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Electrical Power Electronics Lesson Plan Performance Objective At the end of the lesson, students will demonstrate how to use three versions of the power formula to calculate power by completing Lab #1 – Compute Current Using the Power Formula worksheet; Lab #2 – Determine Power Used in a Resistive Circuit worksheet; Lab #3 – Determine the Function of Fuses and Resistor Power Rating worksheet; Student Activity – Compute Power from the Power Formula worksheet; and Power Exam #1 and Power Exam #2. Specific Objectives After completion of this lesson, the student should be able to Match terms associated with power and measurement to their definitions. Match power abbreviations with their terms. State three versions of the formula used to compute electrical power. Arrange in proper sequence the procedures for power measurement using a DC wattmeter. Select true statements concerning resistor wattage rating. List electrical power safety precautions. Distinguish between direct and inverse proportions involved in power formulas. Demonstrate the ability to o Compute current using the power formula. o Determine the power used in a resistive circuit. o Determine the function of fuses and resistor power ratings. Terms Mechanical power - the rate at which work is being done. Electrical power - the rate of electrical energy used by an electrical circuit. Watt - the unit of measurement for power; one volt times one amp. Kilowatt - 1,000 watts. Kilowatt hours - a unit of electrical energy. Fuse - a non-resettable electrical device which protects a circuit from excessive power or current; a blown fuse must be replaced. Circuit breaker - a resettable electrical switch which protects a circuit from excessive power or current. Time The lesson presentation should take approximately 90 minutes. Allow a 45-minute class for the lab. Allow a 45-minute class for student to work problems and take the quiz. Copyright © Texas Education Agency, 2014. All rights reserved. 1 Preparation TEKS Correlations This lesson, as published, correlates to the following TEKS. Any changes/alterations to the activities may result in the elimination of any or all of the TEKS listed. Electronics 130.368 (c) o (5) The student implements the concepts and skills that form the technical knowledge of electronics using project-based assessments. The student is expected to: (A) apply Ohm's law, Kirchhoff's laws, and power laws. 130.368 (c) o (6) The student applies the concepts and skills to simulated and actual work situations. The student is expected to: (A) measure and calculate resistance, current, voltage, and power in series, parallel, and complex circuits. 130.368 (c) o (8) The student learns the function and application of the tools, equipment, and materials used in electronics through project-based assignments. The student is expected to: (A) safely use tools and laboratory equipment to construct and repair circuits; and (B) use precision measuring instruments to analyze circuits and prototypes. Interdisciplinary Correlations Physics 112.39 (c) o (1) Scientific processes. The student conducts investigations, for at least 40% of instructional time, using safe, environmentally appropriate, and ethical practices. These investigations must involve actively obtaining and analyzing data with physical equipment, but may also involve experimentation in a simulated environment as well as field observations that extend beyond the classroom. The student is expected to: (A) demonstrate safe practices during laboratory and field investigations. o (2) Scientific processes. The student uses a systematic approach to answer scientific laboratory and field investigative questions. The student is expected to: (H) make measurements with accuracy and precision and record data using scientific notation and International System (SI) units; and (I) identify and quantify causes and effects of uncertainties in measured data. Copyright © Texas Education Agency, 2014. All rights reserved. 