11.1 Household Magnets11.1 Household Magnets

New ideas for today:New ideas for today:

• Magnetism• Refrigerator magnets• Electromagnets

Observations aboutObservations aboutHousehold MagnetsHousehold Magnets

Two magnets can attract or repeln Magnets can stick to certain metalsn Magnets affect compassesn The earth seems to be magneticn Some magnets use electricity to operate

Magnetic PolesMagnetic Poles

n Two types: north & south

n Like poles repel, opposites attract

n Forces increase with decreasing separationn Analogous to electric charges EXCEPT:

n No isolated magnetic poles ever found!n Net pole on an object is always zero!

Bar magnetBar magnet

Clicker Question:Clicker Question:Suppose you have a long bar magnet with a north pole at one end and a south pole at the other. If you break it in half, will the two new ends:

A. AttractB. RepelC. Neither

Broken bar magnetBroken bar magnet

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Magnetic FieldsMagnetic Fieldsn A magnetic field pushes on

magnetic pole n The magnitude of the field is

proportional to the magnitude of the force on a test pole

n The direction of the field is the direction of the force on a north test pole

But isolated magnetic poles don’t seem to exist!

Bar magnet and compassBar magnet and compass

Torque on a dipoleTorque on a dipole

N

S

•Magnitude of torque proportional to magnetic field

•Torque always acts on dipole to line it up with field

Torque on a dipoleTorque on a dipole

N

S

•Magnitude of torque proportional to magnetic field

•Torque always acts on dipole to line it up with field

FerromagnetismFerromagnetismn Most atoms are magneticn Most materials are not

n Atomic magnetism is perfectly cancelledn Some materials do not have full cancellation

n Magnetism is usually hidden by randomnessn However, ferromagnets can be permanently

magnetized by applied magnetic fields

Refrigerators and MagnetsRefrigerators and Magnetsn A refrigerator’s steel has magnetic domainsn Domains cancel so steel appears nonmagneticn When a magnetic pole is near steel

n it causes some domains to grow, others to shrinkn and the steel develops magnetic polarizationn so that it attracts the magnetic pole

n Magnets thus stick to steel refrigerators

magnetized steel magnetized steel paperclipspaperclips

Ferrous materialsFerrous materials

Electromagnetism IElectromagnetism I

n Magnetic fieldsn Push on magnetic poles

n Electric fieldsn Push on electric charges

Electromagnetism IElectromagnetism I

n Magnetic fieldsn Push on magnetic poles n Bend moving electric charges

n Electric fieldsn Push on electric charges

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TVTV

Earth’s magnetic fieldEarth’s magnetic field

Near surface of the earth: 0.00005 Tesla

Charged particles from the sun spiral around magnetic field lines and crash into the atmosphere near the poles!

Clicker Question:Clicker Question:What is the North Pole?

A. Magnetic north poleB. Magnetic south poleC. Neither

Electromagnetism IIElectromagnetism II

n Magnetic fields created byn Fundamental particles (dipoles) ― electrons,

protons, neutrons…

n Electric fields created byn Charges

Electromagnetism IIElectromagnetism II

n Magnetic fields created byn Fundamental particles (dipoles) ― electrons,

protons, neutrons…n Moving electric charges (current)

n Electric fields created byn Charges

CurrentCurrentn Current is moving positive chargen Current is measured in coulombs/second or

amperes (amps)n Electric fields cause currents to flown Currents produce magnetic fields:

Straight wire fieldStraight wire field

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Magnetic field linesMagnetic field lines

Electromagnet Magnetic

Dipole

Single loop fieldSingle loop field

Solenoid fieldSolenoid field

ElectromagnetElectromagnet

Other forms of magnetism:Other forms of magnetism:

• Paramagnetism• Diamagnetism• Perfect diamagnetism (superconductor)

Liquid OLiquid O22

This is diamagnetismThis is diamagnetism

SupercondutorSupercondutor

See you next class!

For next class: Read Sections 11.2, 11.3