Waterwheel Experiment Teacher's Guide The basic idea of this experiment is to demonstrate the concepts of potential energy and conservation of energy. Also, the apparatus demonstrates how mechanical energy can be converted to electrical energy. The experimental apparatus consists of a wheel connected to a DC motor that outputs a voltage proportional to the speed at which the wheel rotates. The electronic circuit senses the voltage and lights up the LEDs. The LEDs give two pieces of information. First, three different voltage thresholds are set in the circuit, corresponding to each of the LEDs. A given LED does not light until the motor voltage passes the corresponding threshold (lowest for green, highest for red). Second, the voltage the motor outputs oscillates slightly at the frequency of the wheel rotation. As a result, the LEDs flicker, with the flicker frequency being proportional to the wheel speed. So the students have two tools to measure the wheel speed. The apparatus has two deficiencies of which the teacher should be aware. First, the circuit only responds when the voltage on the black wire is less than the voltage on the red wire. As a result, the circuit only responds to one rotation direction of the wheel, even though the motor generates a voltage for either rotation direction. During the assembly, the students are asked to determine which rotation direction fires the circuit. The second problem is that the current for the LEDs actually comes from the batteries, not from the motor (the motor is actually too small to drive the LEDs directly). So it is not really true to say that the mechanical energy of the motor rotation is being converted into electrical energy; in fact, essentially none of the motor's kinetic energy is being used. The first question on the Student Activity Sheet asks the students to investigate how the circuit responds to the speed of the waterwheel. This is a good time for the teacher to ensure that the students understand how the lighting of the LEDs correlates with the speed of the wheel so they can do the rest of the experiment. The second question is sort of an aside that asks them to think about how they could use the waterwheel to do work and to relate it to other machines they may know about. The teacher here might talk about how in the past waterwheels have been used in grain mills or how they are used in hydroelectric plants today. Another related example is the windmills in the hills by Livermore that are being tested for the feasibility of using wind power to generate electricity. As noted above, the kinetic energy of the wheel is not being used to run the circuit, so it is not really valid to use it as an example. With their waterwheel built, the students are now ready to investigate ideas about energy. Kinetic and potential energy are defined and the concept of conservation of energy is introduced. The students are asked to figure out how to make the wheel turn faster by trying to increase the water's kinetic energy at the moment it hits the wheel. By making analogies with other similar situations (roller coasters, bungee jumping!), hopefully the students will try to raise the water higher. Finally, the students are asked to determine if changing the number of vanes on wheel has an effect. Also, they are explicitly asked to fix the height of the water during these changes. This is an important point of scientific procedure -- controlling all your variables and changing only one to see effects, and the teacher should try to emphasize this.