2 o (5) Science concepts. The student knows the nature of forces in the physical world. The student is expected to: (F) design, construct, and calculate in terms of current through, potential difference across, resistance of, and power used by electric circuit elements connected in both series and parallel combinations. Occupational Correlation (O*Net – www.onetonline.org/) Job Title: Electrical Power-Line Installers and Repairers O*Net Number: 49-9051.00 Reported Job Titles: A Class Lineman, Apprentice Lineman Third Step, Class A Lineman, Electric Lineman, Electrical Lineman (Power), Electrical Lineworker, Journeyman Lineman, Lineman, Lineworker, Power Lineman Tasks Adhere to safety practices and procedures, such as checking equipment regularly and erecting barriers around work areas. Test conductors, according to electrical diagrams and specifications, to identify corresponding conductors and to prevent incorrect connections. Open switches or attach grounding devices to remove electrical hazards from disturbed or fallen lines or to facilitate repairs. Climb poles or use truck-mounted buckets to access equipment. Drive vehicles equipped with tools and materials to job sites. Identify defective sectionalizing devices, circuit breakers, fuses, voltage regulators, transformers, switches, relays, or wiring, using wiring diagrams and electrical-testing instruments. Install, maintain, and repair electrical distribution and transmission systems, including conduits, cables, wires, and related equipment, such as transformers, circuit breakers, and switches. Dig holes, using augers, and set poles, using cranes and power equipment. Place insulating or fireproofing materials over conductors and joints. Install watt-hour meters and connect service drops between power lines and consumers' facilities. Soft Skills Active Listening Critical Thinking Complex Problem Solving Coordination Judgment and Decision Making Accommodations for Learning Differences This lesson will accommodate the needs of every learner. You may modify this lesson to accommodate your students with learning differences. Refer to the files found on the Special Populations page of this website. Copyright © Texas Education Agency, 2014. All rights reserved. 3 Preparation Review the Electrical Power slide presentation and the lesson document prior to the lesson. Have worksheets, exams, and review materials ready before class begins. Have calculators and other materials ready. References • Buchla, D. & Floyd, T. (2005). The science of electronics DC/AC. (Chapter 4). Upper Saddle River, NJ: Pearson Prentice Hall. • Floyd, T. (1993). Principles of electric circuits: Electron flow version. New York, NY: Macmillan Publishing Co. • Robertson, L. (1980). Basic electronics I. Mid-American Vocational Curriculum Consortium, Inc. Instructional Aids Electrical Power slide presentation and notes Lab #1 – Compute Current Using the Power Formula worksheet Lab #2 – Determine Power Used in a Resistive Circuit worksheet Lab #3 – Determine the Function of Fuses and Resistor Power Rating worksheet Answer key for Lab #1 – Compute Current Using the Power Formula worksheet Answer key for Lab #2 – Determine Power Used in a Resistive Circuit worksheet Answer key for Lab #3 – Determine the Function of Fuses and Resistor Power Rating worksheet Student Activity – Compute Power from the Power Formula worksheet Answer key for Student Activity – Compute Power from the Power Formula worksheet Power Exam #1 Answer key for Power Exam #1 Power Exam #2 Answer key for Power Exam #2 Introduction Show o A light bulb. Say o Most of you have heard something like “this is a 60 watt bulb”. Ask o Do any of you know what that means? Say o Yes, it does have something to do with the amount of light the bulb will produce, but “60 watts” is actually the power rating of the bulb, representing how much electrical power the light bulb will consume. Copyright © Texas Education Agency, 2014. All rights reserved. 4 Ask o Does anyone know what happens when you leave a light on all day when you are not in the room? Say o Correct! Your electric bill will increase because you have used electrical power over a long period of time. In order to understand how that works, we need to understand electrical power. Let’s get started. Outline MI OUTLINE I. Electrical Power A. Rate description B. Energy consumption C. Energy conversion D. Electron movement E. Voltage explanation II. Power and Energy A. Comparing units of energy and power B. Converting electricity into useful energy C. Identifying useful energy around houses D. Associating power formulas with Ohm’s Law NOTES TO TEACHER Slides 1-4 Begin the Electrical Power slide presentation. Establish a good foundation before continuing with presentation. Slides 5-9 III. Wattmeter and Wattage Rating A. Using a wattmeter B. Measuring watts C. Wattage rating of resistors Slides 10-13 Note that a higher wattage resistor is physically larger. IV. Practice Problem 1 – Power in a Series Circuit A. Use a problem solving sequence B. Study the problem first C. Identify the missing information D. Solve the problem Slides 14-24 The problem solving sequence in Problem 1 is consistent with the problem solving process given in other lessons. V. Practice Problem 2 – Power in a Parallel Circuit A. Calculate power B. Determine which formula C. Solve the problem Slides 25-35 Problem 2 shows two methods using different versions of the power formula. Copyright © Texas Education Agency, 2014. All rights reserved. 