ase 6 - electrical electronic systemsfaculty.ccbcmd.edu/~smacadof/books/a6student...and watt’s...

ASE 6 - Electrical ElectronicSystems

Module 5Ohm’s and Watt’s Laws

AcknowledgementsGeneral Motors, the IAGMASEP Association Board of Directors, and RaytheonProfessional Services, GM's training partner for GM's Service Technical College wish tothank all of the people who contributed to the GM ASEP/BSEP curriculum developmentproject 2002-3. This project would not have been possible without the tireless efforts ofmany people. We acknowledge:

• The IAGMASEP Association members for agreeing to tackle this large project tocreate the curriculum for the GM ASEP/BSEP schools.

• The IAGMASEP Curriculum team for leading the members to a single vision andimplementation.

• Direct contributors within Raytheon Professional Services for their support oftranslating a good idea into reality. Specifically, we thank:

– Chris Mason and Vince Williams, for their leadership, guidance, and support.– Media and Graphics department under Mary McClain and in particular, Cheryl

Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & NancyDeSantis.

– For his help on the Electrical curriculum volume, Subject Matter Expert, KenBeish, Jr., for his wealth of knowledge.

Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEPColleges for their contribution for reformatting existing General Motors training material,adding critical technical content and the sharing of their expertise in the GM product.Separate committees worked on each of the eight curriculum areas. For the work on thisvolume, we thank the members of the Electrical committee:

– Jack Davis, Community College of Baltimore County - Catonsville– Jim Halderman, Sinclair Community College– Megan Kuehm, Community College of Allegheny County– Frank Longbottom, Camden County College– Jeff Rehkopf, Florida Community College at Jacksonville– Randy Peters, Des Moines Area Community College– David Rodriguez, College of Southern Idaho– Ed Schauffler, Longview Community College– Vince Williams, Raytheon

ContentsModule 5 – Ohm’s and Watt’s LawsAcknowledgements .......................................................................................... 2Objectives ........................................................................................................ 4

General Rule of Ohm’s Law ........................................................................................... 5Ohm’s Law Equation ....................................................................................................... 6Ohm’s Law Solving Circle ............................................................................................... 7Voltage Unknown ............................................................................................................ 8Resistance Unknown ...................................................................................................... 9Current Unknown .......................................................................................................... 10Exercise ........................................................................................................................ 12Exercise ........................................................................................................................ 13Experiment .................................................................................................................... 15Experiment .................................................................................................................... 17Experiment .................................................................................................................... 19Experiment .................................................................................................................... 21Exercise ........................................................................................................................ 22

© 2002 General Motors CorporationAll Rights Reserved

ASE 6 - ElectricalElectronic Systems

Module 5 - Ohm’sand Watt’s Laws

5-4

Student WorkbookObjectivesAt the end of this section, the student will be able to:• Explain the Ohm’s law relationship between voltage, current, and

resistance.• Using Ohm’s law, determine voltage, amperage, and ohm values of

sample circuits.• Explain the Watt’s law relationship between voltage and current.• Using Watt’s law, determine power in a circuit.

NATF Area VIA4: A Diagnose electrical/electronic integrity for series, parallel and series-parallel circuits using principles of electricity (Ohm’s Law):




5-5

Student WorkbookGeneral Rule of Ohm’s LawOhm’s law states that current is directly proportional to circuit voltage andinversely proportional to circuit resistance. This means that the amount ofcurrent flow in a circuit depends on how much voltage or resistance thereis in the circuit.Since most automotive circuits work using a 12-volt source, the amount ofcurrent is determined by how much resistance is present in the circuit.Current dose the work that needs to be done. Voltage is the pressure thatmoves the current, and resistance is what slows down the current flow.(restricts current flow)The rules used to understand, to predict, and to calculate the behavior ofelectricity in specific circuits are grouped under the title “Ohm’s Law.”Looking at the Ohm’s Law equation, you can derive the following generalrules.

