* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Developer Notes
Friction-plate electromagnetic couplings wikipedia , lookup
Edward Sabine wikipedia , lookup
Mathematical descriptions of the electromagnetic field wikipedia , lookup
Magnetic stripe card wikipedia , lookup
Giant magnetoresistance wikipedia , lookup
Neutron magnetic moment wikipedia , lookup
Magnetometer wikipedia , lookup
Electromagnetism wikipedia , lookup
Magnetic monopole wikipedia , lookup
Electric machine wikipedia , lookup
Electromagnetic field wikipedia , lookup
Lorentz force wikipedia , lookup
Earth's magnetic field wikipedia , lookup
Magnetotactic bacteria wikipedia , lookup
Magnetic field wikipedia , lookup
Magnetohydrodynamics wikipedia , lookup
Magnetotellurics wikipedia , lookup
Multiferroics wikipedia , lookup
Magnetochemistry wikipedia , lookup
Magnetoreception wikipedia , lookup
Electromagnet wikipedia , lookup
Eddy current wikipedia , lookup
Ferromagnetism wikipedia , lookup
History of geomagnetism wikipedia , lookup
PPT Magnets Developer Notes 1. Need more exercises. 2. I left out units for magnetism because they’re more complicated than we need here. 3. Could use a diagram of the magnetic field of a dipole. It’s hard to draw in Word. Version 01 02 Date 2005/02/08 2005/02/11 03 1. 2. 3. 4. 5. 6. 7. Who dk dk Revisions Initial version, taken from 11.4 charge 03dk.doc Removed incorrect material on F p1p2/r2 Changes due to testing with students Goals Students should know that magnets work primarily with ferrous materials and not others. Students should know that all magnets have two poles, north and south. Students should know that in magnets, opposites attract and likes repel. Students should know that magnets have energy/ can do work. Students should know that a magnetic field is defined as going from south to north. Students should be able to visualize and draw magnetic fields and field lines. Students should know that a magnet placed in a magnetic field has a torque applied in the direction of the field. Concepts & Skills Introduced Area Physics Concept Magnets Standards Addressed Time Required Warm-up Question Presentation This activity introduces magnets, which will reappear later in electromagnetism. This activity is placed before charge because magnets and charge are similar, but magnets are accessible tactilely. This way, the students will have images to work with as they explore charge. Magnets and charge are alike in many ways: 1. Both are di-poles, where opposites attract and likes repel. 2. Both act through space. 840962969 1 of 12 PPT Magnets 3. Both attract non-charged objects by inducing fields in them. 4. Both can pass a charge to another object through contact. 5. Both have energy. Magnets are just a lot easier to see than electrons and protons. Here is the sequence of concepts: 1. Some objects are not magnetic. 2. There are two poles on magnets. 3. Opposites attract and likes repel. 4. Either pole attracts a non-magnetized object (by induction). 5. Some objects are magnetically neutral. 6. A magnet can pass a charge to a non-magnetized object. 7. An object can be de-magnetized. Make sure you’re not doing this activity on a ferrous surface. Stations 1. Magnetic materials Most students probably know that magnets attract ferrous materials, but just in case there are those who don’t know, here is an opportunity to play with it.. Some students may know that some other materials are magnetic, too. 2. Magnetic field There are several stations where the students draw fields. It is important for them to get the idea of field and field lines. A magnet (compass) placed in the field will have a torque applied to align its north end in the direction of the field, toward the north. 3. Strength of field This is just to reinforce the feel of how strong magnets are with another view. 4. Two disk magnets and filings The goal here is to see the field lines connecting between two magnets when opposite poles are showing, and to see the lines not connecting when the same poles are showing. 5. Different magnetic fields The goal here is to see different shapes of magnetic fields. 6. Magnetic shielding This will reinforce which materials are magnetic. It also shows induced magnetism in ferrous objects, and it shows the alteration of field lines by magnetic materials. 7. Levitating magnets This should show that a magnetic field is inverse squared. 8. Inductance This shows that a magnetic charge can be temporarily induced in another object. 9. Transfer This shows that a magnetic charge can be permanently (more or less) transferred to another object. 