To understand how electroscope is related to Coulomb Force Law.
- A charged probe
- A Braun electroscope
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.
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.
|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
Instructor: Stephen Gedney
Department: University of Kentucky, EE Department
Created by: Tang, wee-hua and S. Gedney
Created on 25th June 1998, Last modified 10th September, 1998