1 L 28 Electricity and Magnetism [5] magnetism magnetic forces applications Why are magnets magnets?

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1 L 28 Electricity and Magnetism [5] magnetism magnetic forces applications Why are magnets magnets? Slide 2 2 Magnetism two sources of magnetism permanent magnets electromagnets the earths magnetic field how does a compass work the north pole is really a south pole! Van Allen radiation belts Slide 3 3 Permanent magnetism certain minerals (magnetite, Fe 3 O 4 ) are naturally magnetic a piece of loadstone will attract bits of iron a magnet produces a magnetic field in the space around it, just like the Sun has a gravitational field that holds the planets in their orbits the magnetic field can be visualized with iron filings Slide 4 4 Earths g-field Solenoidal Coil Slide 5 5 Permanent magnets Are made from alloys of some of the rare earth elements like neodymium and samarium and cobalt. Always have a north and a south pole like poles repel and unlike poles attract if you break a magnet in half you get 2 magnets cannot have just a north or just a south pole N S N S N S Slide 6 6 If you pass current through a loop of wire you get a magnet Oersted discovered this Ampere figured out the formula relating currents to magnetic fields (Amperes Law) Bar Magnet Electromagnet Slide 7 7 Homemade magnets Duracell + You can think of the nail as a collection of little magnets that are randomly aligned. The magnetic field of the coil aligns these little magnets giving a larger field than that of the coil alone. We say that the nail becomes magnetized, but the effect is not permanent. COIL Iron nail Slide 8 8 N S atomic magnets NORTH SOUTH UNMAGNETIZED PARTIALLY MAGNETIZED Slide 9 9 Magnetic materials some materials are naturally magnetic or can be magnetized and retain their magnetism ferromagnetic materials other materials (iron) can be magnetized temporarily by placing them near magnets some materials have essentially no magnetic properties copper, aluminum, plastics... heat can destroy magnetism (Curie effect) Slide 10 10 The earth is a big magnet The earths north geographic pole is the south pole of a big magnet. A compass needle is attracted to the earths north geographic pole The earths magnetism is due to currents flowing in its molten core (not entirely understood!) the magnetic north pole is inclined about 14 from the geographic north pole, or by about 600 miles. Slide 11 11 Report: Earth's magnetic field fading Slight chance of flipping magnetic poles The strength of the earths magnetic field had Decreased10% over the last 150 years. At this rate, the field will disappear altogether in 1,500 to 2,000 years. CNN Report Dec. 12, 2003 North South time Slide 12 12 Sun Earth Connection: space weather SUN solar eruption earth Northern Lights solar eruption Slide 13 13 Solar eruptions Slide 14 14 Particles stay on the magnetic field lines electron that originated on the sun part of the solar wind. The earths magnetic field traps charged particles. During periods of intense solar activity, satellites can be at risk. Slide 15 15 Van Allen Radiation Belts Slide 16 16 Magnetic forces magnetic fields can cause charges to turn around. Slide 17 17 Magnetic deflection of electrons in a TV tube. Slide 18 18 Magnetic forces on wires N S N S Wire pushed OUT Wire pulled IN Slide 19 19 Forces on parallel wires Opposites repel Likes attract Slide 20 20 Applications Magnets are not just for holding things on the refrigerator! Slide 21 21 The electric motor When a current is present in a coil, it experiences a torque and rotates. Slide 22 22 Magnets make speakers work The force between the permanent magnet and the voice coil moves the speaker cone Slide 23 23 MAGLEV Trains Slide 24 24 Magnetic tape recording


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