1103 Period 8: Energy Conservation and Efficiency

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1 Name Section 1103 Period 8: Energy Conservation and Efficiency 8.1 How Is Energy Conserved? 1) Energy carts: Your instructor will demonstrate two rolling carts colliding with a barrier. Both carts have the same mass and the same frictional force with the table top. a) Compare the velocities and the distances each cart travels after hitting the barrier. b) Which cart has more kinetic energy after it hits the barrier the cart that rolls a shorter distance or a longer distance? How do you know? c) Now watch the carts, without their outer covers, collide with the barrier. How can you explain the difference in the behavior of the carts after they hit the barrier? d) Explain how the energy conversions differed for the two carts. 8.2 How Is Friction Involved in Conservation of Energy? 2) A truck does work: In this activity, we measure the work done against the force of friction by a toy truck pulling a block connected to a spring scale. a) Once the truck and block are moving at a constant velocity, how much force does the truck exert on the block? b) Once the truck reached a constant velocity, how far did it pull the block? 43

2 c) Calculate the amount of work the truck did to pull the block that distance. d) Place 300 grams (0.3 kilograms) of mass on top of the block. How much force does the truck exert on the moving block with 0.3 kg of mass on it? e) How much work must the truck do to move the block with 0.3 kg of mass the same distance as in question 2? f) Why does the truck do more work when mass is added? g) Suppose that the truck required 25 joules of energy input to pull the block and the 0.3 kg of mass. The truck s work output is the joules of work you found in part e) above. What fraction of the 25 joules of work input is useful work output? h) What happened to the remaining portion of the 25 joule work input? 8.3 Energy Conservation and Efficiency 3) Energy input to a waterwheel: Your instructor will use the gravitational potential energy of water pouring over a waterwheel to raise a 50 gram (0.05 kilogram) mass. a) How many milliliters of water flowed from the small bucket? b) Each milliliter of water has a mass of 1 gram. What is the mass (in kg) of this water? c) How high (in meters) is the water in the small bucket above the waterwheel? d) From these data, calculate the gravitational potential energy of the water. 44

3 Name Section 4) Work output and efficiency of a waterwheel: As the waterwheel turns, it raises a 50 gram mass. The work output of the waterwheel equals the increase in gravitational potential energy of the mass. a) To what height (in meters) was the 50 gram (0.05 kilogram) mass raised? b) What is the gain in gravitational potential energy of the mass? c) Not all of the input energy (work in) results in output work. The efficiency of the waterwheel is the ratio of the work out to the work in. What percentage of the energy input of the water flowing over the waterwheel became the energy output that raised the mass? This percentage is the efficiency of the waterwheel. d) What happened to the rest of the energy input? How was energy conserved in this process? 8.4 What Is the Efficient of a Series of Energy Conversions? 5) Efficiency of the water wheel: To turn the waterwheel, water was first carried up the stairs and placed into the small bucket. Next, we find the efficiency of raising the water. a) Suppose that it required 10 joules of energy input (work in) to place the same amount of water into the small bucket that flowed down the tube to the waterwheel. The output energy (work out) is the gravitational potential energy of this water, which you found in part 3.d. Find the efficiency of the process of placing water into the small bucket. b) What is the efficiency of the entire process, which includes the efficiency of placing water in the small bucket and the efficiency using the gravitational potential energy of the water to raise the 50 gram mass? 45

4 6) Efficiency of a solar cell: Efficiency applies to many energy conversions. Next, we consider the efficiency of a solar cell that converts radiant energy into electrical energy. a) Connect a small light bulb to a solar cell. Shine the large flood light onto the solar cell so that the small bulb lights. b) Predict whether the small light bulb will light when another small bulb that is connected to batteries shines on the solar cell. Then try the activity and write the outcome in the answer blank. Will the small bulb light? Prediction: Answer: c) Explain why the small bulb did or did not light when a small bulb shines on the solar cell. d) What form of energy goes into the solar cell? e) What form of energy lights the bulb? f) What form of energy is wasted when the bulb lights? 7) Efficiency of multiple energy conversions: Find the efficiency of the conversion processes that turn radiant energy shining on the solar cell into visible light from the small bulb. a) If a solar cell requires 100 joules of radiant energy to produce 25 joules of electrical energy, find the efficiency of this step of the conversion process. b) If 25 joules of this electrical energy are converted into 5 joules of visible light in the small bulb, find the efficiency of this step of the process. c) Use the answers to parts a) and b) to find the overall efficiency when this solar cell lights the small bulb. d) Group Discussion Question: When the solar cell lights the small bulb, the majority of the radiant energy shining on the solar cell is wasted. List the places where energy is wasted. Which forms of energy are wasted? 46

5 Name Section Period 8 Exercises: Energy Conservation and Efficiency Write answers to the questions below. Show your mathematical steps and the units of the quantities. This sheet with your answers should be turned in at the beginning of Period Finding efficiencies: In a hydroelectric generating plant, water falling over a dam spillway turns a turbine that generates electricity. a) For every 1 kg of water than falls 20 meters onto the turbines, 100 joules of electricity are generated. What is the energy input into this process? What is the energy output from this process? b) What is the efficiency of this process? c) Some of this electricity is used to light a compact fluorescent bulb. For every 18 joules of electricity provided to the bulb, 15 joules of radiant energy are produced. What is the efficiency of this process? d) What is the overall efficiency of producing electricity at this hydroelectric plant and using this electrical energy to light the bulb? 2. Understanding conservation of energy: When a toy truck moves at a constant velocity across a level surface, all the work output is done against the force of friction. a) When the truck pulled the block at a constant velocity on the level floor, into form of energy was the truck s energy converted? b) If the truck accelerated the block, into what forms of energy was its energy converted? c) If the truck pulled the block at a constant velocity up a ramp, into what forms of energy was the truck s energy converted? d) In which of the three situations described above (level constant velocity, level accelerated motion, uphill constant velocity) was energy conserved? 47

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