatus that will be used to experimentally verify Coulomb's law is illustrated below in Pic 1. It basically consists of a polystyrene ball (or sphere) covered with a uniform conductive graphite coating. The sphere is suspended by a thin thread from two sides of the top of the box so that it is free to move in one dimension. There are also two additional graphite coated spheres fastened to guide blocks with small insulating rods. These balls will be charged and then used to deflect the suspended ball. A scale is provided to measure the deflection, thus allowing us to verify Coulomb's law.

Begin by inductively charging the sphere fastened to the guide block. To this end, first vigorously rub the vinyl strip with the wool square. This creates a "free" static charge on the surface of the insulator through a process known as triboelectrification. The suffix tribo means to rub in Greek, thus triboelectrification simply means to electrify (or charge) by rubbing, or by contact. Interestingly, it is not friction that results in the charging process, but rather a chemical reaction that occurs between the two dissimilar materials. By rubbing the two materials together a larger surface area is contacted resulting in a greater exchange in charge.

Next, touch the coated sphere mounted on the guide block to the surface of the charged plastic strip. The charge on the strip will polarize the charge on the coated sphere. Even though it is polarized, the conducting shell has a net charge of zero since the total number of electrons and protons are equal resulting in a nuetrally charged sphere. Let us assume that the charged plastic strip has a negative charge on it. Then, when the sphere is close to the charged plastic, negative charges will be repelled by the strip away from the strip, while positive charges are attracted to it.

Next, briefly touch the sphere with your finger on the side of the ball away from the strip. Your body essentially grounds the sphere, draining the electrons (or negative charge) at the top of the sphere. This leaves a net positive charge on the sphere. Then, when the sphere is removed from the vicinity of the stip, the positive charges will uniformly distribute themselves about the sphere. In this process, it is said that the sphere has been charged by induction.

The charged ball is then inserted into the chamber through one of the holes in the base. Gently slide the charged ball near to the suspended ball. Just before they touch, the uncharged ball will be drawn toward the charged ball at a very short distance because the uncharged ball becomes polarized. Observing the figure below, the ball in the chamber is initially neutrally charged. However, when the charged ball is placed near the neutrally charged ball, it will attract the negative charges and repel the positive charges. This "polarizes" the neutrally charged ball. Next, push the charged ball a little closer until the touch. When they touch, the charge will be equally distributed between the two balls as shown below.

Once the balls have touched, the charge carried by the sphere glued to the guide block is distributed equally between the two spheres and their like charges cause them to immediately spring apart. The balls quickly reach an equilibrium. That is, the balls remain a fixed distance apart at which the mechanical force due to gravity acting on the pendulum is equal but opposite to the electrical force due to the charges. Diagram 1 below illustrates the force diagram for the two balls. We will develop a formula from this diagram to express the electrostatic force between the two balls as a function of the suspended balls weight and displacement from its center axis.

Results:

DATA:

The measurement commences as follows: The charged ball is placed a distance r from the ball on the pendulum. This distance is recorded. The deflection d of the pendulum is then recorded. The electrical force in N/kg is then calculated from the above equation. This information is tabulated below.

The graph of the repulsive force, Fe, as a function of the separation of the two balls, r, is shown in Figure 1. The curve is observed to be linear, which indicates that the repulsive electric force, Fe, is inversely proportional to the square of distance, r²,between them.

Next, we will the dependency of the electrical force between the two balls on the charges of the balls. Thus, recharge the two balls and position the guide block ball (A) near the suspended ball (B) so that the suspended ball is deflected by a distance d. Next, the charge on ball (B) is halved. This is simply done by touching (B) with an uncharged ball (C) that has been glued to an insulating rod. The charge on (B) will be equally distributed between (B) and (C) thus leaving (B) with one half of its original charge, as shown in pic 4 below. Remove ball (C) and note the new deflection d of (B).

Table 2 provides the recorded data from this experiment. Fig. 2 illustrates the graph of the repulsive electrical force, Fe, between the two balls as a function of the product of the charges on the two balls, Q1*Q2. Since the graph is linear, it is concluded that the repulsive force, Fe, is indeed directly proportional to the product of the charges, Q1*Q2.

Finally, the figures below illustrate some pictures of the experiment in process.

Some pictures of the experiment's procedures
Pic 1: A chamber with mirror, ruler, and glass window, two guide blocks with graphite coated spheres, a wool square, and a vinyl strip.
Pic 2: A guide block is pushed near to the suspended graphite coated sphere in the chamber
Pic 3: The suspended ball is repelled, which is due to coulomb force, when the charged guide ball is closed to it.
Pic 4: An uncharged ball (C) is touched with the suspended ball (B), thus leaving (B) with one half of its original charge.

Any comments?

Instructor: Stephen Gedney
Department: University of Kentucky, EE Department
Created by: Tang, wee-hua and S. Gedney

Created on 25th June 1998, Last Modified September 10, 1998