10. Two poles This shows that all magnets have a north and a south pole. 840962969 2 of 12 PPT Magnets 11. Opposites or likes Here are drawings of magnets showing that opposites attract. The first assumption is that the two sides of the magnet are opposite. a. Assume likes attract. Then a stack of three magnets looks like this: N S S N N S If you move the left-most magnet to the right-most position, they attract, indicating that opposites attract. The pattern is inconsistent, so it can’t be right. But if you assume opposites attract, then a stack of three magnets looks like this: N S N S N S If you move the left-most magnet to the right-most position, they attract, and the pattern is consistent. b. Stack three magnets. Assume that likes attract, so it looks like this: N S S N N S Remove the middle magnet, and opposites attract and the pattern is inconsistent, so it can’t be right. But if you assume opposites attract, then it looks like this. N S N S N S Remove the middle magnet and the pattern is consistent. c. Stick two magnets together. Separate the two magnets so that the attracting sides are up. Try one side of the third magnet against the attracting sides of the first magnets. It will attract one and repel one, so the two sides must be different, and opposites attract. Stated simply: A and B attract. A and B are different. So opposites attract. 12. Repulsion The goal here is to give the students a feel for how things act when they repel each other. It is a tactile way to understand repulsion of charge. Assessment Observe: Levitate a magnet. Get two ring magnets and place them over a post, like a pencil or chopstick so that one is levitated above the other. Explain: pole, north, south, equilibrium Observe: Rub a magnet on a paper clip to magnetize it (paper clip M). Place a small, round magnet in the middle of another un-magnetized paper clip (paper clip U). Touch the ends of paper clip U to the ends of M. Only one end of M should be attracted, but it should be attracted to both ends of U. Now reverse the side of the magnet on paper clip U and repeat. The other end of M should be attracted to both ends of U. Explain: pole, north, south, induce Writing Prompts 840962969 3 of 12 PPT Magnets Relevance Equipment Small disk magnets, about 1 cm in diameter, work well for this activity. They should be unmarked. The poles should be on the two flat sides rather than across the diameter for most of the activities. Radio Shack, PN 64-1883, $0.50 each. Ring magnets work well – Radio Shack PN 64-1888, $0.60 each. Flinn AP5664 ring magnets look like they would work, $0.25 each. Ring magnets break a little easier than disk magnets. Neodymium magnets are too strong; they’re dangerous and can demagnetize other magnets. Most refrigerator magnets don’t work because there are multiple poles on each face. Ben Franklin carries some magnets; they aren’t facemagnetized, but they could be used for mapping fields. Bar magnets are useful for some activities. It is good if they are marked for N and S. Flinn AP9262, 150 mm, $7 each, or AP9264, 100 mm, $9/pair(or each?). An unmarked bar magnet is Flinn AP9261, 50 mm, $5. Small compasses, about 2 cm in diameter, are inexpensive and work well. Flinn AP 5285, $0.80 each, or AP 6387, $20 for 30. Iron filings are messy but straightforward. Get the kind that don’t rust. Flinn 10059, 500 g, $7. You only need about 5 g per station. You’ll need to put them in a container that’s good for sprinkling, easy to put them back into, and can be sealed. Pepper shakers work, although the traditional glass with metal top tend to spill when opened. Plastic sealable ones by Rubbermaid (Servin’ Saver) work well. Plastic bottles with flip-up squirt caps also work. Use jumbo paper clips because they’re big and heavy enough to hit on a corner and demagnetize; standard paper clips are not. Stations 1. Magnetic materials This just ensures that all of the students know that magnets work primarily with ferrous materials and not others. For the various materials, you should include a variety of metals and some non-metals. Coins, paper clip, tin can, soda can, steel (common) nail, aluminum nail, copper wire, aluminum foil, cloth, paper, wood, plastic bottle, styrofoam, pencil lead, 2. Magnetic field The goal here is to map a magnetic field directly. Other stations show a variety of magnetic fields, and combinations of fields, but this one forces the students to draw one accurately, with the direction of the field. 3. Strength of field The goal here is to be able to visualize varying strengths of fields. The magnets should be the same shape but different strengths. They need a common third element to test against, which could be a piece of steel or another magnet. 840962969 4 of 12 PPT Magnets 4. Two disk magnets and filings The goal here is to see magnetic fields that attract and repel. The two magnets should be identical. 