PHYS 102

Lecture 9

December 9, 2009

Video 20 - "Electrostatics - Interaction of Charge", Video 21 - "Distribution of Charge on a Conductor",
Video 22 - "Induction of Charge".
Design and operation of an electroscope.
Nine basic experimental results:

1.  Electric charges are of two kinds, positive and negative.
2.  Like charges repel each other and unlike charges attract each other.
3.  Substances differ in electrical conductivity:  (a) conductors - have many "free" electrons,
     (b) insulators - have very few free electrons, (c) semiconductors - have some free electrons.
4.  The surplus charge on a solid or hollow conductor resides entirely on its exterior surface.
5.  Electric charge concentrates at edges and pointed surfaces on a conductor.
6.  When charges are separated by contact (rubbing things together), equal amounts of positive and negative
     charge are produced.  This is the Law of Conservation of Charge.  (Charge can be moved around but it cannot
     be created or destroyed.)
7.  Electric forces act at a distance.
8.  A body can be charged by conduction or induction.
9.  Higher humidity increases charge "leakage".

Demos:  (a) charged cassette case picks up shredded paper, (b) charge an electroscope by conduction and later by induction using various items - glass rod (+) and silk (-), PVC (-) and fur (+), (c) a charged balloon (rubbed on fur) "hangs" on a wooden door - and maybe other things, (d) a charged rod is brought near a falling thin stream of water - and deflects it - via induction.