Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ch. 21: Magnetism Magnetism • • • • magnetic poles produce magnetic forces poles always exist in pairs (N and S) opposite poles attract, like poles repel there are no magnetic “monopoles” Magnetic Fields magnetic field lines (B-field) always point from N to S Big and Little Magnets • currents within the mantle produce the earth’s field • in atoms, orbiting and spinning electrons produce tiny magnetic fields • Fe, Ni, and Co are the most magnetic elements Electric Current and B-Fields • Hans Christian Oersted (1820) first noticed that an electric current will deflect a compass needle • first right hand rule Electric Current and B-Fields • a current in a coil (or solenoid) produces an electromagnet • second right hand rule B I Magnetic Domains • domains are clusters of billions of iron atoms with aligned fields • domains will align in a B-field • permanent magnets have been exposed to very strong fields Magnetic Force • a charged particle moving perpendicular to a B-field feels a force • Fmagnetic q v B 1 Tesla (T) = 1 N/(C · m/s) = N/(A·m) • third right hand rule: Cloud Chamber Images Magnetic Force Problem • A proton moving at 1200 km/s (in the solar wind) runs perpendicular into the earth’s magnetic field (B = 5.5 x 10-5 T). How much force is applied to the proton? Mass Spectrometer • mass spectrometer: an instrument that measures the mass of charged particles • used to identify elements present in a sample Magnetic Force on a Wire • a current-carrying wire in a B-field will feel a force perpendicular to the wire Fmagnetic q v B Fmagnetic B I L • How much force is applied to a 5-cm long wire carrying 12 A of current when it is placed in a 3 mT magnetic field? Chapter 22: Induction and Alternating Current Magnetic Fields and EMFs • electromagnetic induction: the production of a current caused when a conductor is moved through a magnetic field (or the magnetic field is changed) • emf: electromotive force; an increase in PE per charge (voltage) that pushes charges through a conductor; emf produces a current • Use the 3rd right hand rule to determine direction of current. Lenz’s Law • Lenz’s law: the magnetic field of an induced current opposes the change in the applied magnetic field • energy is conserved due to this “magnetic friction” Faraday’s Law [ AB cos ] emf N t • • • • N = number of loops A = area B = magnetic field t = time Generators and Motors • generator: converts KE to electrical energy (current) • spinning a coil in a B-field causes an AC to form • the “commutator” determines if AC or DC Electric Motors • motor: a device that converts electric energy (AC or DC) to KE Transformers • transformer: converts AC to higher or lower voltage (step up or step down) • V2 = V1N2 / N1 • Electricity is transmitted at high V, low I (due to “I2R loss”) then stepped down • 230kV to 20kV to 120V