GRADE: Eighth

CONTENT STANDARD: Physical Science

CONTENT TOPIC: Transformation of Energy

CONCEPT: Electricity and magnetism are related forms of energy.

CONTENT OBJECTIVES: 8C1.00 To understand how electricity and magnetism are produced, measured and used

INSTRUCTIONAL OBJECTIVES: The learner will:

1.01 define these terms: electric energy, resistance, armature, turbine, ampere, ohms, volt, watt, kilowatt-hour.
1.02 explain how electricity is produced in relation to the atom.
1.03 examine what causes electric current to flow and what determines how much current flows through a circuit.
1.04 compare and contrast alternating current and direct current.
1.05 analyze the ways electric current is measured and the instruments used to measure it.
1.06 breakdown the structure of a dry cell and an electric generator.
1.07 generalize the vast number of ways electricity is used.

I. Electricity
A. How electricity is produced
B. How electricity is measured
C. How electricity is used

TN COMPONENT OF SCIENCE: Unifying Concepts of Science

TN GOAL:

To enable students to acquire scientific knowledge by applying concepts, theories, principles and laws from life/environmental, physical, and earth/space science.

2.2 FORM AND FUNCTION - Form may determine the function of a material or a system, and function may alter form.

2.2a How an object functions is related to its form.

BENCHMARK: There are different forms of energy such as heat, mechanical, chemical, electrical, nuclear, and light.

TIME REQUIRED:

Three to five instructional periods

Articles made of plastic (combs, rulers, pens), new 1.5 volt dry cells, old dry cells, flashlight, several battery operated appliances (radios, small toys), strips of copper, strips of zinc, a roll of copper bell wire, lemons, voltmeter, ammeter, galvanometer, small box, compass, hollow cardboard tube, bar magnets, horseshoe magnet, 2-inch iron bolt with nut and two washers, pliers, flashlight bulbs with socket

Ampere, armature, current electricity, electric energy, electromagnet, kilowatt-hour, lines of force, magnetic field, magnetism, ohms, resistance, static electricity, turbine, volt, watt

Electricity is a form of energy, similar to magnetism, that occurs everywhere. Electricity must be produced. Some electricity is made by friction. (Provide students with plastic articles: comb, ruler, fountain pen, pencil.) Rub your plastic article briskly against your hair. Bring it near a pile of tiny pieces of torn-up paper. When two different materials rub together, they may acquire static electricity. Observe how the paper is attracted to the plastic. Some electricity is produced by nature during a thunderstorm. A small amount of electricity is also made by chemical action, as in dry cells and storage batteries (Provide dry cells for students to insert in various battery-operated things like radios, flashlights, small toys.) However, most of the electricity we use today is produced by generators which are turned by turbines. Our study of electricity will help you understand how it is produced, how we measure electricity and how it is used in many different ways.

You remember that matter is made up of very small particles called atoms. Atoms are made up of particles, three of which are protons, neutrons, and electrons. Electrons have a negative electric charge and move very rapidly around the nucleus of an atom. Ordinarily the negative electric charge of the electrons is equal to the positive electric charge of the protons found in the nucleus. If the number of electrons and protons are equal, we say the atom is balanced. If this charge is disturbed, the electrons will move from one atom to another and the atom becomes unbalanced or charged. This movement, or flow, of electrons is an electric current. Everything has electrons in it but they are not always flowing. An electric current flows only when a force makes the electrons move in a stream. To make an electric current flow, you need a source of electrical energy. Sometimes this source is a dry cell or battery. Dry cells change the chemical energy stored in them into electrical energy. However, most of the electrical energy needed today is supplied by generators.

You have already learned there are two kinds of electricity: static and current. Static electricity is electricity that is not moving. It is produced by friction. When you rub your shoes on a rug and then touch something, you feel a shock. (Have a student demonstrate this.) This was caused by a flow of electrons. The electrons flowed from your shoes through your body into the object you touched. Rubbing a comb through your hair, or with a piece of wool, may generate static electricity. When electrons jump you may see a spark and hear a crackling noise. Electricity was first produced by friction but the amount that can be produced this way is usually small and man needed much larger amounts.

