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Watts in the Water? Student Generated Power Physical Science Energy Conversions Grades: 8-12 How powerful are your students? How can we measure their power? Here is oneway, which helps them understand different forms of energy. Objectives: To measure the energy a student can put into a small volume of water. To explore different ways heat can be added to water. To introduce units of heat energy, and relate them to units of electrical energy, and ultimately to -student power-. Materials; for each pair of students: One stoppered, standard-sized test tube. Clock or stop watches Thermometer Student Directions: 1. Students work in pairs. Each pair is given a test tube into which they pour 10 to 20 ml of water. 2. They should measure and record the initial temperature and mass of their water. (1 ml = 1gram) 3. When the signal is given, students begin doing whatever they wish with their test tube to warm it up. They can shake it, rub it, sit on it, whatever they want, so long as they only use their body. 4. After five minutes time is up! Take and record the final temperature. Calculations: 1. The heat gained by the water is found by subtracting the initial temperature from the final temperature (Tf - Ti) The temperature change is used to find the change in heat energy, which is measured in calories. 2. The temperature change is converted into calories by multiplying each degree gained by the number of grams of water. If 10 g of water gained 8 degrees, then 80 calories of heat was added to the water. (Note: Calories in food are actually Kilocalories, which are 1000 calories each). 3. Work is defined as a force causing movement. The work done in raising the temperature of water can be found by multiplying the calories added to the water by 4.19 joules (joules is a unit of work). 80 calories X 4.19j = 335.2 joules. 4. Power is defined as work over time, and is expressed in watts (335.2 j /300 seconds = 1.12 watts) This is getting a bit thick, for non-physics majors. Let's take a look at the last step, and what it is really saying. Since the student in the example did 335 joules of work over 300 seconds, he did 1.12 joules of work each second, which is then defined as 1.12 watts. Relating this to the more familiar electrical watt, a 100 watt bulb performs 100 joules of work each second. Extension: You can finally convert watts to kilowatts, then to kilowatt hours, and see what we pay for electrical power. The electric company sells us power by the Kilowatt hour (Kwh), meaning 1000 watts of power in use for an hour. In other words, if a 100 watt bulb is on for ten hours, you have used one kilowatt hour. A Kwh might cost $.25. If each student in your class is capable of generating 1 watt of power, then 33 of them would need to work for 30 hours to produce the equivalent energy contained in one kilowatt hour! (1 watt x 33 x 30.3 = 1 kilowatt hour) Maybe electricity is not so expensive after all! I got this idea from Mark Nelson, of Tucson, Arizona.