5. Different magnetic fields The goal here is to see a variety of magnetic fields. A disk magnet magnetized across the diameter is more interesting here than one that is magnetized on its faces. Any variety of magnets will work. 6. Magnetic shielding The goal is to see that magnetism passes through some materials, but not through others. The materials it doesn’t pass through are magnetized themselves. The field is induced in them. The sheets should be the same shape. The hardest to cut is the steel, so find it first and cut the others to match. A radius or length of 5-10 cm is about right. Aluminum foil is a little thin for this activity, but pie tins work, or a soda can can be cut fairly easily. Any plastic will do, or wood, or anything else you like. It’s also fun to try different shaped ferrous objects, like keys, paper clips, hooks, etc. 7. Levitating magnets The goal is to show that magnetic force is an inverse squared function with distance. You can buy kits for this, or just use ring magnets and make a post. The post must be a close fit with the hole in the magnets so that they don’t tilt too much. Some ball point pen tubes work pretty well, or roll up a piece of paper to the right size and tape it. 8. Inductance The goal is to show that magnetism can be temporarily induced in another object. 9. Transfer The goal is to show that magnetism can be (semi-) permanently transferred to another object. 10. Two poles The goal is to show that magnets always have two poles. You could also include broken magnets, if you have them, to show that they all have two poles. 11. Opposites or likes The goal is to show that opposites attract and likes repel. There are several ways to prove it. See the teacher’s section. This is a bit of a tricky station and could be worked on after all of the stations are done, if necessary. Bar magnets could be used, but they are generally marked with north and south. Also, they are not as symmetrical. Disk magnets can only be put together two ways; the one on top can only be flipped over, and it is attracted or repelled. Bar magnets can be stacked four ways; the one on top can be flipped over and it can be flipped end for end. 12. Repulsion The goal is to give students an image they can use when they are thinking about electrons spreading over a surface. It is the same principle, but magnets are easy to see and fun to play with. Answers to Exercises 1. Is magnetism a form of energy? Explain. [Yes, magnetism is a form of energy because it can do work. It can apply a force to an object and make it move a distance.] 2. Is Earth a magnet? Explain. [Yes, Earth is a magnet. Compasses are made of magnets, and they are attracted in a particular direction to Earth, so Earth must be a magnet, too.] 840962969 5 of 12 PPT Magnets 3. Can you pick up an aluminum can with a magnet? How about a tin can? What’s the difference? [You can’t pick up an aluminum can with a magnet, but you can pick up a tin can. Tin cans have iron in them, but aluminum cans don’t. Later we’ll see that you can magnetize an aluminum can by passing a current through it.] 4. The north end of a compass points to Earth’s magnetic pole is in northern Canada. Does the north or south end of a compass needle point north? [Opposites attract. You can’t tell from the information here. Actually, Earth’s geographic north is a south magnetic pole.] Answers to Challenge/ extension 1. 840962969 6 of 12 PPT Magnets Background You know that all objects have mass and that every object attracts every other object gravitationally. There is just one kind of mass and it always attracts other mass - it never repels it. You have probably played with magnets, and you know that magnets attract each other, but they can also repel each other, so magnetic force is different from gravitational force. Problem Explore magnetism. Prove whether likes or opposites attract. Materials At the stations Procedure Stations When you work with iron filings, sprinkle them gently. Don’t use too many filings, but use enough to see the magnetic field lines. Tapping the paper gently or lifting the corners a bit might help the filings to show the fields better. To put the filings back in the container, double the paper into a U shape and pour them. 1. Magnetic materials 1 magnet AR various materials a. Touch the magnet to the objects. Which objects are attracted to the magnet? b. Based on what you have seen, are all objects attracted to magnets (magnetically)? c. Touch one of the objects that was attracted to the magnet to the other objects. Which objects are attracted to it? d. Are all objects that are attracted to magnets attracted to each other (magnetically)? e. Try some other objects near you. What type of object is attracted to magnets? Can you figure out the rule? f. Can magnets apply force without physically touching another object? 2. Magnetic field 1 bar magnet 1 small compass a. Lay the bar magnet on a piece of paper and trace around it. b. Place the compass anywhere on the paper. Make dots at the north and south end of the compass needle. Remove the compass and make an arrow between the dots, with the head of the arrow at the north end. c. Place the compass so that one end of the needle is on one of the ends of the arrow you just drew and make a dot at the other end of the needle. Draw the arrow. Keep repeating until you connect the dots to the compass on both ends. Draw at least eight lines. 3. Strength of field 1 strong magnet 1 weak magnet 1 piece of steel 840962969 7 of 12 PPT 4. 5. 6. 7. 