Assuming the resistance does not change:• As voltage increases, current increases• As voltage decrease, current decreases

Assuming the voltage does not change:• As resistance increases, current decreases• As resistance decreases, current increases




5-6

Student WorkbookOhm’s Law EquationOhm’s Law can be expressed as an algebraic equation in which:

“E” stands for electromotive force or voltage.“I” stands or intensity or amperage“R” stands for resistance or ohms

If you know two parts of the Ohm’s Law equation, you can calculate thethird part.

• To determine volts, multiply amps times ohms.• To determine amps, divide volts by ohms• To determine ohms, divide volts by amps.




5-7

Student WorkbookOhm’s Law Solving CircleThe Ohm’s law solving circle is an easy way to remember how to solveany part of the equation. To use the solving circle, cover any letter that youdon’t know. The remaining letters give you the equation for determiningthe unknown quantity

Figure 5-1, Ohm’s Law Solving Circle




5-8

Student WorkbookVoltage UnknownIn this circuit, the source voltage is unknown. The resistance of the load is2 ohms and the current flowing through the circuit is 6 amps. Since thevoltage is unknown, the equation to solve for voltage is amps times ohms.So, multiplying 6 times 2 equals 12. Therefore, the source voltage in thecircuit is 12 volts.

Figure 5-2, Voltage Unkown




5-9

Student WorkbookResistance UnknownIn this circuit, the resistance is unknown. The current flow through thecircuit is 6 amps and the source voltage is 12 volts. Since the resistance isunknown, the equation to solve for ohms is voltage divided by amperage.So, 12 divided by 6 equals 2. Therefore the resistance in the circuit is 2ohms.

Figure 5-3, Resistance Unknown




5-10

Student WorkbookCurrent UnknownIn this circuit, the current is unknown. The resistance of the load is 2 ohmsand the source voltage is 12 volts. Since the amps are unknown, theequation to solve for amperage is volts divided by ohms. So, 12 divided by2 equals6. Therefore, the current flow in this circuit is 6 amps.

Figure 5-4, Current Unkown




5-11

Student Workbook

Figure 5-5, Watt’s Law Solving Circle

This is the Watt’s circle, it is similar to the Ohm’s circle. The “P” representsPower in Watt’s, the “I” represents intensity in Amps, “E” representselectromotive force in Voltage. When any of the two values are known,cover the unknown value, and calculate the same way you did when usingOhm’s law.




5-12

Student WorkbookExercise




5-13

Student WorkbookExercise

1. What is the current through the circuit in amps?

2. What is the formula used?

Figure 5-6, Calculate Current




5-14

Student Workbook

Figure 5-7, Voltage Affects Current




5-15

Student WorkbookExperimentObjective: Demonstrate that if the resistance remains the same, voltagedirectly affects current and wattage.

On your project board, assemble the circuit shown on the previous page.With the circuit connected to the 12-volt power tap, measure the voltageacross the resistor and the current flowing through the resistor (remember,your meter must be connected in series with the circuit to measurecurrent). Then calculate the power in watt's Record the results. Repeatthese two measurements (voltage and current) with the circuit connectedto the 5-volt power tap.

Measure

Resistance

(Ohm’s)

Voltage Drop

Across R1

Current

(Amps)

Through R1

Calculate

Power

(Watts)

12 volt

supply

5 volt supply

Why did the current change? Why did the wattage change?




5-16

Student Workbook

Figure 5-8, Current Performs Work (Experiment 5-2)




5-17

Student WorkbookExperimentObjective: Demonstrate that electrical current flow performs work.On your project board, assembly the circuit as shown on the previouspage. With the circuit connected to the 12-volt power tap, measure thevoltage across the bulb. Then measure he current flowing through the bulb(remember, your meter must be connected in series with the circuit tomeasure current). Then calculate the power in watt’s. Record the results.Repeat these two measurements (voltage and current) with the circuitconnected to the 5-volt power tap.