Current electricity, a flow of electrons through a conductor, provides this for us. Two kinds of electric current are used to operate appliances. One kind is direct current (D.C.). This kind always flows in the same direction through the wire and is supplied by dry cells. The other kind is called alternating current (A.C.). Alternating current flows first in one direction and then in the other, changing its direction many times every second. This kind of current is supplied by generators. Let us find out how electricity is produced by chemical action in a dry cell.

Inside a dry cell, a strip of zinc is made into the shape of a small can. This forms the outside part of the dry cell and becomes the negative terminal. There is a carbon rod in the middle which is connected to the brass cap on top. This is the positive terminal. Inside the can is a paste like mixture of manganese dioxide and powdered carbon which has been soaked in ammonium chloride. Blotting paper which has been soaked in ammonium chloride lines the zinc can to prevent bubbles from forming. A chemical reaction between the acid paste and the zinc releases electrons. The flow of electrons to the carbon rod makes an electric current. Dry cells should not be used continuously. They are best used where small amounts of current are needed for short periods of time. (Have students do Activity One, Two and Three under Active Participation.)

Now let us find out how power companies make electric current from generators. Magnets are used inside the generator. Magnets have strong magnetic fields around them. You know this is so because they can pick up pieces of iron. The magnetic field is filled with lines of force. If you move a coil of wire through these lines of force, electrons will flow in the wires. An electric current is produced. (Have students do Activity Four and Five under Active Participation.)

Power plants have one or more large generators, sometimes called dynamos. The power in a generator depends on the number of lines of force cut each second. Electromagnets are used because they can be made stronger. Modern generators have a large number of electromagnets. The electromagnets surround a moveable iron core wrapped in coils of wire. This is called the armature and when it is turned rapidly, an enormous number of lines of force will be cut each second. Therefore, a great amount of electricity will be generated. Power is needed to turn the generator. Motion is the key to generating electricity. Most generators today are driven by huge turbines, a kind of wheel with blades. The blades of the turbine can be turned with moving air or moving water, or steam. Most of the steam comes from water heated by coal, oil, gas and nuclear energy. (If at all possible, have a small, hand generator in class. Have students turn the crank to produce an electric current).

In some ways, an electric current flowing through a circuit acts much like water flowing through pipes. To measure how much current is flowing through a circuit, we use a unit called an ampere. A sixty watt light bulb uses about one-half ampere. A current of about five amperes flows through an electric toaster while about ten amperes flows through an electric iron. An instrument called an ammeter measures the flow of electrons past a certain point per second. When a current of one ampere is flowing, electrons are going by at the rate of about six billion per second. How much current is flowing through a circuit will also depend on the amount of opposition that the current meets in flowing around the entire circuit. This opposition to the flow of current is called resistance. Every electric current meets some resistance as it flows, some friction that holds the moving electrons back. Resistance is calculated in units called ohms. Wires made of copper, wires which are larger and wires which are shorter have a low resistance and let a large current flow. If the wire is small or long or made of iron, it has a higher resistance and lets a smaller current flow. You calculate the resistance by dividing the number of volts by the number of amps.

How much current flows through a circuit also depends on how much pressure is needed to push the electrons through the circuit. A volt is a measure of electrical pressure. The higher the voltage the more current flows through the wire. A pressure of one volt makes a current of one ampere flow through a resistance of one ohm. A new dry cell produces about 1.5 volts. A car storage battery gives about six volts. In a household circuit, the pressure is usually 115 to 120 volts. To measure voltage we use an instrument called a voltmeter. The rate of using energy or doing work is called power. A unit called a watt is used to measure the power of an electric current. A light bulb may be marked 60 watts, a toaster, 600 watts and an iron, 1200 watts. The power in watts equals the pressure in volts multiplied by the current in amperes (watts = volts X amperes). Watts tells you how fast electrical appliances use energy. Electrical energy is measured in a unit called a watt-hour, which is the amount of energy used at the rate of one watt for one hour. Since this is a rather small amount we ordinarily use a larger unit called a kilowatt-hour, which is equal to 1,000 watt-hours. On the outside of your home near the main switch or fuse box, you will find a kilowatt-hour meter.

