Unit 2: Understanding Common Energy Conversion Systems. Lesson Objectives: Introduction. Lesson

Transcription

1 Unit 2: Understanding Common Energy Conversion Systems Lesson 4 - Electric Energy & Power Approximate Lesson Length: 3 hours Subject Line for SCI24U2L4 Lesson Objectives: Upon completion of this lesson the student will be able to: learn how to measure electric energy and electric power; examine how to calculate the amount of electric power used by electric appliances; examine how to calculate the number of kilowatt hours used. learn how to calculate the efficiency of electric devices identify the electric transformation systems in selected electric appliances; determine how utility companies bill customers for the electric energy used; investigate the efficiency of light bulbs Lesson Links Please click on any of the links below to take you to the specific section of the lesson. Electricity Energy & Power Efficiency Lesson Review Assignment Introduction In today's lesson, you will learn how to solve mathematical problems involving work and energy. You will also learn about the importance of efficient appliances. Back To Top Lesson file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (1 of 8)19/10/2008 7:24:07 PM

2 Electricity In your last lesson, you learned that electric power involves the movement of electrons from the power source to the customer. Electricity is produced at a power plant. It is transported via power poles and metal cables what many call a wire-grid system. There are some specific terms are used when talking about electricity. The speed of movement of electrons is called current and is measured in amperes. The electric force moving the electrons is called voltage and is measured in volts. The product of the current and the voltage is power. Power is measured in watts. The amount of electric power needed by customers is called the load. Customers pay for the electric load in kilowatt-hours (1000 watt hours). The number of kilowatt-hours used appears on their utility bills. Back To Top Energy & Power Energy is the ability to do work. Work is defined as force (F) multiplied by distance (d) over which the force is applied. work = force x distance work = F x d Force is measured in newtons. When a newton of force is applied for one metre of distance, one joule of work is done. The joule is the unit used in measuring work. A wagon is pushed with a force of 5 newtons a distance of 3 meters. How much work is done? Use the formula for work that you know and substitute in the values you are given. Solution: work = F x d work = 5 N x 3 m work = 15 N.m or 15 joules Power is the rate at which energy is transferred. The following formula can be used to calculate power. power = energy/time Please refer to pages to see how the formula can be used to calculate power. file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (2 of 8)19/10/2008 7:24:07 PM

3 An aquarium light bulb uses 7 J of electric energy in one second. How many watts of power does this light bulb consume is 3 s? Solution: What you know: power =?; energy = 7J; time = 1s; power = energy/time What you want to find out? - How much power is consumed? Use the formula: power = energy/time = 7J/1s = 7 J/s Express as watts: The light bulb uses 7W of power in 1s. Amount used in 3s = 7 W/s x 3s = 21W If the bulb uses 7J of electric energy in one second, then it must use three times as much in 3 seconds. power = 3s x 7J = 21W If energy is the variable that you want to find, you need to isolate it in the equation. You would then use the equation: energy = power x time How much electric energy does a 100 W light bulb use if it is left on for one hour? Solution What you know: energy =?; power = 100W; time = 1h; energy = power x time What you want to find out? - How much energy is used? Use the formula: energy = power x time = 100 W x 1 H = 100 W.h Express as watt hours: The light bulb uses 100 W.h of energy The watt hour is a small unit of energy. Electric utility companies use an energy unit called the kilowatt hours (kw.h). Read over page 110 in your text on calculating kilowatt hours. 1000W = 1 kw How much electric energy does a 1000 W table saw use when run for 3 hours? Solution What you know: energy =?; power = 1000 W; time = 3 h What you want to find out? - How much energy is used? Use the formula: energy = power x time = 1000 w X 3 h = 3000 W.h Divide by 1000 to change W.h to kw.h 3000/1000 = 3 kw.h State your findings - the table saw would use 3 kw.h Back To Top Efficiency file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (3 of 8)19/10/2008 7:24:07 PM

4 Efficiency is a term used that describes how completely the input energy is converted to accomplish the desired output energy. You have already learned that most energy conversion systems have wasted energy as part of the output energy. For example, we turn on a light because we want it to light up the room, not to heat the room. Yet, some of the energy that goes into the light bulb is lost as heat energy. Ideally we want to keep the amount of wasted energy to a minimum. This conserves energy which decreases the release of greenhouse gases into the environment and saves you money on your monthly utility bill. Appliances use about one third of the amount spent on energy used to run the average home each year. The true cost of an appliance is greater than its showroom price tag the purchaser must also consider energy costs and possible water costs when calculating potential savings. Refrigerators, washing machines, clothes dryers, and dishwashers account for the major appliance energy costs in the home. Replacing an older, less energy-efficient appliance with a newer model will almost certainly save money in the long run, even though the initial expense of the purchase may seem great. This is because newer appliances must conform to higher standards of energy efficiency. You can determine the efficiency of an electric device by dividing useful energy output by the total energy input, and then multiplying by 100. Efficiency is always expressed as a percent. Percent efficiency of an electric device = useful energy output/total electric energy input x 100 An incandescent light bulb uses J of electric energy. It emits only 900 J of useful energy. What is the efficiency of the light bulb? Solution What you know: useful energy output = 900 J; total electric energy input = J percent efficiency of device = useful energy output/total electric energy input x 100 What you want to find out? What is the efficiency of the incandescent light bulb? Use the formula: percent efficiency of device = useful energy output/total electric energy input x 100 =900J/30 000J x 100 = 3% (Remember that in algebra anything divided by itself is equivalent to 1. In this equation, joules/joules = 1. In effect the joules cancel each other.) Summary: The light bulb is 3% efficient Back to Top Lesson Review You have been introduced to some important formulas in this lesson. Let's review... Work = Force x Distance Power = Energy/Time Percent efficiency of an electric device = useful energy output/total electric energy input x 100 file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (4 of 8)19/10/2008 7:24:07 PM

