Household Magnets Household Magnets other. If you break this bar in - PDF document

Household Magnets 1 Household Magnets 2 Introductory Question Introductory Question Suppose you have a long, thin bar magnet with Suppose you have a long, thin bar magnet with � a north pole at one end and a south pole at the a north pole at one end and a south pole at the Household Magnets Household Magnets other. If you break this bar in half, the two other. If you break this bar in half, the two new ends–one on each half of the bar new ends new ends new ends one on each half of the bar one on each half of the bar will one on each half of the bar–will will will attract attract A. A. repel repel B. B. neither attract nor repel neither attract nor repel C. C. Household Magnets 3 Observations about Household Magnets 4 Observations about 5 Questions about 5 Questions about Household Magnets Household Magnets Household Magnets Household Magnets � They attract or repel, depending on orientation They attract or repel, depending on orientation � Why do any two magnets attract Why do any two magnets attract and and repel? repel? � � Magnets stick only to certain metals Magnets stick only to certain metals � Why must magnets be close to attract or repel? Why must magnets be close to attract or repel? � � Magnets affect compasses Magnets affect compasses � Why do magnets stick only to some metals? Why do magnets stick only to some metals? � � The earth seems to be magnetic The earth seems to be magnetic � Why does a magnetic compass point north? Why does a magnetic compass point north? � Some magnets use electricity to operate Some magnets use electricity to operate � Why do some magnets use electricity? Why do some magnets use electricity? Household Magnets 5 Household Magnets 6 Question 1 Question 1 Magnetic Pole (Part 1) Magnetic Pole (Part 1) � Why do any two magnets attract Why do any two magnets attract and and repel? repel? � Objects that attract or repel magnetically carry Objects that attract or repel magnetically carry � portions of a physical quantity called magnetic portions of a physical quantity called magnetic � Why don’t they attract Why don’t they attract or or repel? repel? pole or simply “pole” pole or simply “pole” � Why do their orientations affect their forces? Why do their orientations affect their forces? � Pole comes in two types: � Pole comes in two types: Pole comes in two types: Pole comes in two types: � Poles of the same type repel Poles of the same type repel � Poles of different types attract Poles of different types attract � 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 2) Magnetic Pole Magnetic Pole (Part 3) (Part 2) (Part 3) � Magnetic pole Magnetic pole � Every magnet has equal north Every magnet has equal north and and south poles south poles � is a conserved quantity is a conserved quantity � They have magnetic polarizations, not net poles They have magnetic polarizations, not net poles � is analogous to electric charge is analogous to electric charge � A typical bar or button magnet is a magnetic dipole A typical bar or button magnet is a magnetic dipole � There is, however, one big difference: There is, however, one big difference: Th Th i i h h bi diff bi diff � A dipole has one north pole and one south pole A di A dipole has one north pole and one south pole A di l h l h h h l l d d h h l l � no isolated magnetic pole has ever been found! no isolated magnetic pole has ever been found! � A fragment of a magnet A fragment of a magnet � the net pole on any object is always exactly zero! the net pole on any object is always exactly zero! � 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 Introductory Question Introductory Question (revisited) Question 2 Question 2 (revisited) Suppose you have a long, thin bar magnet with Suppose you have a long, thin bar magnet with � Why must magnets be close to attract or repel? Why must magnets be close to attract or repel? � a north pole at one end and a south pole at the a north pole at one end and a south pole at the other. If you break this bar in half, the two other. If you break this bar in half, the two new ends new ends one on each half of the bar new ends new ends–one on each half of the bar one on each half of the bar–will one on each half of the bar will will will attract attract A. A. repel repel B. B. neither attract nor repel neither attract nor repel C. C. Household Magnets 11 Household Magnets 12 Magnetic Forces Magnetic Forces (Part 1) 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 Th Th These magnetostatic magnetostatic forces are i f forces are � their net forces depend on distance and orientation � th ir n t f r 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 2