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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 • 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 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! History • Not a modern concept/Known since ancient times • Greeks (600 BC): Magnesia stone would attract iron • China: used for navigation – http://science.discovery.co m/videos/what-theancients-knew-i-shortschinese-magneticcompass.html • Lodestone (magnetite) 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; May 24, 1544 – November 30, 1603 used in navigation Preliminary Compasses • Leaf, Water, Wire • http://dsc.discovery.com/videos/man-vs-wild1-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? 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 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... 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 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! Bar Magnet Horseshoe Magnet • Think of bending a bar magnet – what would happen? Multiple Bar Magnets Look Familiar? 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 Homework • Magnetic Field Lines Handout 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 What Would a Magnet Look Like if Cut? 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? Creation of Magnets • Ferromagnetism: the phenomenon by which materials become and remain magnetized 1. placing molten paramagnetic material in the vicinity of a large magnetic field and allowing it to cool 2. passing an electric field through the material - More later… 3. rubbing a paramagnetic material with a magnet – demo 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 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 Homework • Domain Theory Handout 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 • 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 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! 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 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 Magnetic Declination Map 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 Circle 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 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 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 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 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... 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