PURPOSE: A simple circuits to encourage thought about the relationships between resistance, current, volatge and power.
DESCRIPTION: The circuit shown above at left connected as shown with the switch open. What will happen when the switch is closed? Let students discuss each case before actually doing the demo.
(Left photo) With the switch open the two lamps in series are equally bright and the lamp at left is off. Q: What happens to the three lamps when the switch is flipped? A: Nothing. The potential is the same at the two points connected by the switch.
(middle photo) A third lamp is connected into the circuit when the switch is closed. Before the switch is closed both lights are equal in intensity. Q: What happens to the intensity of the lights when the switch is closed? It appears that one lamp becomes very bright while the other two become very dim.
(right photo) To show how the power is distributed to the two dim lamps turn off the room lights and cover the brightest lamp with a coffe can. the two parallel lamps share 1/8 the power that the brightest lamp obtains.
EQUIPMENT: Low volatge circuit paradox demonstration.
SETUP TIME: 1/2 hour.
Updated by Jun Qi in 3/13/2000