How Are Water Pressure and Water Flow Related? In this activity, students apply the scientific method as they attempt

Transcription

1 How Are Water Pressure and Water Flow Related? Overview In this activity, students apply the scientific method as they attempt to answer the question, How are water pressure and water flow related? A large plastic soda bottle, representing a reservoir, is punched with holes at specific vertical locations. The holes are plugged with removable stoppers (push-pins) and the bottle filled with water. Students predict which hole, when unplugged, will spray water farther and why. Students apply the scientific method as they consider specific questions, make predictions, and observe and record the results of their experiment. Through their observations, students gain an understanding of the relationships between water pressure, head, and water flow.

2 Core Concepts A hydropower (hydroelectric power) plant uses the energy of motion in flowing water to rotate a turbine which activates a generator to produce electricity. An electrical generator is a device that converts mechanical energy into electrical energy by moving a wire through a magnetic field. Electricity and magnetism are two aspects of a single electromagnetic force. Magnetism produces electricity: Whenever an electrical conductor (such as a copper wire) is moved through a magnetic field, an electric current is produced. The amount of electricity that can be generated by a hydropower plant is dependent on the flow rate (the quantity of water flow in a given time) and the head (the height from which the water falls and the pressure behind that water). These concepts are from the Powering Our Future Energy Education Conceptual Framework (Appendix A). We work best under pressure! 3

3 Learning Objectives ARIZONA DEPARTMENT OF EDUCATION ACADEMIC STANDARDS See Appendix B for the full text of these ADE Standards. Science Mathematics SC06-S1C-01; SC06-S1C-03; SC06-S1C-05; SC06-S1C3-01; SC06-S1C3-0; SC06-S1C4-0; SC06-S1C4-03; SC06-S1C4-05; SC06-S5C3-01; SC07-S1C-01; SC07-S1C-03; SC07-S1C-05; SC07-S1C3-01; SC07-S1C3-0; SC07-S1C3-04; SC07-S1C3-05; SC07-S1C4-03; SC07-S1C4-05; SC08-S1C-01; SC08-S1C-03; SC08-S1C-05; SC08-S1C3-01; SC08-S1C3-0; SC08-S1C3-05; SC08-S1C4-01; SC08-S1C4-03; SC08-S1C4-05 M06-SC1-0; M06-SC1-03; M06-SC1-04; M06-SC1-05; M06-SC1-06; M07-S1C-01; M07-SC1-04; M07-SC1-05; M07-SC1-06; M07-SC1-07; M08-SC1-08 After completing this lesson, students will be able to do the following: Apply the scientific method by making and testing predictions. Analyze data on water pressure and water flow. Express results of an experiment through a graph. Explain how water pressure influences water flow. Describe how flowing water may be used to do work. Define and describe the difference between potential and kinetic energy. TIME NEEDED As demonstration 45 minutes As student experiment 90 minutes TEACHER BACKGROUND INFORMATION Review these sections of the Energy Primer: Renewable Energy Sources subsection on Hydropower Also review Parts One and Two of the presentation, Introduction to Hydroelectric Power. 4

4 Advance Preparation MATERIALS Student Data Sheet: Working Under Pressure photocopy one per student Overhead Transparency: Flowing Water at Work overhead projector calculators For each student group conducting the experiment, the following equipment is required: a two liter soda bottle four push pins ruler (foot long) or meter stick water source (such as pitchers, gallon jugs, or soda bottles full of water) large tray or plastic sheet (to collect water if done indoors) old towels or rags (for spills) GENERAL PREP Prepare and test a sample experimental set-up with the suggested materials so that you are comfortable with the experiment. Have the example set-up ready to use in class. Decide whether students will conduct the experiment in pairs, teams, or view it as a teacher demonstration. (Note: It is recommended that each student or pair of students get the opportunity to prepare and test their own experimental set-up. However, the activity may be conducted by larger student teams, or by the teacher as a large-scale demonstration with the entire class participating and observing. Your choice may depend on your teaching style, your students ability to stay on task while working in teams, class time, or to a lesser extent, availability of materials.) 5

5 Suggested Procedure 1Display the overhead transparency Flowing Water at Work as you introduce this activity. Review the terms potential energy, kinetic energy, flow rate, and head from the overhead transparency. Ask students to recall the introductory presentation in which they learned that the amount of electricity that can be generated by a hydropower plant is dependent on the flow rate (the quantity of water flow in a given time) and the head (the height from which the water falls). Explain that it is important for them to recognize that head is more than just the height from which water is falling. Head is a function of the height from which water falls, and the pressure behind that water. (On the transparency, the water behind the dam is exerting pressure which influences the head of this system.) In this activity, the class will be conducting experiments which demonstrate how head, water pressure and water flow are related. These factors relate to how much work can be done with the water or how much electricity can be generated at a hydropower plant. 3Other items to point out on the overhead include: the location of the turbine in relation to the top of the dam the measurement of head (the distance from the water surface to the turbine) how electricity is generated (The turbine spins the shaft that goes into the generator and the shaft rotates a magnet inside coils of copper wire to generate electricity.) 6

