Coulomb'S Law - Projection
To demonstrate Coulomb's law semi-quantitatively.
Sphere with bifilar suspension, sphere on lucite block, transparent scale, overhead projector, charging◙materials (glass with silk and hard rubber with fur).
Two identical conducting balls, one mounted rigidly to a plastic stand, the other suspended from the overhead projector as shown in the picture, are charged with the same charge. As the fixed ball is slid closer to the suspended ball, the angle of the thread support becomes greater, indicating a greater horizontal force. With a little bit of luck, you can actually show that the horizontal force is inversely proportional to the square of the distance between the centers of the two balls.
John B. Johnston, Coulomb's Law on the Overhead Projector, TPT 17, 60-61 (1979). ◙Richard H. Price and Richard P. Phillips, The Force Between Two Charged Wires, AJP 58, 534-539, (1990). ◙Peter Heering, On Coulomb's inverse square law, AJP 60, 988-994 (1992). ◙Xishun Xie and Xiaoqin Huang, Elegant calculations of the Coulomb force between two hemispherical surfaces with uniform◙charge densities, AJP 62, 952-953 (1994). ◙H. H. Hassan, Comment on "Elegant calculations of the Coulomb force between two hemispherical surfaces with uniform◙charge densities," by Xishun Xie and Xiaoqin Huang [AJP 62, 9