5 NOTES TO TEACHER OUTLINE MI VI. Review Power, Terms, and Safety A. Practice Watt’s Law B. Terms and definitions C. Safety Precautions Slide 35 Have students complete as an independent practice. VII. Power Labs A. Lab #1 – Compute Current Using the Power Formula worksheet B. Lab #2 – Determine Power Used in a Resistive Circuit worksheet C. Lab #3 – Determine the Function of Fuses and Resistor Power Rating worksheet Slides 36-39 Use some of the worksheets for guided practice and others for independent practice. VIII. Student Activity – Compute Power from the Power Formula worksheet IX. Ensure students work many problems before they take the exams. Assessment A. Power Exam #1 B. Power Exam #2 Multiple Intelligences Guide Existentialist Interpersonal Intrapersonal Kinesthetic/ Bodily Logical/ Mathematical Musical/ Rhythmic Naturalist Verbal/ Linguistic Visual/ Spatial Application Guided Practice The teacher may conduct Lab #3 as a demonstration. Lab #1, Lab #3, and the discussion questions after Lab #2 can be guided practice - especially if supplies are limited. The first couple of problems from Student Activity should also be teacher-led. Use some of the worksheets for guided practice and others for independent practice. Independent Practice Lab #2, the rest of the problems from Student Activity, and slide 35 are independent practices. Copyright © Texas Education Agency, 2014. All rights reserved. 6 Summary Review Go over the lab worksheets to reinforce concepts and check for understanding. How did you know that your resistors were ½-watt resistors? Explain. How much power was applied to the 1,000-ohm resistor? (P=I2R) Explain why the fuse blew when 10-volts were applied to the 10-ohm resistor. Explain what happened to the resistor. How much power was being applied after you shorted the fuse? If you had to use a 10-ohm resistor with 10-volts applied, what would you do? Evaluation Informal Assessment The teacher will ask questions and encourage student feedback during the presentation. The teacher will also monitor each student or small group as they individually work to complete the assignments. Formal Assessment The student will be assessed by the accuracy of the completed assignments. The teacher will divide the final assessment into the following two parts. A measurement test of a series of exercises using the breadboards, power supplies, and assorted loads or lamps to measure current, voltage, resistors, and power accurately. The scores on the formal exams (Exam 1 and Exam 2.) Enrichment Extension Using the wattmeter to accurately measure power becomes a vital part of the technical problemsolving skills needed in the workplace. Thinking through a problem to arrive at the answer and working past frustrations strengthens character. Developing leadership skills helps solve challenges throughout life. Leading and following creates a sense of accomplishment that also helps strengthen character. Copyright © Texas Education Agency, 2014. All rights reserved. 7 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Lab #1 – Compute Current Using the Power Formula Equipment and materials Lamp holder with 100-watt bulb Lamp holder with 40-watt bulb 110-volt power source NOTE: Smaller voltage lamps can be used with an appropriate power supply. Procedure 1. Plug both lamps into 110-volt line and turn switches on at the same time. 2. Let lamps heat up for a brief time. 3. Put a hand close to each lamp and feel which one is hotter: ____ 40-watt ____ 100-watt. 4. Determine which lamp is using more power: ____ 40-watt ____ 100-watt. NOTE: The hotter lamp uses using more power. 