8. Magnets 1 bottle of iron filings 1 sheet of paper a. Test the two similar magnets against the steel to see which is stronger. b. Keep the magnets at least 10 cm away from each other. Place the paper over the magnets and sprinkle iron filings on the paper. On a separate sheet of paper, draw a diagram of the magnetic fields around the two magnets and compare them. c. Return the filings to the container. Two disk magnets and filings 2 disk magnets 1 bottle of iron filings 1 sheet of paper a. Place the magnets about 1 cm apart. Place the paper over them and sprinkle iron filings on the paper. On a separate sheet of paper, draw a diagram of the field lines. Return the filings to the container. b. Flip one of the magnets over, leaving them about 1 cm apart, then proceed as above. c. Compare the fields. Different magnetic fields 1 disk magnet 1 ring magnet 1 bottle of iron filings 1 sheet of paper a. Place the paper over one of the magnets. Gently sprinkle iron filings on the paper until you can see lines of the magnetic field. On a separate sheet of paper, draw a picture of the magnet and lines. Return the filings to the container. b. Repeat for the other magnet(s). Magnetic shielding 1 bar magnet 1 sheet of steel 1 sheet of aluminum 1 sheet of plastic 1 bottle of iron filings 1 sheet of paper a. Place the magnet on the table. Place the paper over the magnet. Sprinkle iron filings on the paper. Draw a diagram of the field lines. Return the filings to the container. b. Place the aluminum over the magnet and proceed as above. c. Place the steel over the magnet and proceed as above. d. Place the plastic over the magnet and proceed as above. Levitating magnets 5 ring magnets 1 post 1 ruler Place the magnets over the post so they repel each other (don’t touch). Measure the distances between the magnets. Draw a graph to show whether distance and force have a direct or inverse relation, and whether they have a linear or exponential relation. (Hint: how many magnets are above each space?) Inductance 840962969 8 of 12 PPT Magnets 1 disk magnet 2 straightened jumbo paper clips Before starting, hit both ends of the paper clips two or three times sharply on a hard object, like the corner of a table, to neutralize them. Test both ends of the paper clips against each other. There should be no attraction. a. Stick (and hold) a magnet to the middle of one of the paper clips. Touch the end of that paper clip to the end of the other paper clip. Move the paper clip with the magnet until it attracts the other paper clip. Has the magnetism been passed from the magnet into the paper clip? b. Take the magnet off. Do the paper clips attract each other? c. Does the magnet pass a temporary charge into (induce a magnetic charge in) the paper clip? d. Try to explain how an un-magnetized object can be temporarily magnetized. 9. Transfer 1 disk magnet 2 straightened jumbo paper clips Before starting, hit both ends of the paper clips two or three times sharply on a hard object, like the corner of a table, to neutralize them. Test both ends of the paper clips against each other. There should be no attraction. a. Rub one side of a magnet in one direction three times on the full length of a paper clip. Remove the magnet. Touch one end of that paper clip to each end of the other one. Does it attract both ends? b. Can magnetic charge be passed from a magnet to another object? c. Try to explain how an un-magnetized object becomes magnetized. 10. Two poles 1 disk magnet 2 straightened jumbo paper clips Before starting, hit both ends of the paper clips two or three times sharply on a hard object, like the corner of a table, to neutralize them. Test both ends of the paper clips against each other. There should be no attraction. a. Rub one side of a magnet in one direction only three times on the full length of a paper clip. Touch one end of that paper clip to each end of the other one. Does it attract both ends? b. Now rub the other paper clip the same way. Touch the ends of the paper clips. Try all four combinations. Make notes on which ends are attracted. c. Do you think it is possible to have a magnet with just a north or just a south pole? Why? 11. Opposites or likes 3 magnets Do not write on the magnets! a. Put the flat sides of two magnets together. Do they attract or repel each other? b. Flip one of them over. Do they attract or repel each other? c. Is there more than one type of magnetism? d. Take two magnets and determine which ends attract and which repel. Use the third magnet to prove whether the ends that attract are the same or different. e. Do likes attract or do opposites attract? f. Draw a diagram showing how you proved it. 840962969 9 of 12 PPT Magnets 12. Repulsion 3 disk magnets a. Take all three magnets. Lay them flat on a surface so that they repel each other. Push one of them toward the other two. Notice how they move. Play with them, changing which one you push. Squeeze one with two others. Enjoy. Remember what this looks and feels like for the next activities. Summary 1. In magnets, do likes or opposites attract? 2. The two poles of a magnet are called north and south. If you have a new magnet, how can you tell which end is north? 