This measures and records the number of kilowatt-hours of electrical energy that are used. All the current in the house must flow through the meter. Once a month, a man from the power company reads the meter. He writes down the reading. The power company subtracts last month's reading from this month's reading to get the number of kilowatt-hours you used.

How many times a day do you use electricity? It is unbelievable how we've come to depend on this energy. Most of our homes are lighted with incandescent lights. Electric current passes through a thin wire which becomes red hot and gives off light. Fluorescent bulbs give off light when electric current is carried through mercury vapor in the tube. Electricity makes heat for the home. It is used to cook our food. Make a list of the appliances which use electricity by passing the current through coils of wire or some kind of metal (toasters, irons, water heaters, hair dryers, clothes dryers, electric blankets, etc.). Electric current is used to operate motors. Motors are necessary to operate many of the appliances we use everyday as well as power tools. Electricity is used in communications the telephone, radio, television, tape recorders, video cameras.

1. Have the students in groups. Give each group an old dry cell from which you have already removed the outer covering and then cut in half with a saw. Have students observe the positive pole in the center (carbon rod). Observe the negative pole (the outside zinc container). Between the two poles is the chemical that acts on the plate in the cell. Notice how the chemical has eaten away the zinc.

2. Have students work in groups to demonstrate how to make electricity by chemical action. Give each group a strip of copper, a strip of zinc (you can cut the zinc from the can of a used dry cell), 2 short lengths of copper bell wire and a lemon. Have students puncture one end of the copper strip and one end of the zinc strip. Connect one end of the copper wire through each hole. Roll the lemon on a table several times to loosen the juice inside. Cut two slits in the skin of the lemon and push the copper strip into one slit and the zinc strip into the other slit. Now attach the two free ends of the copper wire to a low voltage voltmeter or a galvanometer. Observe the reading. (If a voltmeter or galvanometer is not available, have students make and use the sensitive meter below.)

3. Have students work in groups to make a sensitive meter, a simple instrument to show electric currents. Have a small box just large enough to hold a compass. Place the compass in the bottom of the box with no lid. Wind twenty to thirty turns of bell wire around and around the center of the box over the compass with the compass needle pointing parallel with the wire. Now connect the free ends of the wire to a dry cell and observe the compass needle. (You can attach the free ends to the hole in the copper and zinc strip inserted in the lemon and observe the compass.)

4. You may have students work in large groups or allow students to do this in front of the whole class. Wind about 50 turns of insulated bell wire around a hollow tube, such as a mailing tube or paper towel tube. Slip the coil from the tube. Connect each end of the wire to the posts of a galvanometer (or you could use your sensitive meter). Hold a bar magnet in one hand and push it through the center of the coil of wire. Have students observe the galvanometer. Now pull the magnet out. Observe the galvanometer. Move the magnet rapidly. Now hold the magnet still and have a student move the wire over it. Watch the galvanometer needle. Is it moving the same as before when you moved the magnet? Take a large horseshoe magnet and wave it back and forth on both sides of the coil. Observe the galvanometer. What is your conclusion about how electric current is made? (When a magnet is moved inside a coil of wire, an electric current is generated and you can generate more current if you move the magnet or coil faster, if you use a stronger magnet or if you use more coils of wire.)