5. Read and answer the following. Determine which lamp is using more current: ____ 40-watt ____ 100-watt. Determine which lamp has the lower resistance: ____ 40-watt ____ 100-watt. Using the formula P=VI, compute the current flowing through each lamp. 6. Return lamps to proper storage area. Copyright © Texas Education Agency, 2014. All rights reserved. 8 Name: _______________________________________ Class: ________________________ Date ___/___/___ Answer Key: Power Lab #1 – Compute Current Using the Power Formula Equipment and materials Lamp holder with 100-watt bulb Lamp holder with 40-watt bulb 110-volt power source NOTE: Smaller voltage lamps can be used with an appropriate power supply. Procedure 1. Plug both lamps into 110-volt line and turn switches on at the same time. 2. Let lamps heat up for a brief time. X 3. Put a hand close to each lamp and feel which one is hotter: ____ 40-watt ____ 100-watt. X 100-watt. 4. Determine which lamp is using more power: ____ 40-watt ____ NOTE: The hotter lamp uses more power. 5. Read and answer the following. X Determine which lamp is using more current: ____ 40-watt ____ 100-watt. Determine which lamp has the lower resistance: ____ 40-watt ____ 100-watt. Using the formula P=VI, compute the current flowing through each lamp. X 6. Return lamps to proper storage area. Copyright © Texas Education Agency, 2014. All rights reserved. 9 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Lab #2 – Determine Power Used in a Resistive Circuit Equipment and materials Adjustable DC power supply (0-30 volts) DC wattmeter (0-20 watts) 330-ohm, 2-watt resistor Ammeter or multimeter DC voltmeter and ohmmeter (multimeter) Procedure Steps 1. Leaving the power off, connect the following circuit (Figure 1.) a. FIGURE 1 2. Double check your circuit for correct wiring. 3. Turn on power. 4. Apply 10 volts across the resistor. 5. Measure the voltage to set it to the indicated value, then measure the current and power and record the values. Data Table OBSERVED _________________ V I P COMPUTED ________________ V I P (Record step 5.) 10 V ____ ____ ____ ____ ____ (Record step 7.) 15 V ____ ____ ____ ____ ____ (Record step 9.) 20 V ____ ____ ____ ____ ____ (Record step 12.) _____Ω Copyright © Texas Education Agency, 2014. All rights reserved. 10 Name: _______________________________________ Class: ________________________ Date ___/___/___ 6. Increase the power supply to 15 volts. 7. Read and record V, I, and P in the Data Table. 8. Increase the power supply to 20 volts. 9. Read and record V, I, and P in the Data Table. 10. Turn off the power supply. 11. Disconnect the circuit. 12. Read its value with the ohmmeter and record in the Data Table. 13. Using the three forms of power formula, compute the power for the V, I, and R values at 10 volts, 15 volts, and 20 volts. 14. Compare the computed values with the wattmeter readings. NOTE: The following questions may be used for discussion: A. What causes the difference between computed values and wattmeter indications? B. Did the resistance remain constant during this experiment? Did the current remain constant? Compare the changing voltage with the power consumed by the resistor. Is the relationship a direct proportion or inverse proportion? C. Would the same power be consumed if the load were reversed? Why? D. When the voltage was doubled (10 V to 20 V), how much did power increase? 15. Return the equipment and materials to the proper storage area. Copyright © Texas Education Agency, 2014. All rights reserved. 11 Name: _______________________________________ Class: ________________________ Date ___/___/___ Answer Key: Power Lab #2 – Determine Power Used in a Resistive Circuit Equipment and materials Adjustable DC power supply (0-30 volts) DC wattmeter (0-20 watts) 330-ohm, 2-watt resistor Ammeter or multimeter DC voltmeter and ohmmeter (multimeter) Procedure Steps 1. Leaving the power off, connect the following circuit (Figure 1.) b. FIGURE 1 2. Double check your circuit for correct wiring. 3. Turn on power. 4. Apply 10 volts across the resistor. 5. Measure the voltage to set it to the indicated value, then measure the current and power and record the values. Data Table OBSERVED ________________ V I P (Record step 5.) 10 V meas ____ meas ____ mW 10 V 30____ mA 300 ____ ____ (Record step 7.) 15 V meas ____ meas ____ 15 V ____ (Record step 9.) 20 V meas ____ meas ____ 20 V (Record step 12.) _____Ω COMPUTED _______________ V I P 45 mA ____ 682 mW ____ mA 1.21 A ____ 60 ____ ____ Measured value 330 Ω +/- 5% Copyright © Texas Education Agency, 2014. All rights reserved. 