11.1 Household Magnets. New ideas for today: Magnetism Refrigerator magnets Electromagnets

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Text of 11.1 Household Magnets. New ideas for today: Magnetism Refrigerator magnets Electromagnets

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11.1 Household Magnets Slide 2 New ideas for today: Magnetism Refrigerator magnets Electromagnets Slide 3 Observations about Household Magnets Two magnets can attract or repel n n Magnets can stick to certain metals n n Magnets affect compasses n n The earth seems to be magnetic n n Some magnets use electricity to operate Slide 4 Magnetic Poles n n Two types: north & south n n Like poles repel, opposites attract n n Forces increase with decreasing separation n n Analogous to electric charges EXCEPT: n n No isolated magnetic poles ever found! n n Net pole on an object is always zero! Bar magnet Slide 5 Clicker Question: Suppose you have a long bar magnet with a north pole at one end and a south pole at the other. If you break it in half, will the two new ends: A. A. Attract B. B. Repel C. C. Neither Broken bar magnet Slide 6 Click me Slide 7 Magnetic Fields n n A magnetic field pushes on magnetic pole n n The magnitude of the field is proportional to the magnitude of the force on a test pole n n The direction of the field is the direction of the force on a north test pole But isolated magnetic poles dont seem to exist! Bar magnet and compass Slide 8 Torque on a dipole N S Magnitude of torque proportional to magnetic field Torque always acts on dipole to line it up with field Slide 9 Torque on a dipole N S Magnitude of torque proportional to magnetic field Torque always acts on dipole to line it up with field Slide 10 Ferromagnetism n n Most atoms are magnetic n n Most materials are not n n Atomic magnetism is perfectly cancelled n n Some materials do not have full cancellation n n Magnetism is usually hidden by randomness n n However, ferromagnets can be permanently magnetized by applied magnetic fields Slide 11 Slide 12 Refrigerators and Magnets n n A refrigerators steel has magnetic domains n n Domains cancel so steel appears nonmagnetic n n When a magnetic pole is near steel n n it causes some domains to grow, others to shrink n n and the steel develops magnetic polarization n n so that it attracts the magnetic pole n n Magnets thus stick to steel refrigerators magnetized steel paperclips Ferrous materials Slide 13 Electromagnetism I n n Magnetic fields n n Push on magnetic poles n n Electric fields n n Push on electric charges Slide 14 Electromagnetism I n n Magnetic fields n n Push on magnetic poles n n Bend moving electric charges n n Electric fields n n Push on electric charges Slide 15 - - TV Slide 16 Earths magnetic field Near surface of the earth: 0.00005 Tesla Charged particles from the sun spiral around magnetic field lines and crash into the atmosphere near the poles! Slide 17 Clicker Question: What is the North Pole? A. A. Magnetic north pole B. B. Magnetic south pole C. C. Neither Slide 18 Electromagnetism II n n Magnetic fields created by n n Fundamental particles (dipoles) electrons, protons, neutrons n n Electric fields created by n n Charges Slide 19 Electromagnetism II n n Magnetic fields created by n n Fundamental particles (dipoles) electrons, protons, neutrons n n Moving electric charges (current) n n Electric fields created by n n Charges Slide 20 Current n n Current is moving positive charge n n Current is measured in coulombs/second or amperes (amps) n n Electric fields cause currents to flow n n Currents produce magnetic fields: Straight wire field Slide 21 Click me Slide 22 Magnetic field lines Electromagnet Magnetic Dipole Single loop field Solenoid field Electromagnet Slide 23 Slide 24 Other forms of magnetism: Paramagnetism Diamagnetism Perfect diamagnetism (superconductor) Liquid O 2 Slide 25 This is diamagnetism Slide 26 Supercondutor Slide 27 See you next class! For next class: Read Sections 11.2, 11.3

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