Exploring the Science of Electricity

Exploring the Science of Electricity

Unit: Mechanical Systems and Technology

Problem Area: Electrical Systems

Lesson: Exploring the Science of Electricity

� Student Learning Objectives. Instruction in this lesson should result in studentsachieving the following objectives:

1 Relate electricity to the structure of elements and atoms.

2 Explain conductors, insulators, and semiconductors.

3 Explain the conventional and electron theories of electrical current flow.

4 Describe how electricity can be generated by friction, heat, light, chemicalreactions, and magnetism.

5 Describe the difference between and applications of direct current (DC) andalternating current (AC) electricity.

� List of Resources. The following resources may be useful in teaching this lesson:

Burkybile, Carl, and Stanley R. Burke. Modern Agricultural Mechanics, ThirdEdition. Upper Saddle River, NJ: Prentice Hall Interstate, 2006.

Johnson, Donald M., Joe Harper, David E. Lawver, and Philip Buriak.Mechanical Technology in Agriculture. Danville, IL: Interstate Publishers,Inc., 1998.

McKenzie, Bruce A., and Gerald L. Zachariah. Understanding and Using Elec-tricity. Danville, IL: Interstate Publishers, Inc., 1982.

Phipps, Lloyd J., Glen M. Miller, and Jasper S. Lee. Introduction to Agricul-tural Mechanics, Second Edition. Upper Saddle River, NJ: Prentice HallInterstate, 2004.

Phipps, Lloyd J., and Carl L. Reynolds. Mechanics in Agriculture. Danville, IL:Interstate Publishers, Inc., 1992.


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Richter, H.P., and W. Creighton Schwan. Wiring Simplified. Minneapolis, MN;Somerset, Wisconsin: Park Publishing, Inc., 1996.

Surbrook, Truman C., and Ray C. Mullin. Agricultural Electrification.Cincinnati, OH: South-Western Publishing Co., 1985.

VAS U3003c. Planning for Electrical Wiring. Urbana, IL: Vocational Agricul-ture Service.

� List of Equipment, Tools, Supplies, and Facilities

� Writing surface

� Overhead projector

� Transparencies from attached masters

� Copies of student lab sheet

� Short piece of insulated copper wire

� Solar calculator

� Terms. The following terms are presented in this lesson (shown in bold italics):

� Alternating current

� Atoms

� Battery

� Conductors

� Conventional theory

� Cycle

� Direct current

� Electricity

� Electrodes

� Electrolyte

� Electromagnet

� Electron theory

� Electrons

� Elements

� Free electrons

� Friction

� Fuel cell

� Hertz

� Insulators

� Magnetic induction

� Neutrons

� Photovoltaic effect

� Polarity

� Primary batteries


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� Protons

� Secondary batteries

� Semiconductors

� Solar (photo) cell

� Synchronous alternator

� Thermocouple

� Thermopile

� Valence

� Interest Approach. Use an interest approach that will prepare the students for thelesson. Teachers often develop approaches for their unique class and student situations. Apossible approach is included here.

Ask the students, “what is electricity, where does it come from, how does it getfrom its source to its place of use, etc.”? Allow students to offer some ideas.Guide students in the direction of the lesson’s objectives. Have students find outwhere electricity in their area is generated and how it gets to their home orschool. Relate to students a common experience of walking on carpet in thewintertime and receiving a “shock” when they touch someone or somethingmetal such as a doorknob. Explain that the shock is actually a transfer ofelectrons which is actually static electricity, from them to the object they touch.

SUMMARY OF CONTENT AND

TEACHING STRATEGIES

Objective 1: Relate electricity to the structure of elements and atoms.

Anticipated Problem: How does electricity relate to the structure of elements andatoms?

I. Elements are substances that cannot be broken down into simpler substances usingordinary chemical methods. All matter is composed of one or more elements. Atomsare the smallest units of an element. They cannot be divided further without losingphysical and chemical properties. Atoms are composed of protons, neutrons, andelectrons.

A. Protons have a positive electrical charge, while neutrons have no electricalcharge. Protons and neutrons make up the nucleus, or center of each atom. Thusthe nucleus of each atom has a positive electrical charge.