Measure

Resistance (Ohm’s)

Voltage Drop Across the bulb

Current (Amps) Through the bulb

Calculate Power (Watts)

12 volt supply

5 volt supply

Why is the bulb not as intense when the 5-volt supply is applied to thecircuit? Did the wattage change?

Note:Resistance readings of the bulb filaments do not represent the actualworking resistance of the bulb when voltage is applied. The resistanceincreases as the bulb warms.




5-18

Student Workbook

Figure 5-9, Resistance Inversely Affects Current (Experiment 5-3)




5-19

Student WorkbookExperimentObjective: Demonstrate that if the voltage remains the same, changes inresistance inversely affect current and wattage.Assemble the circuit as shown on the previous page. With the power off,measure the resistance of R1. Then apply power to the circuit andmeasure the voltage drop across R1 and the current flowing through R1.Then calculate the power in Watt’s. Record the results in the table below.Next, remove power from the circuit and replace R1 with R2. Repeat thethree measurements: resistance (power off), voltage drop, current andcalculate power. Again, record your results in the table below.Replace R2 with R3. Repeat the three measurements: resistance (poweroff), voltage drop, current and calculate power. Again, record you resultsin the table below.

Resistance (Ohm’s)

Voltage Drop

Current (Amps)


R1 = 100 Ohm’s

R2 = 1000 Ohm’s

R3 = 10,000 Ohm’s

If resistance in a circuit increases how does this affect the current flow inthe circuit? Did the wattage change?




5-20

Student Workbook

Figure 5-10, Effects of Resistance on Work Performed (Experiment 5-4)




5-21

Student WorkbookExperimentExperiment Objective: Demonstrate that increasing resistance in a circuitdecreases current flow and reduces the work performed in the circuit.Assemble the circuit shown on the previous page. With the power off,measure the resistance of R1. Then apply power to the circuit andmeasure the voltage drop across R1 and the current flowing throughR1.Then calculate the power in watt’s. Record the results in the tablebelow. Next, remove power from the circuit and replace R1 with R3.Repeat the three measurements: resistance (power off), voltage drop,current and calculate the power(watts). Again, record your results in thetable below.

Is there a difference in the intensity of the LED when the 10,000 ohmresistor is used in place of the 100 ohm resistor? Why? Did the wattagechange?

Resistor Resistance Voltage Drop

Current (Amps)


R1 = 100 Ohm’s

R3 = 10,000 Ohm’s




5-22

Student WorkbookExerciseRead each question carefully and answer by filling in the blanks.1. If the resistance of a typical automotive circuit equaled 6 ohms, the

equation to solve for current would be ______________assuming 12volts.a. 6 ohms divided by 12 volts =______________b. 12 volts divided by 6 ohms =_____________ ampsc. 12 volts x 6 ohms = ____________ampsd. None of the above

2. In a circuit, what can cause an increase in current if voltage stays thesame?a. decrease in resistanceb. increase in resistancec. opend. short to power

3. In Ohm’s Law, “E” stands for ____________a. currentb. resistancec. voltaged. watts

4. In Ohm’s Law, “I” stands for _____________a. currentb. resistancec. voltaged. watts




5-23

Student Workbook5. If the voltage applied to a resistor in increased, the currentwill__________a. decreaseb. increasec. stay the samed. vary

6. A cigar lighter draws approximately 6 amps. A starter motor drawsapproximately 150 amps. Using Ohm’s Law, which has the lowerresistance?a. cigar lighterb. starter motorc. neitherd. insufficient information

7. A terminal in series wit a dome lamp is seriously corroded. The domeis very dim. The overall resistance of the circuit has__________________a. decreasedb. increasedc. stayed the samed. varied

ase 6 - electrical electronic systemsfaculty.ccbcmd.edu/~smacadof/books/a6student...and watt’s...

Documents