Transcript File
Magnetism History of Magnets • (~800 BC) Ancient Chinese and Greeks discovered that certain stones would attract and magnetize iron. • Small slivers of the stone were found to align themselves with the North Pole. • Chinese were the first to use magnets for navigation. • The orienting properties were used to align streets in cities in the North-South / East-West direction. Poles of a Magnet • • • • Magnets have a North and South Pole. Like poles repel. Unlike poles attract. What happens if you break a magnet in half? Will you get two monopoles? – No. N S N S + N S Oddly shaped magnets still have a north and a south Magnets either attract or repel each other South poles are attracted to north poles Like poles repel Unlike poles attract Magnetic Field Lines vs. Electric Field Lines Magnetic Dipole Electric Dipole What happens when you bring a compass near a bar magnet? The north indicator of the compass will point toward the south pole of a magnet. This is also why a compass points north, it's lining up with the earth's magnetic poles. The Earth’s Magnetic Field • The earth has a magnetic field that scientist believe is a result of the dynamo effect due to electrical currents created in the molten iron and nickel outer core. • The Earth's Magnetic Field • Bar Magnet - 3D Sometimes the field completely flips. The north and the south poles swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable. They come at irregular intervals averaging about 300,000 years; the last one was 780,000 years ago. Are we overdue for another? No one knows. The magnetic North Pole is responsible for more than just the direction a compass points. It's also the source of the aurora borealis, the dramatic lights that appear when solar radiation bounces off the Earth's magnetic field. This happens at the South Pole as well. In the southern hemisphere, the lights are called the aurora australas. Source of Magnetic Fields • Electrical Charge in motion. – Currents occur at the atomic level in atoms due to the orbits of electrons around the nucleus. – The intrinsic spin (+1/2, -1/2) is critical in the case of magnetism. Magnetic Domains • A: Iron absent of a magnetic field. • B: Iron in the presence of a magnetic field. • C: A non-magnetic material. A B C Magnetic Domains = groups of atoms with aligned poles Magnets can be temporary (like the needle used in the compass). This nail has its atoms aligned, but the effect is only temporary. You can get this affect by rubbing the nail on a magnet. Neat fact: Hitting the nail can demagnetize it, you are basically scrambling the atoms. Magnetism of Soft Ferromagnetic Materials How does a magnet attract screws, bolts nails, paperclips, etc. when they are not magnetic to start with? – Soft ferromagnetic material align their domains in the presence of an external magnetic field creating a magnetic dipole. • When the magnetic field is removed, the domains re-randomize resulting in no magnetic attraction. They are temporary • Soft ferromagnetic material is attracted to both the North pole and South pole. S N S Types of Magnets • Temporary: When charged particles move through space, they induce a magnetic field (Electromagnets). • Permanent: Electrons have an intrinsic magnetic field that may add together in certain matter to create a magnetic field (Speakers). Temporary Permanent A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Metals that are magnetic: nickel, iron, cobalt Things that are not magnetic: aluminum, plastic, glass Ferromagnetic - a substance such as iron in which the magnetic moments of the atoms spontaneously line up with each other, making a large net magnetic moment. Ferromagnets lose their ferromagnetism when heated above a specific temperature , because the thermal energy melts the magnetic alignment. What else can cause a ferromagnet to lose its magnetism? Focus on SPEAKERS The loudspeakers in your radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone. By varying the electric current through the wires around the electromagnet, the speaker cone moves back and forth. The resulting vibration of the speaker cone will create sound waves, including that from voice and music. Maglev Trains Magnetic + Levitation = maglev Maglev vehicles “float” over an electromagnetically powered fixed steel guideway and are propelled by the current with no motors, wheels, moving parts or additional energy sources. The system is environmentally friendly, energy efficient and runs in all weather conditions. http://dsc.discovery.com/video s/extreme-engineeringseason-1-shorts-maglevtrain.html Make your own compass http://adventure.howstuffworks.com/outdooractivities/hiking/compass2.htm Applications • • • • • • • • Computer disc drives (hard and floppy) VCR and cassette tape Credit cards Speakers Motors (Both AC and DC) Speed sensors Solenoids for relays, valves, etc. Magnetos (piston engine aircraft) Key Ideas • All magnets have North and South Poles • Magnetic field lines originate in the North and end at the south pole. • Magnetic field lines do not cross. • Magnetism exists at the atomic level. • Magnetism is the result of moving charges. • Some magnets are temporary while others are permanent. • Types of Magnetism. – Ferromagnetism. – Paramagnetism. – Diamagnetism. Quiz Time - check out this page first http://www.magnet.fsu.edu/education/tutorials/index.html Check out the *Science Demonstrations link for some neat tricks with magnets. Or check out this video about "LINEMEN" who repair power lines: http://www.youtube.com/watch?v=7gl-_Liz_0o 1. All magnets are surrounded by an invisible force called the ________________ _______________ 2. The north pole of one magnet will be attracted to the ______ pole of another magnet. 3. The poles of the earth can move. True or False? 4. In the far north, radiation from the sun is reflected off the earth's magnetic field and creates patterns of colors. This effect is known as the ___________ ________________ 5. A nail is normally not magnetized. How can you magnetize it? 6. Who discovered that magnets are affected by electricity? 7. You can make an ______________________ by wrapping a wire coil carrying a current around an iron core. 8. What car part is attached to your starter and is needed to start the car? 9. This is a device used to measure electric current. 10. A generator converts ___________ energy into electrical energy.