5 In this lesson we completed the following activities: Section 1 - Electricity Section 2 - Energy & Power Section 3 - Efficiency Back to Top Conclusion In order to feel comfortable using these formulas to make calculations you will need some practice. Make sure you attend our next tutorial where we will go over the steps together. It is also important that you complete all of the assignment with this lesson. Remember to send me an if you have any questions. Back to Top Assignment Part A: Read pages in your text and answer the following questions in a word document. Word Document Calculating Power Worksheets Download this worksheet to your computer and complete it. Send the completed worksheet to your teacher. What Did You Discover? Part B: Please use your science.connect2 Student Multimedia CD-ROM to answer the following questions. Launch the Electrical Energy applet on your computer. In order to do this, open the CD, click on "PC", then click on "menu.exe". Next click on Electrical Energy. Follow the instructions on each screen. Use the information on the applet to answer the questions below. file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (5 of 8)19/10/2008 7:24:07 PM

6 1. Using Electrical Energy (a) What is an energy transformation system? (b) Describe the energy transformation system in a light bulb. (c) Describe the energy transformation system in a toaster. (d) Describe the energy transformation system in a radio. (e) Describe the energy transformation system in a fan. 2. Generating Electrical Energy (a) Describe the energy transformation system in a generator. (b) Write a statement that describes the relationship between the speed of the spinning wire and the amount of electric energy produced 3. Electrical Power (a) Define the term electrical power. (b) Write the chemical formula for calculating power. (c) Identify the unit of power. (d) Define one watt of power. (e) How much power does a 100 watt light bulb use every second? (f) How much power does a 40 watt light bulb use every second? (g) How much power does a 20 watt compact fluorescent light bulb use every second? (h) Complete the following chart. Size of Light Bulb 100 watt incandescent 40 watt fluorescent 20 watt compact fluorescent Amount of Power used in 1 hour (W) (i) Which of the light bulbs from (h) uses the most power in one hour? 4. Electrical Energy (a) What two factors determine how much energy appliances use? (b) What is the formula for calculating this energy? (c) How much energy does a one watt electrical appliance use if it is left on for one hour? (d) What units do utility companies use to calculate your electricity bill? 5. Kilowatt Hours (a) Plug each light bulb in and determine how much it costs to operate it for 720 hours. Double the rate to see how increasing the cost of energy would affect you. Light Bulb Power Rating (Watts) Time (Hours) Energy Consumed file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (6 of 8)19/10/2008 7:24:07 PM W.h kw.h Energy Cost $0.61 per kw.h Energy Cost 0.12 per k.wh

7 incandescent 100 incandescent 40 compact fluorescent 20 (b) Which light bulb from (a) uses the most energy? (c) What would happen to your utility bill if the power company doubled the amount it was charging per kilowatt hour? 6. Efficiency of Electrical Appliances (a) Why is no device is 100 percent efficient? (b) Identify the undesirable form of energy produced by an incandescent light bulb. (c) Consider an electrical device that is 75 percent efficient. How much energy is converted into a desirable form of energy? How much energy is converted into undesirable forms of energy? 7. Measuring Efficiency A 100 watt incandescent light bulb and a 20 watt compact fluorescent light bulb each produce similar amounts of light energy. (a) What is the temperature of the air just above the 100 watt incandescent light bulb after 10 minutes of use? (b) What is the temperature of the air just above the 20 watt compact fluorescent light bulb after 10 minutes of use? (c) Which light bulb converted more energy into thermal energy? (d) Which light bulb is less efficient? Explain how you know. (e) Read the Did You Know? What is the approximate efficiency of an incandescent light bulb? (f) What is the estimated efficiency of a compact fluorescent light bulb? 8. Cost of Efficiency a) Comparison of Operating Costs Annual Operating Cost Comparison of Operating Costs Electrical Energy (2190 hours) Energy cost at $0.11/kW.h Initial cost of light bulb Cost of replacement bulbs Total Cost Incandescent Annual Operating Costs Fluorescent (b) Which light bulb uses the most energy during the 2190 hours? (c) Which light bulb cost the most to operate during this time period? (d) How much less does it cost to run a compact fluorescent light bulb for 2190 hours? Part C: Review pgs Complete questions file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (7 of 8)19/10/2008 7:24:07 PM

8 Total Marks = 45 Please make sure you have handed in the following: Attach your word document to the dropbox that contains your answers to parts A - C. Back to Top Rocky View Virtual School - Revised August 30, 2006 file:///c /Documents%20and%20Settings/Student/My%20Documents/Teacher/SCI24D2L/LPConnect/SCI24U2L42006.htm (8 of 8)19/10/2008 7:24:07 PM