Household Magnets Household Magnets Magnets stick only to certain - PDF document

Household Magnets 2 Observations about Household Magnets 1 Observations about Household Magnets Household Magnets � They attract or repel, depending on orientation They attract or repel, depending on orientation Household Magnets Household Magnets � Magnets stick only to certain metals Magnets stick only to certain metals � Magnets affect compasses Magnets affect compasses � The earth seems to be magnetic The earth seems to be magnetic � Some magnets use electricity to operate Some magnets use electricity to operate Household Magnets 3 Household Magnets 4 5 Questions about 5 Questions about Question 1 Question 1 Household Magnets Household Magnets � Why do any two magnets attract Why do any two magnets attract and and repel? repel? � Why do any two magnets attract Why do any two magnets attract and and repel? repel? � Why must magnets be close to attract or repel? Why must magnets be close to attract or repel? � Why don’t they attract Why don’t they attract or or repel? repel? � Why do their orientations affect their forces? Why do their orientations affect their forces? � Why do magnets stick only to some metals? Why do magnets stick only to some metals? � Why does a magnetic compass point north? Why does a magnetic compass point north? � Why do some magnets use electricity? Why do some magnets use electricity? Household Magnets 5 Household Magnets 6 Magnetic Pole (Part 1) Magnetic Pole Magnetic Pole Magnetic Pole (Part 2) (Part 1) (Part 2) � Objects that attract or repel magnetically carry Objects that attract or repel magnetically carry � Magnetic pole Magnetic pole portions of a physical quantity called magnetic portions of a physical quantity called magnetic � is a conserved quantity is a conserved quantity pole or simply “pole” pole or simply “pole” � is analogous to electric charge is analogous to electric charge � Pole comes in two types: � Pole comes in two types: Pole comes in two types: Pole comes in two types: � There is, however, one big difference: Th There is, however, one big difference: Th i i h h bi diff bi diff � Poles of the same type repel Poles of the same type repel � no isolated magnetic pole has ever been found! no isolated magnetic pole has ever been found! � Poles of different types attract Poles of different types attract � the net pole on any object is always exactly zero! the net pole on any object is always exactly zero! � The two types are named “north” and “south” The two types are named “north” and “south” = − north pole south pole = − net pole north pole south pole 1

Household Magnets 7 Household Magnets 8 Magnetic Pole Magnetic Pole (Part 3) Question 2 Question 2 (Part 3) � Every magnet has equal north Every magnet has equal north and and south poles south poles � Why must magnets be close to attract or repel? Why must magnets be close to attract or repel? � They have magnetic polarizations, not net poles They have magnetic polarizations, not net poles � A typical bar or button magnet is a magnetic dipole A typical bar or button magnet is a magnetic dipole � A dipole has one north pole and one south pole A dipole has one north pole and one south pole A di A di l h l h h h l l d d h h l l � A fragment of a magnet A fragment of a magnet � has a net pole of zero has a net pole of zero � retains its original magnetic polarization retains its original magnetic polarization � is typically a magnetic dipole is typically a magnetic dipole Household Magnets 9 Household Magnets 10 Magnetic Forces (Part 1) Magnetic Forces Magnetic Forces (Part 2) Magnetic Forces (Part 1) (Part 2) � Two poles push or pull on one another Two poles push or pull on one another � Since a magnet is a dipole (or more complicated) Since a magnet is a dipole (or more complicated) � with forces that are exactly equal in magnitude with forces that are exactly equal in magnitude � it has both north and south poles it has both north and south poles � but exactly opposite in direction. but exactly opposite in direction. � it simultaneously attracts and repels a second magnet it simultaneously attracts and repels a second magnet � These These magnetostatic Th Th magnetostatic forces are i f forces are � th ir n t f r � their net forces depend on distance and orientation their net forces depend on distance and orientation th ir n t f r d p nd n di t n d p nd n di t n nd ri nt ti n nd ri nt ti n � their net forces decrease precipitously with distance their net forces decrease precipitously with distance � proportional to the amount of each charge proportional to the amount of each charge � they may also experience net torques they may also experience net torques � inversely proportional to (distance between charges) inversely proportional to (distance between charges) 2 � The forces increase as the separation decreases The forces increase as the separation decreases ⋅ ⋅ permeability of free space pole pole force = 1 2 ⋅ 4 π (distance between poles) 2 Household Magnets 11 Household Magnets 12 Question 3 Question 3 Magnetism in Atoms Magnetism in Atoms � Why do magnets stick only to some metals? Why do magnets stick only to some metals? � Magnetism is due primarily to electrons Magnetism is due primarily to electrons � Electrons are intrinsically magnetic Electrons are intrinsically magnetic— —they’re dipoles they’re dipoles � Atoms contain electrons, so atoms can be magnetic Atoms contain electrons, so atoms can be magnetic � When electrons assemble into atoms, Wh When electrons assemble into atoms, Wh l l bl i bl i � their magnetic dipoles often cancel one another their magnetic dipoles often cancel one another � but this cancellation is usually incomplete but this cancellation is usually incomplete � so most atoms are magnetic. so most atoms are magnetic. 2