12 Name: _______________________________________ Class: ________________________ Date ___/___/___ 6. Increase the power supply to 15 volts. 7. Read and record V, I, and P. 8. Increase the power supply to 20 volts. 9. Read and record V, I, and P. 10. Turn off the power supply. 11. Disconnect the circuit. 12. Read its value with the ohmmeter. 13. Using the three forms of power formula, compute the power for the V, I, and R values at 10 volts, 15 volts, and 20 volts. 14. Compare the computed values with the wattmeter readings. NOTE: The following questions may be used for discussion: A. What causes the difference between computed values and wattmeter indications? o Differences in actual resistor values and power supply voltage B. Did the resistance remain constant during this experiment? Did the current remain constant? Compare the changing voltage with the power consumed by the resistor. Is the relationship a direct proportion or inverse proportion? o It should remain nearly constant, although resistance value does change with temperature. o Direct C. Would the same power be consumed if the load were reversed? Why? o Yes, a resistor has no polarity. D. When the voltage was doubled (10 V to 20 V), how much did power increase? o By a factor of 4, a function of V squared 15. Return the equipment and materials to the proper storage area. Copyright © Texas Education Agency, 2014. All rights reserved. 13 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Lab #3 – Determine the Function of Fuses and Resistor Power Rating (NOTE: The teacher may want to conduct this lab as a demonstration.) Equipment and materials Variable power supply (minimum 10 V, 1-amp capability) 1,000-ohm, ½-watt resistor 10-ohm, ½-watt resistor (expendable) One ½-amp fuse (expendable) DC ammeter (1-amp capability) Procedure 1. Connect the power supply, switch, 1,000-ohm resistor, ammeter, and ½-amp fuse in a series as shown in Figure 1. FIGURE 1 2. Turn on the power supply. 3. Adjust to 10 volts. 4. Turn on switch. 5. Read and record the current indication on the ammeter. 6. Turn the power supply to zero. 7. Open the switch. 8. Replace the 1,000-ohm resistor with the 10-ohm resistor. (CAUTION: Set range switch, if applicable, to 1 ampere or more.) 9. Close the switch. 10. Adjust the power supply to 10 volts. 11. Observe the fuse and record what you observe. _____________________ 12. Connect a wire across the fuse and observe the 10-ohm resistor, and record what you observe. ___________________________________ Copyright © Texas Education Agency, 2014. All rights reserved. 14 Name: _______________________________________ Class: ________________________ Date ___/___/___ Answer Key: Power Lab #3 – Determine the Function of Fuses and Resistor Power Rating (NOTE: The teacher may want to conduct this lab as a demonstration.) Equipment and materials Variable power supply (minimum 10 V, 1-amp capability) 1,000-ohm, ½-watt resistor 10-ohm, ½-watt resistor (expendable) One ½-amp fuse (expendable) DC ammeter (1-amp capability) Procedure 1. Connect the power supply, switch, 1,000-ohm resistor, ammeter, and ½-amp fuse in a series as shown in Figure 1. FIGURE 1 2. Turn on the power supply. 3. Adjust to 10 volts. 4. Turn on switch. 10 mA 5. Read and record the current indication on the ammeter. ____________ 6. Turn the power supply to zero. 7. Open the switch. 8. Replace the 1,000-ohm resistor with the 10-ohm resistor. (CAUTION: Set range switch, if applicable, to 1 ampere or more) 9. Close the switch. 10. Adjust the power supply to 10 volts. The fuse should blow because I = 1 A 11. Observe the fuse and record what you observe. __________________________________________ 12. Connect a wire across the fuse and observe the 10-ohm resistor, and record what you observe. The 10 ohm resistor should overheat and burn up without the fuse showing that the fuse protects ________________________________________________________________________________ the circuit. ________ Copyright © Texas Education Agency, 2014. All rights reserved. 