Electrostatic Demonstration
James Evans
Joyce Gridley
7-09-02
Objective:
Show the relationship between electric field strength and distance of a “point charge”.
Calculate the electrostatic force at different distances from a “point charge”
Equipment:
Dowel rod
“Exploded” protractor
Ring stand
Clamp
String
“Pith ball” – about the size of a golf ball
Van de Graff generator
Procedure:
Construct a deflection meter using the dowel rod and exploded protractor. Attach the string to the ball. Hang from the end of the dowel rod so the deflection can be measured.
Part 1: Place the pith ball within the electric field created by the Van de Graff generator. Show how the ball will be deflected by different amounts depending on the distance from the point charge. When the ball gets close to the Van de Graff generator it will pick up a charge and be repelled. Once repelled it will lose charge and be attracted to the generator again. This will create a “pendulum” effect.
Part 2: Place the pith ball far enough away from the generator so that a constant deflection can be obtained. Measure the deflection. Repeat for a different distance.
When the ball is in equilibrium the sum of the forces will be zero. Knowing the mass of the ball and the angle of deflection the electrostatic force can be calculated.
Using the mass of the pith ball and the angle of deflection, you can calculate the electrical force from the static charge using the following relationship.
tan(angle) = F electrical/F gravity
mg tan(angle) = F electrical
By measuring the distance between the center of the sphere and the center of the pith ball, you can use the assumption that an equal and opposite charge is induced on the pith ball and calculate the charge(q).
F e = k q2/r2
or
q =( F e r2 /k )^(1/2)
When movement has stabilized the force total is zero.
Force Diagram (Sum F =0)