3. Can a magnet exist with just a north pole? 4. If you place a compass in a magnetic field, which way will the north pole point? 840962969 10 of 12 PPT Magnets Reading Magnets (lodestones) were discovered long ago. More than 2,000 years ago, the Greeks found that some stones attracted pieces of iron. About 900 years ago, the Chinese started using magnets as compasses. Magnets are useful because they can apply force from a distance – they don’t actually have to touch. Today, magnets have many, many uses. Magnets are still used for navigation. Magnets are used to make electric motors. Magnets are used to remove metal from rubbish to recycle it. Magnets are used to levitate trains and reduce friction. Magnets are extremely useful. Materials Most materials are not very magnetic. The most common magnetic material is iron, or anything with iron in it, like steel. A few other elements are strongly magnetic – cobalt, nickel, and gadolinium. Domains Magnets have very small sections called domains that are small magnets themselves. The domains can change size or orientation. If they are random, the object will not be magnetic. If they align with each other, it will be magnetic. Magnets attract unmagnetized objects by inducing a magnetic pole in them – some of the domains change because the magnet attracts the opposite pole of the domains, inducing magnetism. Rubbing an object with a magnet can change the domains to make align them. Hitting the magnet or heating it allows the domains to change. Depending on the field around the magnet, hitting it or heating and cooling it can magnetize or demagnetize it. Magnetism and gravity Magnetism differs from gravity because there are two poles in every magnet, north and south. Opposite poles attract each other, and like poles repel. No magnets have ever been found with just one pole. If you break a magnet in half, you’ll end up with two smaller magnets, each with a north and a south pole. Magnetism and gravity have some similarities. Both gravity and magnetism act through space. Every mass has a gravitational field around it, and every other mass is pulled toward it. The bigger the mass and the closer another mass is, the bigger the force. The fundamental unit for gravity is mass, while for magnetism the unit is pole strength (p). A magnetic pole has a magnetic field around it. The stronger the pole, and the closer another pole is, the stronger the force. Look at the equations for the force of gravity and the force of magnetism. They are very similar. Gravity Newton’s Law of Gravitation F = Gm1m2/r 2 F = kp1p2/r 2 Magnetics Magnetic force law If the magnetic pole strength is doubled, the force is doubled. Force has a direct linear relationship to pole strength. If the distance between two poles is doubled, the force is divided by four. Magnetic force has an inverse squared relationship to distance. 840962969 11 of 12 PPT Magnets Fields A magnetic field is a way of describing the strength and direction of the magnetic force. Magnetic fields are defined as going from north to south. That means that a magnet (compass) placed in a magnetic field will have a torque applied so that the north end is forced in the direction of the field, south. A magnetic field is a vector because it has a quantity (strength) and a direction. Magnetic fields are represented by a capital B. N S Magnetic force is stronger than gravitational force At short distances, magnetic force is much stronger than gravitational force. Magnetic force decreases faster than gravitational force, however, so that at very long distances, gravity dominates. The further away a compass gets from a magnet, the closer together the two poles of the magnet appear, either in distance or in angle, so the less torque there is on the compass. Additionally, the force decreases with distance. There’s only one kind of gravitational force, so it always pulls in the same direction, and decreases only due to distance. Summary Magnetism and gravity are very similar in some ways, but there are two big differences. There is only one type of gravity, but every magnet has two poles, north and south. All mass attracts all other mass, but opposite poles attract and likes repel. At short distances, magnetism is much stronger than gravity, but at long distances, gravity is stronger. Exercises 1. Is magnetism a form of energy? Explain. 2. Is Earth a magnet? Explain. 3. Can you pick up an aluminum can with a magnet? How about a tin can? What’s the difference? 4. Earth’s north magnetic pole is in northern Canada. Does the north or south end of a compass needle point north? 5. How could magnets be used to make a train run with almost no friction? Challenge/ extension 1. Is there a magnetic field around you? Where does it come from? How is it generated? Glossary magnet – an object that attracts iron. pole – a part of a magnet the exerts a magnetic force. A magnet has two poles, north and south. Opposite poles attract and like poles repel. 840962969 12 of 12