Purpose:

To understand how electroscope is related to Coulomb Force Law.

Accessories:

• A charged probe
• A Braun electroscope

Discussion:

An electroscope is a device to determine or measure the presence of electrostatic charges. The device's operation is simply based on the Coulomb Force Law.

Experiment Description:

The Braun electroscope that is used with various experiments in class is a deflection arm electroscope, as illustrated in Figure 1. The electroscope is constructed of a metal disc conected to a metal rod inside a circular cylindrical ring. The metal rod is insulated from the outer ring by a rubber gasket. the purpose of the outer ring is to shield the electroscope from external charge influences (more on that later). A deflection arm which has a metal pin through its center of gravity is connected directly to the metal rod. The deflection arm is free to rotate about its center axis. The metal rod is also bent such that the gravitational force acting on the deflection arm will result in the arm to reside vertical, yet on the right side of the metal rod at the top and the left side at the bottom. THis is seen in the figure below.

Asume a positive-charged probe is touched to the metal disc, then the positive charges will be induced on the surface of the metal rod and the deflection arm. Then Coulomb forces result in a repulsive force between the like positive charges. This then results in a clockwise directed torque on the deflection arm at the top and the bottem. The deflection arm is then rotated to a certain distance until the coulomb force is in equilibrium with the gravitational force (F=mg) acting on the arm. What actually happens, is that the arm goes through a simple harmonic motion, which is quickly damped out due to frictional forces, until such an equilibrium is reached. Of course, the amount of deflection will be proportional in some manner to the amount of charge induced on the electroscope.

A classical "gold leaf" electroscope is shown in Figure 2. The design consists of a metal disc on top, a metal rod, and two strips of gold leaf at its lower end. The leaves are protected from air current and a simple scale in degree is provided for measurement. Placed the charged probe near to the metal disc, the leaves would diverge because of the Coulomb Force, Fe. It is noted that this type of electroscope could be easily made at home. But rather than using gold leaf, a very thin aluminum foil would suffice.

Results:

An uncharged Electroscope	The Electroscope is charged now. The deflection arm rotated for a certain distance until the Coulomb Force is equaled to the Gravitaional Force

Electroscope Experiment

Any comments?

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