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Basic Magnetism

Basic Magnetism. November 2009 Physics 30S/40S. How Do We Use Magnets?. What is a Magnet?. A magnet is an object which can attract either iron or steel (other magnets) Magnets are designated with poles Poles: extreme ends of a magnet

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Basic Magnetism

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  1. Basic Magnetism November 2009 Physics 30S/40S

  2. How Do We Use Magnets?

  3. What is a Magnet? • A magnet is an object which can attract either iron or steel (other magnets) • Magnets are designated with poles • Poles: extreme ends of a magnet • when a bar magnet is allowed to rotate freely, the pole that seeks the northerly direction, is called the north magnetic pole, the opposite the south magnetic pole

  4. Law of Magnetic Poles • Like in electricity, opposite poles attract and similar poles repel • North attracts south • Two north poles will repel • Attraction and Repulsion? • Magnets are able to exert a force on each other; called a magnetic force • Unlike in electricity, it is not possible to separate two poles on a magnet • There is no north without south!

  5. History • Not a modern concept/Known since ancient times • Greeks (600 BC): Magnesia stone would attract iron • China: used for navigation • http://science.discovery.com/videos/what-the-ancients-knew-i-shorts-chinese-magnetic-compass.html • Lodestone (magnetite)

  6. Sir William Gilbert • De Magnete(1601) • Earth is like a bar magnet • Earth has a core of iron • Magnets can be cut • Domains • Presented magnetism to Queen Elizabeth; used in navigation May 24, 1544 – November 30, 1603

  7. Preliminary Compasses • Leaf, Water, Wire • http://dsc.discovery.com/videos/man-vs-wild-1-shorts-3-min-mexico-water-compass.html • A suspended magnet will point geographically north • Used for navigation • Sign of physics to come • Why do we rub the wire against hair?

  8. Magnets Today • Artificial magnets are most common • ferrite - iron alloys • rare earth magnets - very strong alloys • alnico - alloy made of aluminum, nickel, iron, and cobalt • very stable • supports 100 times it’s own weight (and costs about $20/pound) • used in guitar pickups, microphones, loudspeakers and cow magnets

  9. Geomagnetism – Short Overview • Compasses can be used in navigation – why? • Earth is essentially a giant magnet • Naming conventions: • North is South? • Why is Earth a magnet? More to come later...

  10. What is a Magnetic Field? • The magnetic field is the region of space around a magnet where another magnet will experience a force • Vector quantity: magnitude and a direction • Magnitude: measured in Tesla or Gauss • 1 T = 10,000 G • Direction: the direction which the north pole of a test compass would point; that is, towards south

  11. Mappings • We can create a map of the magnetic field around an object • More concerned with direction of field as opposed to magnitude • Magnitude: More and denser field lines indicate a stronger field • Direction: Use arrows to indicate direction of field • Remember: arrows point towards south pole!

  12. Bar Magnet

  13. Horseshoe Magnet • Think of bending a bar magnet – what would happen?

  14. Multiple Bar Magnets

  15. Look Familiar?

  16. Notes on Drawing Field Lines • Arrows always point towards the magnetic south pole • When drawing the field lines, remember law of opposite poles • Repulsion will push away field lines • Attraction will pull field lines tighter • Remember drawing the fields for static charges? • Bar magnet is like a dipole • Multiple bar magnets are like strings of dipoles

  17. Homework • Magnetic Field Lines Handout

  18. Domain Theory • Magnets are composed of small regions (called domains) which behave as miniature bar magnets • Domains are around 1 μm • Domains can be aligned to produce a net magnetic field or unaligned so that no net field is observable

  19. What Would a Magnet Look Like if Cut?

  20. Types of Magnetic Material • Ferromagnetic: permanent magnet • Paramagnetic: magnetic only in the presence of a magnetic field • How can we explain these two types of magnets in terms of domain theory?

