SPH3U UNIVERSITY PHYSICS

Transcription

1 SPH3U UNIVERSITY PHYSICS ELECTRICITY & MAGNETISM L (P ) Activity: INSTRUCTIONS A. Cover a bar magnet with a sheet of acetate. Carefully sprinkle iron filings around the bar magnet onto the acetate and look for any patterns formed by the iron filings, especially near the poles of the magnet. September 30, U4-1 Activity: INSTRUCTIONS B. Use compasses to indicate the direction of the field in the space above the bar magnet. September 30, U4-2 1

2 Activity: INSTRUCTIONS C. Repeat the iron filing procedure for (i) like poles and (ii) opposite poles close together. September 30, U4-3 Activity: QUESTIONS 1. From what area of the magnets do field lines seem to originate? To what region do they seem to return? September 30, U4-4 Activity: QUESTIONS 2. Do magnetic field lines ever cross each other? September 30, U4-5 2

3 Activity: QUESTIONS 3. What do you notice about the spacing of the field lines as you move away from the poles? What does this spacing indicate about the strength of the magnetic field? September 30, U4-6 Magnetite As early as 600 B.C, the Greeks discovered that a certain type of iron ore, later known as lodestone, or magnetite, was able to attract other small pieces of iron. Lodestone consists mainly of iron oxide, a mineral that was first found near Magnesia, in Greece, hence the term magnetism. September 30, U4-7 Magnetite When pivoted and allowed to rotate freely, a piece of lodestone would come to rest in a north-south position. Because of this property, lodestone was widely used in navigation. Today, however, lodestone is hardly ever used for its magnetic property. Artificial magnets are made from various alloys of iron, nickel, and cobalt. September 30, U4-8 3

4 Magnetic Forces When the N-pole of one magnet is brought near the N-pole of another freely swinging magnet, the magnets repel each other, as shown. Similarly, two S-poles repel each other. On the other hand, N-poles and S-poles always attract each other. These observations lead to the law of magnetic poles. LAW OF MAGNETIC POLES 1. Similar magnetic poles repel. 2. Opposite magnetic poles attract. September 30, U4-9 As you know, when a N-pole and a S-pole are brought close to each other, they begin to attract even before they touch. A magnetic field is the three-dimensional region of space surrounding a magnet that will exert a force on magnetic objects. Magnetic fields are invisible and are more intense at the poles. September 30, U4-10 MAGNETIC FIELD 3-D space around a magnet that exerts a force on magnetic objects is invisible is more intense at the poles September 30, U4-11 4

5 Magnetic Field Lines A magnetic field is represented by a series of lines around a magnet, showing the path the N-pole of a small test compass would take if it were allowed to move freely in the direction of the magnetic force. These magnetic field lines have several unique properties. September 30, U4-12 Magnetic Field Lines Magnetic field lines: always point from N to S outside a magnet and S to N inside (i.e. they form a closed loop), never cross one another, and are closer together where the magnetic field is stronger (poles). September 30, U4-13 Magnetic Field Lines MAGNETIC FIELD LINES point from N to S outside and S to N inside (i.e. form a closed loop) never cross one another are closer together where the magnetic field is stronger (i.e. the poles) September 30, U4-14 5

6 1. Sketch the magnetic field around and through a magnet. September 30, U Describe two methods that could be used to detect the presence of a magnetic field. compass & iron filings September 30, U What does the magnetic field look like when: (a) two opposite poles are brought close together? (b) two like poles are brought close together? September 30, U4-17 6

7 4. Sketch the magnetic field patterns expected when the following poles are brought close together. (a) N and S (b) N and N (c) S and S September 30, U The north pole of a compass points north. What does this indicate about Earth s geographic north? geographic south? Earth s geographic north is actually a magnetic south and Earth s geographic south is actually a magnetic north. September 30, U4-19 Many technologies, such as Maglev trains, particle accelerators, and magnetic resonance imaging (MRI) systems, use magnetic fields. The Maglev train, for example, uses magnetic fields for both levitation and forward motion. The bottom of the train is attracted upward toward the bottom of the track. The forward motion is caused by both attraction and repulsion forces between the track and the train. September 30, U4-20 7

8 A magnetic resonance imaging (MRI) system uses incredibly strong magnetic fields to produce very detailed images of the inside of the human body. These detailed images provide doctors with important information that can be used to diagnose a disease or provide information for surgery. MRI scans can even detect cancerous cells before they become tumours. September 30, U4-21 Magnetic fields are also found in many technologies that we use daily. Magnets can hold pictures on the side of a refrigerator; magnets inside electric motors help the motors spin; electric bells use magnets to ring at the end of your class; and the hard drive in your computer stores information using magnetic fields. September 30, U4-22 In the northern parts of the northern hemisphere, magnetic fields cause the northern lights or aurora borealis. Charged particles, such as electrons and protons, streaming from the Sun spiral in toward these poles, and collide with atoms in the upper atmosphere. These energized atoms then give off energy in the form of visible light with a variety of colours. Sometimes large solar storms cause the auroras to increase and result in the disruption of our communications systems for hours or even days. September 30, U4-23 8

9 Even living creatures can use magnetic fields to their benefit. Spiny lobsters migrate over hundreds of kilometres. Scientists suspected that the spiny lobsters used Earth s magnetic field to guide their way. They removed some lobsters and placed them in a tank where the scientists could change the magnetic field. With the magnetic field changed, the lobsters changed their path. Other animals, like birds, also use the magnetic field of Earth to help navigate while migrating. September 30, U4-24 APPLICATIONS OF MAGNETIC FIELDS everyday L fridge magnets, motors, electric bells, hard drives technology L maglev trains, particle accelerators, MRI biological L living creatures for navigation Earth L protect us from cosmic rays (causes aurora borealis) September 30, U4-25 DYK? Earth s magnetic poles have actually flipped directions in the past. Approximately 780,000 years ago, the north pole slowly moved towards the south pole until the poles changed over. Today, scientists have noticed a steady decline in the strength of Earth s magnetic field. Some scientists believe that another flip is coming soon, perhaps in the next 1000 years. September 30, U4-26 9

10 DYK? 6. Suppose Earth s magnetic field disappeared. How would the northern lights be affected? What would happen to animal navigation? What would happen to life on Earth? September 30, U4-27 TEXTBOOK P.552 Q.5 U Check Your Learning September 30, U