15 Answer Key: Review Questions Directions: The following questions should be used for discussion. Explain how you know that your resistors were ½-watt resistors. o The size of the resistor - bigger than a quarter watt, smaller than a 2 watt. How much power was applied to the 1,000-ohm resistor? (From lab 3, P=I2R) o 100 mW Explain why the fuse blew when 10 volts were applied to the 10-ohm resistor. o The total current would be larger than the fuse rating. Explain what happened to the resistor. How much power was being applied after you shorted the fuse? o The total power would be larger than the power rating of the resistor. If you had to use a 10-ohm resistor with 10-volts applied, what would you do? o Use a higher power resistor and a higher current fuse. Copyright © Texas Education Agency, 2014. All rights reserved. 16 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Student Activity – Compute Power from the Power Formula Directions Study the following schematics and answer the questions below them. Use the formulas for power: P=VI when current and voltage are known; P=I2R when current and resistance are known; and P=V2 /R when voltage and resistance are known. A. I = 25 mA R = 150 Ω 1. State the power formula(s) needed to solve for power. 2. Solve for P. B. R = 100 Ω R = 250 Ω V = 16 V 1. State the power formula(s) needed to solve for power. 2. Solve for P. C. I = 60 mA I = 20 mA R = 40 Ω 1. State the power formula(s) needed to solve for power. 2. Solve for P. Copyright © Texas Education Agency, 2014. All rights reserved. 17 Name: _______________________________________ Class: ________________________ Date ___/___/___ Answer Key: Power Student Activity – Compute Power from the Power Formula Directions Study the following schematics and answer the questions below them. Use the formulas for power: P=VI when current and voltage are known; P=I2R when current and resistance are known; and P=V2 /R when voltage and resistance are known. A. I = 25 mA R = 150 Ω 1. State the power formula(s) needed to solve for power. P=I2R 2. Solve for P. P= 93.75 mW B. R = 100 Ω R = 250 Ω V = 16 V 1. State the power formula(s) needed to solve for power. P= V2 /R 2. Solve for P. P = 1.024 W C. IT = 60 mA I2 = 20 mA R1 = 40 Ω 1. State the power formula(s) needed to solve for power. P1 = I2R, P2 = VI 2. Solve for P. P1 = 64 mA, P2 = 32 mA Copyright © Texas Education Agency, 2014. All rights reserved. 18 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Exam #1 I. II. Match the terms to the correct definitions. 1. Electrical power A 1,000 watts 2. Watt B Energy, in kilowatts, multiplied by the time in hours 3. Kilowatt C The unit of measurement for power 4. Kilowatt hours D The rate of doing work by electrons moving through a resistive circuit Match the symbols or abbreviations with their correct definition. 5. Fuse A The rate of doing work 6. Circuit breaker B An electrical device which protects a circuit from excessive power or current 7. Power C Measured in foot-pounds without any reference to time 8. Work D An electrical switch which protects a circuit form excessive power or current III. Solve the following problems. 9. Find the power, given that the total load for the circuit is 455Ω and the applied voltage is 40 volts. A 3.516 W B 35.164 W C 5.18 W D .88 W Copyright © Texas Education Agency, 2014. All rights reserved. 19 Name: _______________________________________ Class: ________________________ Date ___/___/___ 10. What is the power in a circuit with the following closed loop condition: the current is 32.5 mA, the circuit of four loads different resistance values, and the applied voltage is 32.5 volts? A 1000 W B 2W C 1.056 W D .965 W 11. If the following circuit has RT = 24000Ω with an applied voltage of 25 volts, what is the power to this circuit? A 2.6 W B 1.524 W C 26 mW D 5.2 mW 12. Solve for unknown power when I=2 milliamps and R=3 kilo-ohms. P=____ A 12 W B 1.5 W C 12 mW D .15 W 13. Solve for unknown power when V=110 volts and I=2 amperes. P=_____ A 50 W B 18.18 W C 220 W D 55 W 14. Solve for unknown power when I=16 milliamps and R=8 megaohms. P=____ A 2048 W B 20.48 W C .509 W D 509 W 15. Solve for unknown power when V=22 volts and I=2mA. P=_____ A 4.4 W B 44 mW C 1.1 W D 11 W Copyright © Texas Education Agency, 2014. All rights reserved. 20 Name: _______________________________________ Class: ________________________ Date ___/___/___ 16. What is the power in a circuit with the following closed loop circuit condition: the current is 325 mA, the circuit of four loads has different resistance values, and the applied voltage is 120 volts? A 44.5 W B 36.9 W C 39 W D 55.4 W 17. Find the unknown variable for power when these values are given: applied voltage is 12 volts and the total resistance is 5.60 MΩ. A 25.7 mW B 25.7µW C 2.1 µW D 17.6 mW 18. Find the unknown variable when these values are given: current is 6 µA and the total resistance is 560 MΩ. A 2 mW B 17.6 W C 11.2 mW D 5.66 W IV. Arrange in proper sequence the following procedure for power measurement using a DC wattmeter. Use letters A-E, where A represents Step 1 and E represents Step 5. 