5. When we want a very strong magnet we use an electromagnet, a magnet powered by electric energy. Have students work in groups. Give each group an iron bolt about 2 inches long which has a nut and two washers. Place a washer at each end and screw the nut on to the bolt a short way. Leaving about twelve inches of wire sticking out, wind several layers of bell wire on the bolt between the washers. Cut the wire (leaving about 12 inches sticking out at the end). Wind some tape at the ends of the bolt to keep the wire from unwinding. Now connect the two free ends of the wire to a strong dry cell. Touch one end of your electromagnet to paper clips, tacks, and other iron objects. Experiment with fewer coils of wire, or more dry cells to determine that the strength of electromagnets are increased two ways. (Increase number of coils of wire, increase electric current)

6. Remember, to measure electric current you consider three things: the pressure on the electrons (measured in volts with a voltmeter), the actual number of electrons that flow through the circuit (measured in amperes with an ammeter) and the resistance to the movement of the electrons (not measured but calculated in ohms).

A. If you have available a voltmeter, have students attach a short piece of copper wire to each post of the voltmeter. Connect the wire from the positive post to the negative post of a 1.5 volt dry cell. Connect the wire from the negative post to the positive post of the dry cell. Does the voltmeter show 1.5 volts? Try connecting another dry cell in a series circuit. Did the voltmeter show 3.0 volts?

B. If you have available an ammeter have students connect a copper wire to the negative post of the ammeter and then to the positive post of a dry cell. Attach another copper wire from the negative post of the dry cell to a flashlight bulb with socket. Attach another piece of copper wire from flashlight bulb to the positive post of the ammeter. (The ammeter must be placed in the circuit - be sure students connect it correctly.) What does the ammeter show? (Reading of one ampere). Connect another bulb in the circuit. What does the ammeter show? (A drop in amps because an added bulb added resistance.)

C. (Have students find the plate on the back or bottom of various appliances and record the volts, amps, watts stamped there. Have students calculate as many problems as necessary to understand measuring electricity. Have them write and use these formulas: 0 = V/A; A = V/O; V = AO; P = VA; E = PT. Remember that O-ohms, resistance; A-amperes, current; V for volts, pressure; P for power, watts and kilowatts; E for energy; and T for time.)

Example problems:

1. A light bulb uses 1.2 amps of current and 120 volts. What is the resistance? (100 ohms)

2. A toaster has a resistance of 10 ohms and uses 220 volts. How much current flows through the toaster? (22 amps)

3. A car headlight uses 4 amps of current in a 12-volt circuit. Find the resistance. (3 ohms)

4. How much power is used by an electric dryer in a 120-volt circuit using 30 amps of current? (3600 watts)

5. How much energy is used to run the 3600 watt electric dryer for 2 hours? (7,200 watts or 7.2 kilowatt hours) If the electric company charges you five cents a kilowatt-hour, how much will the dryer cost to run? (7.2 X $.05 = $.36)

You have learned that everything has electricity in it because everything is made of atoms. Static electricity is produced by friction but to be useful to us electricity must be made to flow through a conductor. We call this flowing current electricity. Current electricity can be produced by chemical action, as in dry cells, and by generators. Electricity is so important to us we cannot imagine life without it. Lighting and heating, cooking our food, our transportation and communication, our entertainment - all depend on this marvelous source of energy.

ampere - unit used to measure the amount of current flowing through a circuit; it is measured by an instrument called an ammeter

armature - the movable part of a generator consisting of coils of wire around a soft iron core

current electricity - the flow of electrons through a conductor

electricity or electric energy - a form of energy that causes electric current to flow

kilowatt-hour - unit used to measure electric energy; it is measured by a kilowatt-hour meter

ohms - a unit used to measure the amount of resistance in a circuit

resistance - opposition to the flow of electrons

static electricity - electricity that is not moving

turbine - a kind of wheel with blades that are turned by water, air, or steam; it is used to spin the armature inside a generator

volt - unit used to measure the amount of electrical pressure in a circuit; it is measured by an instrument called a voltmeter

watt - unit used to measure the power of an electric current

This is the time this file has been accessed since 04/04/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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