B. Electrons have a negative electrical charge and orbit the nucleus of an atom inrings or shells. Since unlike charges attract each other, the electrons are held in


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orbit by the positively charged nucleus. The outer ring of electrons is called thevalence ring. As an example, the element copper, has 29 protons in its nucleusand 29 electrons which orbit its nucleus. The electrical charges cancel each other.The electrons actually orbit the nucleus in rings. The first or inner ring can hold amaximum of 2 electrons, the second ring, a maximum of 8 electrons, the thirdring, a maximum of 18, the fourth ring, a maximum of 32, the fifth ring, amaximum of 50, and the sixth, a maximum of 72. Generally, the rings closest tothe nucleus are filled before additional rings are started. Thus, copper has 1electron in the fourth ring or valence ring (2 + 8 + 18 + 1). The number ofelectrons in the valence ring has a key role in determining the electricalcharacteristics of the element.

Show students a piece of copper wire. Explain that it is the most commonmaterial used to conduct electricity. Using the notes from above, explain whycopper is such a good conductor. Use TM–A to see a diagram of what theatomic structure of copper looks like. The bottom illustration on TM–A showshow the free electrons are able to move from one copper atom to another.

Objective 2: Explain conductors, insulators, and semiconductors.

Anticipated Problem: What are conductors, insulators, and semiconductors?

II. Conductors, insulators, and semiconductors are important in electricity andelectronics. The following will help explain each:

A. Conductors are materials that allow electricity to flow through them easily.Copper, aluminum, silver, and gold are good conductors. Generally, nearly allmetals are good electrical conductors. Any element or material having atoms withthree or fewer electrons in its valence ring will be a conductor.

B. Insulators are materials that do not allow electricity to flow through them undernormal conditions. Materials such as rubber, plastic, porcelain, and glass are allgood electrical insulators. Any element or material composed of atoms having fiveor more electrons in the valence ring will be an insulator. Insulators are used toconfine the flow of electricity to desired paths.

C. Semiconductors are materials that are neither good conductors nor goodinsulators. Semiconductors are manufactured from elements having atoms withfour electrons in their valence rings. Silicon and germanium are widely used inmaking semiconducting materials.

Ask students to identify several materials that they would classify as conductors.Ask them what properties make something a conductor. Then ask students toidentify several materials that they would classify as insulators. Ask them whatproperties make something an insulator. Use the notes above to relate thecharacteristics scientifically. Using notes above, discuss how semiconductors fitsomewhere in the middle in terms of characteristics.


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Objective 3: Explain the conventional and electron theories of electrical current flow.

Anticipated Problem: What are the conventional and electron theories of electricalcurrent flow?

III. Electricity is the flow of electrons from atom to atom in a conductor. There are twodifferent theories to help describe electricity:

A. The conventional theory says that electricity is the flow of positively chargedparticles through a conductor. This is the older theory and was developed beforescientists discovered the existence of electrons. With this theory, it was assumedthat current flow in an electrical circuit was from positive to negative.

B. The electron theory is the accepted model of electrical current flow, however,because of tradition, the conventional theory is still widely used. Either theory maybe used as long as it is used consistently. The electron theory says thatelectricity is the flow of electrons through a conductor. Remember conductorshave three or fewer electrons in their valence ring. When there are so fewelectrons in the valence ring they are not held very tightly. These free electronscan be dislodged if sufficient external force is applied. An example of externalforce is a battery, which has a positive terminal and a negative terminal. Sinceunlike charges attract each other, if a copper wire were attached to the twoterminals, the negatively charged free electrons would be dislodged and pushed bythe negative terminal and pulled by the positive terminal of the battery, causingthe electrons to flow. This would continue until the battery discharges.

Explain how the flow of electricity is actually the flow of electrons from one atomto the next. Students need to understand that some external force actuallycauses the flow to occur. Using the notes above, explain the difference betweenthe conventional theory and the electron theory. Use TM–B to illustrate thedifference in the two theories. Students should know from previous scienceclasses that like charges repel and unlike charges attract. Apply this concept toexplain why the electron theory is the preferred theory in understandingelectricity.

Objective 4: Describe how electricity can be generated by friction, heat, light, chemi-cal reactions, and magnetism.

Anticipated Problem: How can electricity be generated by friction, heat, light, chemicalreactions, and magnetism?

