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Lecture 8: Magnets and Magnetism

Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

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Page 1: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Lecture 8:Magnets and Magnetism

Page 2: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnets•Materials that attract othermetals

•Three classes: natural, artificialand electromagnets

•Permanent or Temporary•CRITICAL to electric systems:

– Generation of electricity– Operation of motors– Operation of relays

Page 3: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnets

•Laws of magnetic attraction and repulsion–Like magnetic poles repel each other–Unlike magnetic poles attract each other–Closer together, greater the force

Page 4: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic Fields and Forces•Magnetic lines of force

– Lines indicating magnetic field– Direction from N to S– Density indicates strength

•Magnetic field is region whereforce exists

Page 5: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic TheoriesMolecular theory ofmagnetism

Magnets can be split intotwo magnets

Page 6: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic TheoriesMolecular theory ofmagnetism

Split down to molecularlevel

When unmagnetized,randomness, fields cancel

When magnetized, order,fields combine

Page 7: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic 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 other

Page 8: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Electromagnetism

•Movement of electric chargeinduces magnetic field

•Strength of magnetic fieldincreases as current increasesand vice versa

Page 9: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Right 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 field

DO NOT USE LEFT HAND RULE IN BOOK

Page 10: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Example

RE (V)

Draw magnetic field lines aroundconduction path

Page 11: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Another Example•Draw magnetic field linesaround conductors

Conductorcurrent into page

Conductorcurrent out of page

Page 12: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Conductor coils•Single conductor not very useful•Multiple winds of a conductor required for mostapplications,

– e.g. electromagnet, motors, solenoids•Strength of magnetic field now dependent oncurrent magnitude and number of turns

Page 13: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Right Hand Rule (Coil)•Imagine grasping coil with right hand•Fingers in direction of current flow(not electron flow)•Thumb points in direction of magneticfield through coil

DO NOT USE LEFT HAND RULE IN BOOK

Createselectromagnet

Page 14: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Example•Draw magnetic field lines through andaround coil

Page 15: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic 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 rules.

Choose which one is bestfor you.

I like the one on the right.

Page 16: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

ExampleDraw direction of force on conductors

N S N S

Page 17: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Another ExampleWhat about mutual force on conductors due toinduced magnetic fields?

Force Repels Force Attracts

Page 18: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic 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.

– 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 flux

Page 19: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic Circuits•Permeability of air is low (high reluctance)•Permeability of soft iron is high (lowreluctance)

Majority of reluctance inair gap

Page 20: Lecture 8: Magnets and Magnetism - Montana State University...Electron theory of magnetism •Electrons spin as they orbit (similar to earth) •Spin produces magnetic field •Magnetic

Magnetic CircuitsR

E (V)

IAnalogouscircuit