Coulomb's Law

brief summary of topics discussed:

(time 0:00) discussion of electric forces and what these forces are responsible for, eg

twang steel wire supporting several bricks

demonstrate incompressibility of water by breaking filled test tube with a plunger

(2:30) charge large ball from VdG and use it to charge a large suspended balloon, demonstrate repulsion, then charge ball from oppositely charged VdG and demonstrate attraction

(3:40) inverse square law

lamp illuminating books placed at d, 2d, 3d

analogy - "butter gun" for buttering toast

(5:30) Cavendish's torsion balance, illustration

(6:00) "spring balance" to measure repulsive force between two small charged spheres

calibrate with 0.1 gm mass

charge spheres by means of an electrophorus

distances of 1, 2, 3 'spans' give forces of 23.7, 5.4 and 2.5 respectively, multiplies force for 2 spans by 4 and 3 spans by 9 to obtain 23.7, 21.6 call it 22, and 22.5 which is very close 'in fact we would expect less because charge leaks off and besides the 3 span measurement is too small to be reliable'

(13:00) effect of amount of charge on force at constant distance:

charge balls, measure force with: q on both balls; reduce charge on one ball by half by touching it with an uncharged ball; reduce charge on the other ball: force = 18+, between 8 and 9, and approximately 4

(16:40) discussion a teh blackboard: inverse square law implies no electric effect inside a hollow charged conducting sphere, solid angle argument

(22:00) experimental test: large copper sphere

hold two small balls touching outside of and near copper sphere, separate and test their charge with an electroscope, repeat starting with the two balls inside the copper sphere

test also by touching a single sphere outside the copper sphere and testing charge with the electroscope and then touching the sphere inside the copper sphere

(23:245) remove half of the copper sphere

single ball touched outside and tested with the electroscope, and then touched close to the rim inside (2 trips required)

(25:40) Van de Graaff generator to charge assistant standing on an insulated platform and again with the assistant inside a metal cage

(27:30) summary

(29:21) credits