Magnetic Fields! Ch 29 - Magnetic Fields & Sources! Magnets...! Earth s Magnetic Field!

Transcription

1 Mgnetic Fields Ch 29 - Mgnetic Fields & ources 1. The mgnetic field line hs the direction of the mgnetic field s its tngent t tht point. 2. The number of lines per unit re is proportionl to the mgnitude of the mgnetic field t tht point. 3. The direction of the mgnetic field is defined s the direction the north pole of compss needle would point when plced t tht position. 1.) Mgnets... 3.) Erth s Mgnetic Field...re mde of ferromgnetic elements: iron, coblt, nickel, gdolinium... Mgnets he north pole nd south pole. 2.) Physlet )

2 Mgnetic Domins Exmple 1 dentify the missing ector in these digrms. F F 5.) 7.) Definition: Mgnetic Field F = q F = qsin The units of mgnetic field re the Tesl, = / C m/s. 1 T = 10,000 Guss. Erth s mgnetic field = µt. Wht hppens when chrged prticle moes // to the mgnetic field? When chrged prticle moes t some ngle reltie to the mgnetic field, the mgnetic force cts perpendiculr to both nd, s predicted by the Right Hnd Rule. 6.) q x fine print The direction of force gien by the RHR is for positie test chrge. Adjust s necessry to predict the direction of force on negtie chrge,. For electric fields, the F e ws prllel to the E field; for mgnetic fields, the F is perpendiculr to the field. F cts only when the chrge is in motion. F does no Work on the prticle when it s treling through constnt field (becuse F nd x re perpendiculr to ech other). 8.)

3 Exmple 2 A proton moing upwrd with speed of 5.0e6 m/s in mgnetic field feels force of 8.0e-14 to the west. When moing horizontlly to the, it feels zero force. Find the mgnitude nd direction of the mgnetic field in this region. Currents produce Mgnetism? 1820, Hns Christin Oersted: moing chrges produce mgnetic field. The direction of the field is determined using RHR. 9.) 11.) Chrges moing in field F = q (when nd re perpendiculr) F c = m 2 r r = m q F Exmple 3 Predict the orienttion of the compss needles. r = m q, so r = q m = q m And... since T=2π/, you should esily be ble to clculte the period. 10.) 12.)

4 Demo sed on wht we e lredy lerned, wht do you think hppens in this sitution? Exmple 4 A 12 cm length of wire crrying 30-Amp current runs through the poles of mgnet t n ngle of 60 reltie to the mgnetic field (s shown). f the mgnetic field is 0.90 T between the poles of the mgnet, wht force does the wire feel, nd in which direction? 13.) 15.) Force on current-crrying wire F = Q drift A q L Q = qnal F = qnal drift = nq drift A F = L F = Lsin" Exmple 5 A squre loop of wire(10-cm ech side) hngs erticlly, s shown here. When the current in the wire is A counterclockwise, scle supporting the wire mesures downwrd force of 3.48e-2 down. Find the mgnitude of the mgnetic field. 14.) 16.)

5 Limittions of F=Lx df = ds or df = L d F = b " ds or F = " L d b b Torque on Current Loop A F = L = b = rf sin" = 2( 2 bsin") = Asin" = A # L b 17.) 19.) Torque on Current Loop Torque on Current Loop b Top View ide View F = L = b = r " F # = 2 & % ( F = b = A $ 2' Another wy of expressing this torque is by using the expression = µ ", where µ is" A the mgnetic moment of the coil:" µ=a f the loop of wire hs more thn " single coil, the torque is expressed s" = n µ " 18.) 20.)

6 Exmple 6 A rectngulr coil of dimensions 5.40 cm x 8.50 cm consists of 25 turns of wire, crrying current of 15.0 ma.. Clculte the mgnetic moment of the coil 1.72e-3 Am 2 b. f mgnetic field of T is pplied prllel to the plne of the loop, wht is the mgnitude of the torque cting on the loop? 6.02e-4 m c. Clculte the mgnitude of the torque on the coil when the T mgnetic field mkes ngles of 60 nd 0 with µ. 5.21e-4 m, 0 Electric Genertors Electric genertors conert mechnicl energy (work proided by n externl source) to electricl energy. Motors nd genertors, in most cses, he the sme physicl structure. 21.) 23.) Electric Motors Electric motors conert electricl energy to kinetic energy, nd re creted by plcing current-crrying loop in n externl mgnetic field. There re number of different wys of doing this, but here is one common type: Helicl motion Most of our exmples of moing chrge he inoled prticles with elocity perpendiculr to the mgnetic field. Wht if this elocity is not perpendiculr to the field? 22.) 24.)

7 The Lorentz Force F Lorentz = F E + F F Lorentz = q E + q Mss pectrometer = E 1 r = m q 2 m q = r 1 2 E prticle source slit P r E Physlet ) 27.) Velocity elector Cyclotron prticle source slit E y djusting the mgnitudes of E nd, we cn get: q = qe = E 26.) 28.)

8 The Hll Effect 3 Types of Mgnetism Ferromgnetism - mteril hs permnent mgnetic moment, due to microscopic domins in which moments re ligned. Prmgnetism - mterils hs smll mgnetic susceptibility, tht only becomes eident when plced in n externl mgnetic field Dimgnetism - mteril does not he permnent mgnetic moments. n the presence of n externl mgnetic field, wek mgnetic moment is induced in direction opposite to the externl field. 29.) 31.) The electron & its properties Chemistry & physics students redily tlk bout the electron, the negtiely chrged prticle tht surrounds the nucleus of the tom nd which is responsible for crrying chrges through wires. ts existence wsn t een suggested until the 1890s, howeer. A cuum tube such s the one shown here cn be used to demonstrte tht the cthode (negtie) terminl produces cthode rys... but wht re those rys, relly? 30.)