IV. An external force must be applied to cause free electrons to flow through aconductor. This force is the production or generation of electricity, which can begenerated in the following ways:

A. Friction is caused when two or more materials rub against each other. When thishappens, some of the free electrons from one material are transferred to the


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other material, causing one of the materials to have a positive electrical chargeand the other to have a negative electrical charge. When the negatively chargedmaterial touches a material with a neutral or positive charge, the excess electronswill flow to the second object. This can be demonstrated when you walk acrosscarpet in the wintertime. As you walk across the carpet, the soles of your shoesrub over the carpet which transfers electrons from the carpet to your body. Whenyou touch someone else or a doorknob, the excess electrons discharge throughyour hands resulting in a static electricity shock. Friction is not a practical methodof generating electricity.

B. Heat or temperature differences can be used to generate electricity using athermocouple. A thermocouple consists of two dissimilar metals, such as ironand nickel, joined together to form two junctions. When heat is applied to one ofthe junctions, the difference in temperature between the junctions causes freeelectrons to flow from the iron wire into the nickel wire and away from the hotjunction toward the cold junction. To increase output, several thermocouples areoften combined to form a device called a thermopile. These are often used inflame detectors, furnace safety valves, and in precision heat measurementdevices.

C. Light from the sun can also be used to generate electricity. It requires a solar (orphoto) cell for converting the light into electricity through a process known as thephotovoltaic effect. Certain materials (such as gallium, silicon, and cadmiumsulfide) will convert light energy into electrical energy through the photovoltaiceffect. The solar cell is made of a thin disk of silicon to which other chemicalshave been added. When light strikes the disk, electrons move from one side to theother side. The electrons move through the conductors and provide electricalenergy to power the circuit load(s). Solar energy is used to power such things ascalculators and electric fence controllers.

D. Reactions between certain chemicals can be used to produce electricity. First, abattery is made of two or more chemical cells connected together. Each cell iscomposed of two dissimilar metal plates called electrodes. They are separatedfrom each other and immersed in an electrolyte. An electrolyte is a chemicalsolution that contains positively and negatively charged atoms called ions. Using achemical cell that is composed of a zinc plate, a copper plate, and an electrolytesolution of sulfuric acid and water as an example, the acid/water solution reactswith the zinc plate causing it to lose positively charged ions. Each positive ion lostleaves behind two electrons. Thus, the zinc plate soon takes on a negative charge.It then becomes the negative terminal of the cell. As the positive ions movethrough the electrolyte, they collide with the copper plate. The positive ions attractfree electrons from the copper plate. Thus, the copper plate soon takes on apositive charge due to this loss of electrons and becomes the positive terminal ofthe cell. If we connect a conductor to the two terminals, electrons will flow throughthe conductor from the zinc plate to the copper plate. This flow will continue untilthe difference in charge between the two plates has dissipated. Once thishappens, the cell will be discharged. Batteries may be classified as primarybatteries (carbon-zinc, alkaline, and mercury) which cannot be recharged or as


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secondary batteries (lead-acid and nickel-cadmium) which can be recharged. Asecond chemical reaction to produce electricity is a fuel cell, which is similar to abattery, but different in the way the chemicals are supplied. In a battery, thechemicals are built in. In a fuel cell, the chemicals are pumped into the cell froman external source. Currently, fuel cells are limited to military and spaceapplications.

