|Physical Science||Motion and Forces 8B1.00||Process Of Science||Observing 1.1 ab|
CONTENT STANDARD: Physical Science
CONTENT TOPIC: Motion and Forces
CONCEPT: Magnetism is a form of energy because its forces can do work.
CONTENT OBJECTIVES: 81.00 To understand how magnetism exerts force on objects.
INSTRUCTIONAL OBJECTIVES: The learner will:
TN COMPONENT OF SCIENCE: Process Of Science
BENCHMARK: By incorporating prior knowledge with the process of observations a better understanding of one's environment may develop.
1.1b The human senses and technological instruments are used to gather information from the environment.
BENCHMARK: Scientific investigation is enhanced through the use of technology.
The nail, the steel, and the iron filings were all made to act like magnets. When rubbed with the magnet, the particles that made up these objects lined up in a certain way. The north magnetic poles of each particle point in one direction and the south magnetic poles point in the opposite direction. When the particles are all lined up, the nail, the steel, and the iron filings all formed a magnet. When you shook the test tube and disturbed the particles you no longer had a magnet. The iron will soon lose most of its magnetism but the steel will hold its magnetism longer. Try another experiment to help you see that a magnet is made of magnetized parts. (Have students do Activity Two under Active Participation.)
The law of magnets states that like poles repel each other and unlike poles attract each other. We found the same to be true of electric charges. A magnet exerts a force that we cannot see. A force is a push or pull. It is this force that pulls opposite poles of magnets together and pushes like poles apart. (Have students do Activity Three under Active Participation.)
The lines that the iron filings follow are called the lines of force. We can't see the lines of force - they are invisible around the magnet. If we could see them they would show us the strength and direction of the magnet's force. The area around the magnet where the force acts is its magnetic field.
Look at your drawing. The lines follow the lines of force around the magnet. There are lots of lines leaving or entering the ends, or poles, but very few lines are near the center of the magnet. At the beginning of this lesson we tried to pick up objects with the center of a bar magnet. It did not have much pull. This is because the tiny particles in the magnet are all in a line. The north ends of each particle point in the same direction and the south ends point in the opposite direction. The needle of a compass is a magnet. The needle always points approximately north and south. (Have students do Activity Four under Active Participation.)
You found out by these experiments that a compass is simply a magnet suspended so that it is free to turn on a pivot. And when you suspended a bar magnet or floated a magnetized needle they acted exactly as the compass needle. In both cases, the magnets arranged themselves in a north-south position. Some force we cannot see affects the magnets. It makes them take a definite position. This force seems to come from the earth. The earth is like a huge magnet. It has north and south magnetic poles which are strong enough to attract compass needles. A magnetic field surrounds the earth. A compass points in the same direction as the lines of force in the earth's magnetic field. Scientists have found that magnetism and electricity are related. We learned in our study of electricity that magnets can be used to generate electricity. An electric current can also be used to produce magnetism. A wire carrying an electric current has magnetic properties. (Have students do Activity Five under Active Participation.)
The wire carrying the electric current has magnetic properties: it picked up the iron filings and since the iron filings formed circles around the wire you knew there were lines of force around the wire. A single wire does not have very much magnetic force. In order to increase the force you would need to use more wire. Winding a wire in a coil brings all the lines of force closer together. It is like having many wires side by side. In this way it is possible to make a much stronger magnet. (Have student do Activity Six under Active Participation.)
You found that a coil of wire carrying electricity can do all the things a permanent magnet can do. However, an electric wire coiled around an iron core makes a much stronger magnet. We call this kind of magnet an electromagnet. Electromagnets are very useful because their magnetism can be turned on and off. You can also control the strength of an electromagnet while you can't control the magnetic force of a permanent magnet. You increased the strength of your electromagnet by increasing the number of turns of wire in the coil or by increasing the current.
The discovery of the electromagnet is one of the most important man has made. They play a vital role in our lives. They are used in many of our machines. They can be used to produce motion. Electromagnets are used in electric motors which run many of our appliances such as irons, clocks, refrigerators, and dishwashers. Every electric doorbell contains an electromagnet as do our telephones and telegraphs. Electromagnets are used to generate electricity. Most of the electricity we use today is produced by large generators that use powerful electromagnets. A strong electromagnet is also part of a crane. The iron core will pick up magnetic material when the current is on. This makes it possible to lift heavy loads of iron and move them from one place to another. The load can be dumped when the electricity is stopped. Automatic circuit breakers are replacing fuses. In this device, a magnetic coil is used to turn off a switch when too much current flows. The whole electrical industry is dependent on the electromagnet and all its possibilities have not yet been discovered.