19. __Read and record power value in watts indicated on wattmeter. 20. __Turn on circuit power. 21. __Connect the voltage (E) terminals of wattmeter across load with power off. 22. __Turn off circuit power and disconnect wattmeter. 23. __With circuit power turned off, connect the current (I) terminals of wattmeter in series with circuit load. Copyright © Texas Education Agency, 2014. All rights reserved. 21 Name: _______________________________________ Class: ________________________ Date ___/___/___ V. Select the correct answer. 24. Which of the following is a true statement concerning resistor wattage rating? A Most resistors do not have a wattage rating. B A wattage safety factor of 1 should be used when choosing resistors. C The wattage ratting indicates the maximum amount of power that a resistor can handle before it burns up. D Smaller wire wound resistors are capable of dissipating the heat generated by higher power levels. 25. Which of the following statements is a correct statement relative to either direct or inverse proportion A A directly proportional relationship is one by which a change in one quantity produces the opposite direction of change in another quantity. B An inversely proportional relationship is one by which a change in one quantity produces no effect on another quantity. C An inversely proportional relationship is one by which a change in one quantity produces the same direction of change in another quantity. D A directly proportional relationship is one by which a change in one quantity produces the same direction of change in another quantity. Copyright © Texas Education Agency, 2014. All rights reserved. 22 Name: _______________________________________ Class: ________________________ Date ___/___/___ Answer Key: Power Exam #1 I. Match the terms to the correct definitions. 1. Electrical power D A 1,000 watts 2. Watt C B Energy, in kilowatts, multiplied by the time in hours 3. Kilowatt A C The unit of measurement for power 4. Kilowatt hours B D The rate of doing work by electrons moving through a resistive circuit II. Match the symbols or abbreviations with their correct definition. 5. Fuse B A The rate of doing work 6. Circuit breaker D B An electrical device which protects a circuit from excessive power or current 7. Power A C Measured in foot-pounds without any reference to time 8. Work C D An electrical switch which protects a circuit form excessive power or current III. Solve the following problems 9. Find the power, given that the total load for the circuit is 455Ω and the applied voltage is 40 volts. A 3.516 W B 35.164 W C 5.18 W D .88 W Copyright © Texas Education Agency, 2014. All rights reserved. 23 Name: _______________________________________ Class: ________________________ Date ___/___/___ 10. What is the power in a circuit with the following closed loop condition: the current is 32.5 mA, the circuit of four loads different resistance values, and the applied voltage is 32.5 volts? A 1000 W B 2W C 1.056 W D .965 W 11. If the following circuit has RT = 24000Ω with an applied voltage of 25 volts, what is the power of this circuit? A 2.6 W B 1.524 W C 26 mW D 5.2 mW 12. Solve for unknown power when I = 2 milliamps and R = 3 kilo-ohms. P=____ A 12 W B 1.5 W C 12 mW D .15 W 13. Solve for unknown power when V = 110 volts and I = 2 amperes. P=_____ A 50 W B 18.18 W C 220 W D 55 W 14. Solve for unknown power when I = 16 milliamps and R = 8 megaohms. P=____ A 2048 W B 20.48 W C .509 W D 509 W 15. Solve for unknown power when V=22 volts and I=2mA. P=_____ A 4.4 W B 44 mW C 1.1 W D 11 W Copyright © Texas Education Agency, 2014. All rights reserved. 24 Name: _______________________________________ Class: ________________________ Date ___/___/___ 16. What is the power in a circuit with the following closed loop circuit condition: the current is 325 mA, the circuit of four loads has different resistance values, and the applied voltage is 120 volts? A 44.5 W B 36.9 W C 39 W D 55.4 W 17. Find the unknown variable for power when these values are given: applied voltage is 12 volts and the total resistance is 5.60 MΩ. A 25.7 mW B 25.7µW C 2.1 µW D 17.6 mW 18. Find the unknown variable when these values are given: current is 6 µA and the total resistance is 560 MΩ. A 2 mW B 17.6 W C 11.2 mW D 5.66 W IV. Arrange in proper sequence the following procedure for power measurement using a DC wattmeter. Use letters A-E, where A represents Step 1 and E represents Step 5. 