Description
Induced charge is used to rotate a long 2"x4" wood board balanced on a watch glass.
1. Balance a long 2"x4" board on a watch glass that has been placed on a stool in front of the class.
2. Have two students (one standing at each end of the balanced board) charge up their respective balloon using a piece of rabbit fur.
NOTE: Students could use the old fashion method of rubbing balloon on their hair, but hair styling gels, etc. will interrupt the transfer of negative charge from their hair to their balloon.
3. Have students position balloons on opposite sides of the board: near to the board, but not touching the board (see figure).
4. As board starts to turn to the balloon, instruct student to slowly, and continually move the balloons away from board. Board's rotation speed will actually increase as the balloons exert their respective torques.
NOTE: Negatively charged balloons induce a positive microscopic surface charge on the nearby board. Opposite charges will attract each other, i.e. exert forces of attraction on each other's masses.
VARIATION: Charge up a balloon and have it statically cling to the wall.

Equipment
watch glass, balloons, 6-8 foot long 2"x4" board
Support Equipment
Stool