B. Have students fill the test tube or small jar half full of iron filings. Put the cork or lid on. Hold the glass horizontally. Rub the glass ten times in the same direction with one end of a magnet. Watch the iron filings. Do not jar the test tube. Bring a compass near one end of the test tube. What do you observe? (response) Did the iron filings act like a magnet? (yes) Now shake the glass of iron filings and bring the compass close again. Did the iron filings act like a magnet? (No)
2. Working in small groups have students magnetize a straight piece of iron wire (or you can use a nail but it will be harder to cut). Rub the iron ten times in the same direction with one end of a magnet. Test it to see if you have a weak magnet. Find out which end is north by bringing it near a compass. Now cut the iron in half with pliers or wire cutters. Test each half with the compass. Does each half act the same as the whole piece? (yes) Test each half's magnetism. (Each piece should be a magnet but not as strong as the whole piece.) Why did the iron behave like this? (because all the tiny particles that make up the iron are lined up in the same direction so each piece will be a magnet and each will have a north and south pole.)
3. Working in small groups have students put two bar magnets on a table with the north pole of one about an inch from the south pole of the other. Put a stiff paper (about ten inches square) over the magnets. Sprinkle a thin layer of iron filings on the paper. Tap the paper gently. Do the iron filings arrange themselves in lines? (yes) Now very carefully draw pencil lines along the lines of the iron filings. These lines represent the forces of the magnets. Did your lines go from pole to pole? (Yes - the lines of force went between the two magnets; they seemed to join the two magnets together.) Now repeat the experiment but put the north poles of the magnets about an inch apart. Put another piece of paper over the magnets and sprinkle with iron filings. Very carefully draw pencil lines. Compare your line drawing with the first one you did. How are they different? (The lines of force did not go from pole to pole - the lines from one pole seemed to lead away from the lines from the other pole.) Notice the lines on both drawings that go from the North Pole to the South Pole in each magnet. On the individual magnet where are the lines the fewest? (In the center of the magnet.) What does this mean? (The center of the magnet does not have as much force or pull.)
4. Examine the direction a compass needle points:
B. Tie a piece of string around the center of a bar magnet. Hold the ends of the string by your fingers so that the magnet is suspended and evenly balanced. Put a compass on the table and when the bar magnet is perfectly still check the direction it points with the compass. (The magnet will align itself in a north-south position.)
C. Magnetize a sewing or darning needle by rubbing it with a magnet. Fill a glass with water and float a cork on the water. Carefully lay the magnetized needle across the cork so that it is balanced. When they are still, check the direction the needle is pointing with a compass. (Be sure to keep the compass well away from the needle.)
5. Have students work in small groups. Give each group a dry cell, eight to twelve inches of copper wire, and iron filings. Connect only one end of the wire to the dry cell. Hold the wire near the iron filings. What happened? (Nothing) Now connect the other end of the wire to the dry cell and hold the wire near the iron filings. What happened now? (A wire carrying current picks up iron filings.) Observe how the iron filings act on the wire. (The iron filings form circles around the wire.) Why? (Because there are lines of force around the wire) Disconnect one wire so that the iron filings drop off. Now reconnect it and test the strength of your magnet by seeing how many paper clips or nails it will pick up.
6. Give the students a much longer length of copper wire and a large iron nail. Wind about twenty turns of wire around the nail leaving about the same length sticking out at each end of the nail. Connect these two ends of wire to the dry cell. You now have an electromagnet. Touch one end of the nail to some clips. What happens? Now take one end of the wire off the dry cell and watch your paper clips on the nail. (Electromagnets attract objects only when electricity is running through them.) Next wind forty turns of wire around a nail. Can it pick up more paper clips? (Yes) Now connect this same nail and wire to two dry cells. How many more paper clips can it pick up? (response) In what ways did you increase the strength of your electromagnet? (You increased the number of turns of wire and you increased the amount of electric current.)
electromagnet - a magnet made by passing an electric current through coils of wire wound around an iron core
lines of force - the imaginary lines around a magnet that represents the strength and direction of the magnetic force
magnetic field - the area around a magnet where its force acts
magnetism - a force of attraction that a magnet has
poles - the place on a magnet where the force is the strongest
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
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