metal cutting processes -...

Copyright © Texas Education Agency, 2014. All rights reserved.

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Electrical Power Electronics 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.

Lesson Plan


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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.

Preparation


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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.

http://www.onetonline.org/


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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

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.

Introduction


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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

III. Wattmeter and Wattage Rating

A. Using a wattmeter B. Measuring watts C. Wattage rating of resistors

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

V. Practice Problem 2 – Power in a Parallel Circuit

A. Calculate power B. Determine which formula C. Solve the problem

NOTES TO TEACHER Slides 1-4 Begin the Electrical Power slide presentation. Establish a good foundation before continuing with presentation. Slides 5-9 Slides 10-13 Note that a higher wattage resistor is physically larger. Slides 14-24 The problem solving sequence in Problem 1 is consistent with the problem solving process given in other lessons. Slides 25-35 Problem 2 shows two methods using different versions of the power formula.


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Multiple Intelligences Guide

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.

Existentialist

Interpersonal

Intrapersonal

Kinesthetic/

Bodily

Logical/

Mathematical

Musical/ Rhythmic

Naturalist

Verbal/

Linguistic

Visual/ Spatial

Application

MI

OUTLINE

VI. Review Power, Terms, and Safety A. Practice Watt’s Law B. Terms and definitions C. Safety Precautions

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

VIII. Student Activity – Compute Power from the Power

Formula worksheet

IX. Assessment A. Power Exam #1 B. Power Exam #2

NOTES TO TEACHER Slide 35 Have students complete as an independent practice. Slides 36-39 Use some of the worksheets for guided practice and others for independent practice. Ensure students work many problems before they take the exams.


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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?

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.) Extension

Using the wattmeter to accurately measure power becomes a vital part of the technical problem-solving 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.

Summary

Evaluation

Enrichment


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Name: _______________________________________ Class: ________________________ Date ___/___/___

Power

Lab #1 – Compute Current Using the Power Formula

Equipment and materials

Lamp holder with 100-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.


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Name: _______________________________________ Class: ________________________ Date ___/___/___

Answer Key: Power

Lab #1 – Compute Current Using the Power Formula




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 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.

X

X

X

X


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Name: _______________________________________ Class: ________________________ Date ___/___/___

Power

Lab #2 – Determine Power Used in a Resistive Circuit


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.

OBSERVED COMPUTED _________________ ________________

Data Table V I P V I P (Record step 5.) 10 V ____ ____ ____ ____ ____ (Record step 7.) 15 V ____ ____ ____ ____ ____ (Record step 9.) 20 V ____ ____ ____ ____ ____

(Record step 12.) _____Ω


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Name: _______________________________________ Class: ________________________ Date ___/___/___

6. Increase the power supply to 15 volts.

7. Read and record V, I, and P in the Data Table.


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.


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Name: _______________________________________ Class: ________________________ Date ___/___/___

Answer Key: Power

Lab #2 – Determine Power Used in a Resistive Circuit


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.

OBSERVED COMPUTED ________________ _______________

Data Table V I P V I P (Record step 5.) 10 V ____ ____ ____ ____ ____ (Record step 7.) 15 V ____ ____ ____ ____ ____ (Record step 9.) 20 V ____ ____ ____ ____ ____

(Record step 12.) _____Ω

30 mA 300 mW 10 V

15 V 45 mA 682 mW

20 V 60 mA 1.21 A

meas meas

meas meas

meas meas

Measured value 330 Ω +/- 5%


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Name: _______________________________________ Class: ________________________ Date ___/___/___


7. Read and record V, I, and P.


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.


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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.)


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. ___________________________________


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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.)


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. ________________________________________________________________________________

The fuse should blow because I = 1 A

The 10 ohm resistor should overheat and burn up without the fuse showing that the fuse protects the circuit. ________

10 mA


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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.


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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.

1. State the power formula(s) needed to solve for power. 2. Solve for P.

B.


C.


I = 25 mA

R = 100 Ω

R = 250 Ω V = 16 V

R = 150 Ω

I = 60 mA

I = 20 mA

R = 40 Ω


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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.

1. State the power formula(s) needed to solve for power. 2. Solve for P. P= 93.75 mW

B.

1. State the power formula(s) needed to solve for power. 2. Solve for P. P = 1.024 W

C.

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

I = 25 mA

R = 100 Ω

R = 250 Ω V = 16 V

R = 150 Ω

IT = 60 mA

I2 = 20 mA

R1 = 40 Ω

P=I2R

P= V2 /R


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Name: _______________________________________ Class: ________________________ Date ___/___/___

Power Exam #1

I. 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

II. 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


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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 2 W

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


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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.


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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



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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


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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 2 W

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


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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.


26

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


D A directly proportional relationship is one by which a change in one quantity produces the



27

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. ___________________________________________________

_____

_____

_____

_____

_____

_____

_____

_____

_____

_____


28

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 Answer for Power 3. 37 KΩ 10 µA 3.

P=I2R= (.00001)2 x 37000 = .00037 W

4. 2 mA 57 V 4. P= VI= (.002)(57) = .114 W 5. 24 volts 250 KΩ 5. P=V2/R = 242/ 250000 = .0002304 W 6. 100 µA 35 MΩ 6. P=I2R= (.00001)2 x 35000000 = .35 W

7. 12.5 volts 13 µamps 7.

P= VI= (12.5)(.000013) = 162.5 µW

3.7___

mW__

114__

mW__

2.304_

mW__

0.35__

W ___

162.5_

µW___

metal cutting processes -...

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