19. D (Step 4) Read and record power value in watts indicated on wattmeter. 20. C (Step 3) Turn on circuit power. 21. B (Step 2) Connect the voltage (E) terminals of wattmeter across load with power off. 22. E (Step 5) Turn off circuit power and disconnect wattmeter. 23. A (Step 1) With circuit power turned off, connect the current (I) terminals of wattmeter in series with circuit load. Copyright © Texas Education Agency, 2014. All rights reserved. 25 Name: _______________________________________ Class: ________________________ Date ___/___/___ V. Select the correct answer. 24. Which of the following is a true statement concerning resistor wattage rating? A Most resistors do not have a wattage rating. B A wattage safety factor of 1 should be used when choosing resistors. C The wattage ratting indicates the maximum amount of power that a resistor can handle before it burns up. D Smaller wire wound resistors are capable of dissipating the heat generated by higher power levels. 25. Which of the following statements is a correct statement relative to either direct or inverse proportion. A A directly proportional relationship is one by which a change in one quantity produces the opposite direction of change in another quantity. B An inversely proportional relationship is one by which a change in one quantity produces no effect on another quantity. C An inversely proportional relationship is one by which a change in one quantity produces the same direction of change in another quantity. D A directly proportional relationship is one by which a change in one quantity produces the same direction of change in another quantity. Copyright © Texas Education Agency, 2014. All rights reserved. 26 Name: _______________________________________ Class: ________________________ Date ___/___/___ Power Exam #2 1. List two circuit safety precautions and one worker safety precaution. (2pts each) A. Circuit safety precautions i. ii. B. Worker safety precaution with live circuits i. 2. List three uses of Watt’s Law in any order. (2pts each) A. __ ___ __________________________________ B. _________________________________________ C. _________________________________________ Calculate the power and show the formula used (1 pt. - state formula; 1 pt. - fill in values w/ labels; 1 pt. - for the numeric answer; and 1 pt. - for the proper tag). Given Answer for Power 3. 37 KΩ 10 µA 3. _____ ___________________________________________________ _____ 4. 2 mA 57 V 4. ___________________________________________________ _____ _____ 5. 24 volts 250 KΩ 5. ___________________________________________________ _____ _____ 6. 100 µA 35 MΩ 6. ___________________________________________________ _____ _____ 7. 12.5 volts 13 µamps 7. ___________________________________________________ _____ _____ Copyright © Texas Education Agency, 2014. All rights reserved. 27 Answer Key: Power Exam #2 1. List two circuit safety precautions and one worker safety precaution. (2pts each) A. Circuit safety precautions i. Never install a fuse or circuit breaker whose current rating is higher or whose voltage rating is lower than specified for a particular circuit. ii. Never bypass or defeat a fuse or circuit breaker. B. Worker safety precaution with live circuits i. Work with well-insulated tools whenever possible. OR ii. Avoid completing a circuit through the body. 2. List three uses of Watt’s Law in any order. (2pts each) A. When current and voltage are known, P=VI B. When current and resistance are known, P=I2R C. When voltage and resistance are known, P=V2/R Calculate the power and show the formula used (1 pt. - state formula; 1 pt. - fill in values w/ labels; 1 pt. - for the numeric answer; and 1 pt. - for the proper tag). Given 3. Answer for Power 37 KΩ 10 µA 2 P=I R= (.00001) x 37000 = .00037 W 3. 2 57 V mW__ 114__ mW__ 4. 2 mA P= VI= (.002)(57) = .114 W 5. 24 volts 250 KΩ 2 P=V /R = 24 / 250000 = .0002304 W 5. 2.304_ mW__ 100 µA 35 MΩ 2 P=I R= (.00001) x 35000000 = .35 W 6. 0.35__ W ___ 12.5 volts 13 µamps P= VI= (12.5)(.000013) = 162.5 µW 7. 162.5_ µW__ _ 2 6. 2 7. 4. 3.7___ Copyright © Texas Education Agency, 2014. All rights reserved. 28