Transcription of Lecture 8: Magnets and Magnetism - Montana State …
1 Lecture 8: Magnets and MagnetismMagnets Materials that attract othermetals Three classes: natural, artificialand electromagnets Permanent or Temporary CRITICAL to electric systems: Generation of electricity Operation of motors Operation of relaysMagnets Laws of magnetic attraction and repulsion Like magnetic poles repel each other Unlike magnetic poles attract each other Closer together, greater the forceMagnetic Fields and Forces Magnetic lines of force Lines indicating magnetic field Direction from N to S Density indicates strength Magnetic field is region whereforce existsMagnetic TheoriesMolecular theory ofmagnetismMagnets can be split intotwo magnetsMagnetic TheoriesMolecular theory ofmagnetismSplit down to molecularlevelWhen unmagnetized,randomness, fields cancelWhen magnetized, order.
2 Fields combineMagnetic TheoriesElectron theory of Magnetism Electrons spin as they orbit(similar to earth) Spin produces magnetic field Magnetic direction depends ondirection of rotation Non- Magnets equalnumber of electrons spinningin opposite direction Magnets more spin one way than otherElectromagnetism Movement of electric chargeinduces magnetic field Strength of magnetic fieldincreases as current increasesand vice versaRight Hand Rule (Conductor) Determines direction ofmagnetic field Imagine grasping conductorwith right hand Thumb in direction of currentflow (not electron flow) Fingers curl in the direction ofmagnetic fieldDO NOT USE LEFT HAND RULE IN BOOKE xampleRE (V)Draw magnetic field lines aroundconduction pathAnother Example Draw magnetic field linesaround conductorsConductorcurrent into pageConductorcurrent out of pageConductor coils Single conductor not very useful Multiple winds of a conductor required for mostapplications, electromagnet, motors, solenoids Strength of magnetic field now dependent oncurrent magnitude and number of turnsRight Hand Rule (Coil) Imagine grasping coil with right hand Fingers in direction of current flow(not electron flow)
3 Thumb points in direction of magneticfield through coilDO NOT USE LEFT HAND RULE IN BOOKC reateselectromagnetExample Draw magnetic field lines through andaround coilMagnetic Force on MovingCharge A magnetic field has a force on a moving charge Lorentz Force Law (don't need to know, just tellingyou)Two right hand which one is bestfor like the one on the direction of force on conductorsNSNSA nother ExampleWhat about mutual force on conductors due toinduced magnetic fields?Force RepelsForce AttractsMagnetic Circuits Magnetic Flux in circuit similar to current Unit: Maxwells (Mx) = 1 magnetic line of force. Magnetomotive Force (mmf) similar to voltage Unit: Gilberts (Gb) = the mmf that will establish aflux of 1 Mx in a magnetic circuit having areluctance (rel) of 1 unit.
4 In electromagnets mmf is proportional to coilcurrent and number of turns Reluctance (rel) is similar to resistance Material's opposition to magnetic flux Permeance is similar to conductance inverse of reluctance Material's ability to conduct magnetic fluxMagnetic Circuits Permeability of air is low (high reluctance) Permeability of soft iron is high (lowreluctance)Majority of reluctance inair gapMagnetic CircuitsRE (V)IAnalogouscircuit
