# 11.1 Household Magnets. New ideas for today: Magnetism Refrigerator magnets Electromagnets.

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11.1 Household Magnets

New ideas for today: Magnetism Refrigerator magnets Electromagnets

Observations about Household Magnets Two magnets can attract or repel n n Magnets can stick to certain metals n n Magnets affect compasses n n The earth seems to be magnetic n n Some magnets use electricity to operate

Magnetic Poles n n Two types: north & south n n Like poles repel, opposites attract n n Forces increase with decreasing separation n n Analogous to electric charges EXCEPT: n n No isolated magnetic poles ever found! n n Net pole on an object is always zero! Bar magnet

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. A. Attract B. B. Repel C. C. Neither Broken bar magnet

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Magnetic Fields n n A magnetic field pushes on magnetic pole n n The magnitude of the field is proportional to the magnitude of the force on a test pole n 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 compass

Torque 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 dipole N S Magnitude of torque proportional to magnetic field Torque always acts on dipole to line it up with field

Ferromagnetism n n Most atoms are magnetic n n Most materials are not n n Atomic magnetism is perfectly cancelled n n Some materials do not have full cancellation n n Magnetism is usually hidden by randomness n n However, ferromagnets can be permanently magnetized by applied magnetic fields

Refrigerators and Magnets n n A refrigerator’s steel has magnetic domains n n Domains cancel so steel appears nonmagnetic n n When a magnetic pole is near steel n n it causes some domains to grow, others to shrink n n and the steel develops magnetic polarization n n so that it attracts the magnetic pole n n Magnets thus stick to steel refrigerators magnetized steel paperclips Ferrous materials

Electromagnetism I n n Magnetic fields n n Push on magnetic poles n n Electric fields n n Push on electric charges

Electromagnetism I n n Magnetic fields n n Push on magnetic poles n n Bend moving electric charges n n Electric fields n n Push on electric charges

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Earth’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: What is the North Pole? A. A. Magnetic north pole B. B. Magnetic south pole C. C. Neither

Electromagnetism II n n Magnetic fields created by n n Fundamental particles (dipoles) ― electrons, protons, neutrons… n n Electric fields created by n n Charges

Electromagnetism II n n Magnetic fields created by n n Fundamental particles (dipoles) ― electrons, protons, neutrons… n n Moving electric charges (current) n n Electric fields created by n n Charges

Current n n Current is moving positive charge n n Current is measured in coulombs/second or amperes (amps) n n Electric fields cause currents to flow n n Currents produce magnetic fields: Straight wire field

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Magnetic field lines          Electromagnet Magnetic Dipole Single loop field Solenoid field Electromagnet

Other forms of magnetism: Paramagnetism Diamagnetism Perfect diamagnetism (superconductor) Liquid O 2

This is diamagnetism

Supercondutor

See you next class! For next class: Read Sections 11.2, 11.3

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