E. The final method of generating electricity discussed here will be that ofmagnetism. If a bar magnet is suspended freely from a string, the magnet will turnuntil one end points north and the other end points south. The end pointing northis said to be the north pole of the magnet and the end pointing south the southpole. An electromagnet may also be created using electricity. If electricity flowsthrough an insulated conductor that is wound around a metal object, the metalobject will become a magnet. Any time a conductor cuts across a magnetic fieldor a magnetic field cuts across a conductor, electricity will flow in the conductor.Electricity generated in this manner is produced through the process of magneticinduction. This would be a very impractical method of generating electricity.Electricity could also be generated by rotating a magnetic field around a stationaryconductor or by rotating a conductor inside a stationary magnetic field. As therotating conductor cuts across the lines of magnetism in the stationary magneticfield, electrons will be forced to flow through the conductor. As the conductorcontinues to rotate, the conductor will travel parallel to the lines of magnetismand no electrons will flow. As the conductor continues in its rotation, it will cutacross the lines of magnetism in the opposite direction, causing the electrons inthe conductor to flow in the opposite direction. The cycle continues as the rotationcontinues. In a commercial electricity generation power plant, a synchronousalternator is used to produce electricity. This device has a rotating field windingand a stationary winding. A small amount of electricity is supplied to thealternator’s field windings, which produces a magnetic field around the windings(the field windings become an electromagnet). As the field windings are turned (bya turbine driven by an external power source), the magnetic field also turns,cutting across the alternator’s stationary windings. Since the conductors arecutting across a magnetic field, electricity is induced into the stationaryconductors. Power plants may burn coal or use nuclear energy to make steamfrom heated water in order to turn the turbine, or they may use the kinetic energyof falling water to turn the turbine.

Use the notes above to outline the various methods of generating electricity.Recall the example of static electricity to help explain friction. Use TM–C toillustrate temperature differences. Have students look at a solar calculator toobserve that light can be used to generate electricity to power calculators. UseTM–D to illustrate electricity generated from chemical reactions. You mightrelate the use of batteries in the students’ vehicles to this method. Use TM–E toillustrate the generation of electricity through magnetism. Students can use thesuggested resources to aid in understanding the content of the objective.


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Objective 5: Describe the difference between and applications of direct current (DC)and alternating current (AC) electricity.

Anticipated Problem: What is the difference between direct current and alternatingcurrent and what are some applications of each?

V. Electricity may be classified as direct current or alternating current depending on thepattern of flow of the electrons in the circuit.

A. In direct current or DC electricity, the electrons flow in only one direction. Sinceelectrons flow from the negative terminal to the positive terminal in an electricalcircuit, sources of DC electricity must have a fixed polarity. This means onespecific terminal is always negative while the other is always positive.Thermocouples and thermopiles, solar cells, batteries and fuel cells all produceDC electricity. A DC generator may also be used to produce DC electricity throughmagnetism.

B. With alternating current or AC, electricity flows first in one direction, stops,reverses and flows in the opposite direction. Once this occurs, the electricity issaid to have completed one cycle. AC electricity is the type generated by electricpower plants and is what is used in homes, businesses, and other locations. Onecycle of electrical flow is produced with each revolution of the plant’s synchronousalternator. In the U.S., they turn at a speed of 60 revolutions per second.Therefore, current generated in the U.S. completes 60 cycles per second. Theterm hertz (Hz) represents one cycle per second, so in the U.S. our electricity isgenerated and delivered at 60 Hz. In several other countries, 50 Hz AC electricityis the standard.

Use the notes above to explain the difference between DC and AC current.Students can use the suggested resources to aid in further understanding of thecontent of the objective. Have students complete LS–A as worksheet to betterunderstand the science of electricity.

� Review/Summary. Use the student learning objectives to summarize the lesson.Have students explain the content associated with each objective. Student responses canbe used in determining which objectives need to be reviewed or taught from a differentperspective. Questions at end of chapters in the textbook may also be used in thereview/summary.

� Application. Application can involve the following student activities using attached labsheet:

� LS–A: Exploring the Science of Electricity—Worksheet


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� Evaluation. Evaluation should focus on student achievement of the objectives for thelesson. Various techniques can be used, such as student performance on the applicationactivities. A sample written test is attached.

� Answers to Sample Test:

Part One: Matching

1=b, 2=e, 3=h, 4=f, 5=c, 6=g, 7=a, and 8=d

Part Two: Completion

1. secondary, primary

2. electromagnet

3. hertz

4. protons, neutrons

Part Three: Short Answer

1. Use the notes or TM–B as a guide for scoring.

2. With direct current, electron flow is in one constant direction, whereas in alternatingcurrent, electrons flow in one direction and then reverse and flow in the oppositedirection 60 times per second.