6 4Using the sample bottle you created previously, review the experimental set-up. Depending on how you choose to conduct the experiments (as a demonstration or in student teams), adjust your introductory explanation to allow for maximum student inquiry. Regardless of how the experiments will be conducted, point out the following to students before commencing the activity: The two liter soda bottle is considered your reservoir. A line near the very top should be drawn and labeled fill line. You will be attempting to answer the questions: Will water that is falling from higher spray out farther? Why or why not? To answer this question, holes should be punched in the reservoir at four different heights. Explain that the hole height is the distance from the ground (or tray) to the hole. Ask: Where should those holes be placed to best test your prediction? Push pins should remain in the holes throughout the experiment and only be removed for the trial with that hole. (Duct tape may be used to cover the holes if leakage is a problem.) Three trials should be conducted for each hole and the average distance that the water sprays should be calculated. Why? Each trial should be conducted by removing the push pin, noting the distance of the stream of water, replacing the push pin, and recording the data. The three trials for a specific hole should be conducted before moving to another hole. The water in the reservoir should be refilled to the fill line for each trial. Why? A standardized system for noting and measuring the distance of water flow for each trial should be predetermined. Who will observe where the water stream landed and how will it be measured? Unless the experiment is conducted outside, a tray or plastic sheet should be placed under and in front of the reservoirs. Whatever is used, it should not hinder the ability to accurately measure the distance of the water stream. PROCEDURE CONTINUED 7

7 Suggested Procedure (continued) Ask 5Review the concepts of potential and kinetic energy both in general and in the context of this experiment: With the sample bottle, point out that the water behind a particular hole (the water that is about to be released) has gravitational or potential energy. That is, it has energy that is not yet being used. Water that is flowing out of the hole is moving and has motion or kinetic energy. 6 Remind students that head is not only the height from which water falls, but the pressure behind the water. Head is actually a measure of the system s potential energy. Point to a hole on the bottle and explain that the hole height is the distance from the ground (or tray) to the hole. However, the head of the system would actually be a measure from the hole to the fill line. 7 8 Pass out the student data sheet Working Under Pressure to each student. It may be helpful to review the data sheet to ensure students understand the information they should be recording. Before conducting the trials, students should write their predictions on their data sheets. You may want to review the scientific method and describe how this experiment follows the scientific method by asking them to make and test hypotheses. This might be a good time to remind them that power production is directly proportional to the head and the flow of a hydropower system. How might this concept relate to their experiment? 9 10 Have students begin their experiments, guiding them in the initial data collection if necessary. Remind students to refill their bottles to the fill line after every trial. After all of the trials have been completed at all of the hole heights, students should calculate the averages for each trial and graph their results. Have students analyze their graphs and complete their data sheets. Note, you may also choose to analyze the graphs as a class. 8

8 11Once students have completed their data sheets, conduct a wrap-up discussion with the class using the Discussion Questions from their data sheets as a guide. Note: Students may be confused as to why water falling from the higher holes did not spray out the farthest; that head is a function of both the height of falling water and the pressure behind that water as applied by the water itself. * Head is a measure of the potential energy of the system at a given point. In this experiment, the height of the hole is not the same as the head. Instead, head is more accurately measured as the distance from the fill line to the hole. (You might refer back to the overhead transparency Flowing Water at Work and point out the measurement of head as the distance from the water surface to the turbine.) Thus, the holes nearer to the bottom of the bottle (the ones with more pressure on them from the water above) would have the greater head (and the greater potential energy). 1(Optional) To further help you orally assess students grasp of the concepts, ask them to respond to the following questions (which could be written on the board): How are water pressure and water flow related? How are water pressure and head related? When does water have kinetic energy? When does water have potential energy? Note: Head, in its most accurate sense, is too complex a concept to adequately explain in this setting to middle school students. In this lesson, the source of the pressure is only addressed as being a function of the height of the water and the pressure behind that water as applied by the water itself. * 9

9 ASSESSMENT IDEAS Ask students to write down answers to the points in the last step of the Suggested Procedure. In conjunction with their completed student data sheets, this should serve as an adequate assessment of their understanding. EXTENSION IDEAS Alternate designs: Have students come up with their own experimental designs to test the concepts of water pressure and flow. Allow them to bring in and use other materials. See who can come up with a design to have water spray out the farthest. Maximizing head: Have students consider the following: How might the water pressure in the bottle be increased and what might happen to the stream of water as a result? Create an experiment to test this. 30