To show that electrostatic forces are significantly large.
To compare gravitational force with electric force.
To show induced charge.
To show inertia of massive object (2x4 wood) compared to light object (say, a balloon or pith ball)
Any 2x4 piece of wood, 4 ft to 10 ft long or so. The longer, the more impressive, provided you have space in your room for it to rotate.
A watch glass, or a door knob (hardware store) cut in half to make a watch-glass-shaped dish.
Any source of electrostatic charge, such as fur and a plastic rod.
Balance the 2x4 wood horizontally on the watch glass on a table with a hard, smooth surface, so friction between table and watch glass is very low.
1. Bring an uncharged plastic ruler near a 2x4.
2. Charge the plastic rod, and then use it to induce an opposite charge in the wood 2x4 near one end. The resulting attractive force will cause the 2x4 to rotate, usually much to the amazement of those who have not before seen this. The longer the 2x4, the more impressive is its resulting rotation. Speed it up, slow it down and reverse its direction.
Why is the force always attractive? ( A good take-off question for discussion.)
1. What kind of force was there between the ruler and 2x4 in Part 1.
2. Does the 2x4 rotate as a consequence of this force?
3. What kind of force was there between the charged ruler and the 2x4 in Part 2?
4. Does the 2x4 rotate as a consequence of this force?
5. Compare the magnitude of the force in Part 1 with the magnitude of the force in Part 2.