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Sample TestName ________________________________________


� Part One: Matching

Instructions. Match the term with the correct response. Write the letter of the term by the defini-tion.

a. electrons e. electricityb. solar cell f. electrodec. elements g. electrolyted. semiconductors h. thermocouple

_____1. Used for converting light into electricity through a process known as photovoltaic effect.

_____2. The flow of electrons from atom to atom in a conductor.

_____3. The joining of two dissimilar metals, such as iron and nickel, to generate electricity.

_____4. The name given for two dissimilar metal plates within a cell of a battery used ingenerating electricity.

_____5. Substances that cannot be broken down into simpler substances using ordinarychemical methods.

_____6. A chemical solution that contains positively and negatively charged atoms called ions.

_____7. Contain a negative electrical charge and orbit the nucleus of an atom in rings or shells.

_____8. Materials that are neither good conductors nor good insulators.

Part Two: Completion

Instructions. Provide the word or words to complete the following statements.

1. ____________________ batteries are made of lead-acid or nickel-cadmium and can berecharged, while _____________________ batteries are made of carbon-zinc or alkaline andcannot be recharged.

2. An ________________________ is created using electricity. It is the result of electricityflowing through an insulated conductor that is wound around a metal object.

3. The term _______________________ is used to represent one cycle of electricity per second.

4. The nucleus of an atom contains _____________________ which have a positive electricalcharge and ______________________ which have no electrical charge.


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� Part Three: Short Answer

Instructions. Provide information to answer the following questions.

1. Contrast the conventional theory vs. the electron theory of electricity.

2. Explain the difference between direct current and alternating current.


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TM–A

COPPER AS A CONDUCTOR

Atomic structure of copper.

Loosely attached free electrons which movefreely from one atom to another.


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+29

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TM–B

ELECTRICALCURRENT FLOW

The direction of current flow according to the conventional theory.

The direction of current flow according to the electron theory.


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D-Cell – 1.5 V

+–

D-Cell – 1.5 V

+–

TM–C

GENERATION OF ELECTRICITY

THROUGH TEMPERATURE

DIFFERENCES

A thermocouple consists of two dissimilar metals (for example,iron and nickel) joined in two junctions.


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Voltmeter

Ni wire

Fe wire

Cold

Fe-Ni junction

Hot

Fe-Ni junction

Ni wire

Ice water

Candle

TM–D

CHEMICAL CELL FORPRODUCING ELECTRICITY


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Zincplate (–)

Copperplate (+)

Conductors

Ammeter

Electrolyte(sulfuric acidand water)

TM–E

GENERATION OF ELECTRICITY

THROUGH MAGNETISM

A horseshoe magnet, conductor, and light bulb usedto illustrate the process of magnetic induction.

Electricity is generated by rotating a conductorwithin a stationary magnetic field.


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N S N SN S

A B C

A B

DC

N

NN

NS S

S S

LS–AName ________________________________________


Purpose:

Students will enhance their understanding of the science of electricity by using text materialsto answer the following questions.

Fill-in-the-Blanks:

1. ____________________ and _____________________ are elements used in makingsemiconducting materials.

2. Copper contains one _____________ electron in its outer shell making it a very goodconductor. Materials that contain five or more electrons in their outer shell or valencering hold relatively tightly to those electrons, making them act as insulators. Threeexamples of insulators are ___________________, ___________________, and___________________.

3. To increase output, several thermocouples are often combined to form a device called a___________________________, which may be used as flame detectors or furnace safetyvalves.

4. Certain materials (such as gallium, silicon, and cadmium sulfide) will convert light energyinto electrical energy through the ______________________ ___________________.

5. Any time a conductor cuts across a magnetic field (or a magnetic field cuts across aconductor), ____________________ will flow in the conductor.

6. In an electrical power plant, a _____________________ ______________________ is usedto produce electricity.

7. In a _________________________ power plant, the kinetic energy of falling water turnsthe turbine.

8. In the United States, _____________ Hz electricity is generated and delivered toelectricity consumers.

9. _______________________ and ________________________ are the two most commontypes of conductors used in electrical circuits.

10. Electricity produced by friction is often referred to as ____________________ electricity.This phenomena is not a practical method of generating electricity.


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LS KEY–A


Answers:

1. Silicon, germanium

2. free, rubber, plastic, glass

3. thermopile

4. photovoltaic effect

5. electricity

6. synchronous alternator

7. hydroelectric

8. 60

9. Copper, aluminum

10. static


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Exploring the Science of Electricity

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