  21. Creation of Magnets • Ferromagnetism: the phenomenon by which materials become and remain magnetized • placing molten paramagnetic material in the vicinity of a large magnetic field and allowing it to cool • passing an electric field through the material - More later… • rubbing a paramagnetic material with a magnet • demo

  22. Destruction of Magnets • Aim is to break the alignment of the domains! • Dropping • Repeated exposure to opposing strong magnetic fields • Heating • Curie Point (Pierre not Marie!): • Magnetic Heat Machine: • http://www.youtube.com/watch?v=RWrTvB-oK94 • Explanation from Walter Lewin (MIT): • http://www.youtube.com/watch?v=X8ZHQQUusGo

  23. Relation to Chemistry • For the answer to why certain elements are magnetic, while others are not, we turn to Chemistry • Pauli Exclusion Principle: for electrons in a single atom, no two electrons can have the same quantum numbers • Spins of electrons gives certain elements ferromagnetic or paramagnetic properties

  24. Homework • Domain Theory Handout

  25. Geomagnetism – The Details As postulated by Gilbert, the Earth does act like a bar magnet. It has magnetic north and south poles. However, our best explanation of Earth’s magnetic phenomena is described by: DYNAMO THEORY!!!

  26. Dynamo Theory • Molten core is composed primarily of iron; surrounds solid iron core • Fluid motion of iron creates magnetic poles • Poles are not stationary; they wander • Fluid dynamics • Poles are able to flip; on average, this happens every 300,000 years (variable) • Evidence in ocean floor • Last flip occurred 780,000 years ago

  27. Computer simulation: Blue signifies a magnetic south pole; yellow signifies a north pole From: http://www.es.ucsc.edu/~glatz/geodynamo.html See more simulations here!

  28. Location of Magnetic Poles • Earth’s magnetic poles wander • Currently, magnetic pole near geographic north is headed north at about 40 km per year • Located about 600 km north of Resolute Bay • Magnetic pole can move daily; sometimes dramatically (80 km) From: http://gsc.nrcan.gc.ca/geomag/nmp/northpole_e.php

  29. Magnetic Declination • Angle between magnetic pole and geographic north • Must be known for navigation • Dependent on location • Victoria: 20o East • St. John’s: 23o West • (Means that in Victoria, magnetic north is 20o East of geographic north) • For Elm Creek (geographical coordinates: 49° 40' 0" North, 98° 0' 0" West), the magnetic declination is ? • http://geomag.nrcan.gc.ca/apps/mdcal-eng.php

  30. Magnetic Declination Map

  31. Angle of Dip • Earth’s magnetic field is not 2D, but 3D • True pole is actually located in Earth’s core • Results in an angle of dip • Directly above the pole, the angle of dip is 90o • At the equator, the angle is 0o • Can be used to determine latitude Circle of Dip

  32. Magnetosphere • Earth’s magnetic field is not bound to the surface; it extends upwards into space • Magnetosphere: a region of the upper atmosphere beyond 200 km which the motion of charged particles is governed by the Earth’s magnetic field • Solar winds: streams of charged particles that travel away from the sun • extends to 57 000 km towards the sun (10 Earth radii) and hundreds of Earth radii away from the Sun

  33. Auroras • Magnetic field diverts charged particles and protects Earth from radiation • Earth’s magnetic field means that charged particles from solar winds are unable to contact Earth’s surface, except for near the poles • Results in Aurora Borealis and Aurora Australis

  34. Explanation of the Auroras • In 1958, regions of intense radiation were discovered in the magnetosphere by a team headed by Dr. J. Van Allen • contain energetic protons and electrons • create the northern lights • high energy particles from solar winds are trapped in the belts • energetic particles collide with oxygen and nitrogen molecules near the poles • collisions excite the molecules and they emit light we see in the auroras • Interestingly enough, the colours which we see are described by the atomic structure of the atoms • green light - oxygen, pink light - nitrogen

  35. Electromagnetism • Why does electricity keep popping up in the magnetism unit? • Remember how magnetism had to do with electron spins.... And electricity is the movement of electrons, so... Electricity and Magnetism are Related!

  36. Vocabulary • Magnet: object which can attract iron or steel (other magnets) • Magnetic field: the region of space around a magnet where another magnet will experience a force • Domain: small magnet (region) within a larger object • Domain theory: small magnetic domains contribute to the larger magnetic properties of the object • Ferromagnetic: permanent magnet i.e. iron • Paramagnetic: becomes magnetized only in the presence of a magnetic field • Curie point: temperature at which an element loses magnetic properties • Dynamo theory: Molten core is composed primarily of iron which surrounds solid iron core; fluid motion of iron creates magnetic poles • Magnetic declination: angle between pole and geographic north • Angle of dip: the angle beneath Earth’s surface at which the magnetic pole is located • Magnetosphere: a region of the upper atmosphere beyond 200 km which the motion of charged particles is governed by the Earth’s magnetic field

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