Magnetism and Magnetic ForceUnit 10 Lecture 1 AP Physics
A bit of history The ancient Greeks knew about "lodestones (rare natural magnets) with the power to attract iron. One site where such stones were found was near the city of Magnesia in Asia Minor (now Turkey), and from that (perhaps) came the term "magnetism." But they were not the first to understand how to use it.
A bit of historySomewhere around 500 ACE, the Chinese built a primitive compass by placing a lodestone on a boat in a bowl of water. The boat always rotated to point in the same direction, no matter where or how the experiment was performed. The boat was never pulled in any direction: it only rotated.
William GilbertPhysician to Elizabeth IPublished On the Magnet and Magnetic Bodies in 1600. First understood that the earth was itself a magnet. Summarized all of the knowledge of magnetism up to his day.
Magnets: OverviewHas two ends or poles, called N and SOpposite poles attract; like poles repel
Magnets: OverviewHowever, if you cut a magnet in half, you dont get a north pole and a south pole you get two smaller magnets.
Sources of MagnetismA strongly magnetized bar consists of aligned domains.
Sources of MagnetismA broken magnet
Ferromagnetic materialThe domains persist when the external magnetic field is removedAlso called hard magnets or permanent
Paramagnetic materialDomains can align with an external magnetic fieldThe domains do not persist when the external magnetic field is removedAlso called soft magnets
Nonmagnetic materialAtom has no net magnetic field so there can be no domainExhibits no magnetic effectsExample: copper, gold and silver
Magnets: OverviewMagnetism can be destroyed by heat and restored again by cooling.
Magnetic EarthThe earth acts as a giant magnet. Geographic north is a south magnetic poleFreely suspended magnets will align with this earth magnet.A compass will point downwards near the north pole. The magnetic pole is not located at the geographic pole. There is a difference!
The earth as a magnet
20.1 Magnets and Magnetic FieldsMagnetic fields can be visualized using magnetic field lines, which are always closed loops.
20.1 Magnets and Magnetic FieldsA uniform magnetic field is constant in magnitude and direction.The field between these two wide poles is nearly uniform.
Magnetic ForcesHans Christian rsted is credited with discovering the link between current and magnetic forces. (1820) He was giving a demonstration of the battery when he noticed that a compass always reacted when he turned the electricity off and on.
Force of a Charged ParticleA charged particle (or current) experiences a magnetic force when it is moving through, but not parallel with, a magnetic field.
Magnetic ForceThe vector force is given by a vector equation:
The magnitude of the force is given by:
The direction of the force is given by the right hand rule
Right Hand RulePoint your fingers in direction of velocityRotate wrist so you can sweep your fingers in the direction of the magnetic field without moving your palmThumb points in the direction of the Force.
Your turnA positive charge is moving south in a magnetic field that points west. What is the direction of the force on the charge?NorthSouthEast WestUpDownThere is no forceCorrect Answer: 6
Your turnA positive charge is moving upwards in a magnetic field that points south. What is the direction of the force on the charge?NorthSouthEast WestUpDownThere is no forceCorrect Answer: 3
Your turnA negative charge is moving west in a magnetic field that points north. What is the direction of the force on the charge?NorthSouthEast WestUpDownThere is no forceCorrect Answer: 5
Your turnA positive charge is moving east in a magnetic field and the force is downwards. What is the direction of the field?NorthSouthEast WestUpDownThere is no forceCorrect Answer: 2
Your turnA positive charge is moving along the +x axis in a magnetic field that points in the x direction. What is the direction of the force on the charge?+z-z+x -x+y-yThere is no forceCorrect Answer: 7
Your turnAn electron is moving along the +x axis in a magnetic field that points in the z direction. What is the direction of the force on the charge?+z-z+x -x+y-yThere is no forceCorrect Answer: 6
Magnetic Field DefinedThe magnetic field strength is defined in terms of the magnitude of the force that is produced on a charged object.
Magnetic Field Defined
SourceField (Gauss)Earth0.5Appliance10Bar magnet100Human limit2000electro-magnet50,000
Example 1A proton moves at 8.0E6 m/s along the x-axis. It enters a magnetic field of B=2.5 T, directed at an angle of 60 from the x-axis in the xy plane.What is the magnetic force on the proton?What is the acceleration of the proton?
Forces on ConductorsA current-carrying conductor is a stream of moving charges. There will be a force on the conductor if it is placed in a magnetic field.
Representing BArrows are used to represent the magnetic field.
Force on a wireIf the current is not perpendicular to the field B, then the force will be
Motion of Charged ParticleSince F is always perpendicular to v, the motion is circular
ApplicationsAnimals and Magnetism: http://videos.howstuffworks.com/hsw/9511-understanding-magnetism-animals-and-magnetism-video.htmOther topics:http://videos.howstuffworks.com/search.php?terms=magnetism&x=0&y=0&media=video