Magnetism and Magnetic Force Unit 10 Lecture 1 AP Physics

Magnets

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 history Somewhere 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 Gilbert Physician to Elizabeth I Published 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: Overview Has two “ends” or poles, called N and S Opposite poles attract; like poles repel

Magnets: Overview However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.

Sources of Magnetism A strongly magnetized bar consists of aligned domains.

Sources of Magnetism A broken magnet

Ferromagnetic material The domains persist when the external magnetic field is removed Also called “hard” magnets or permanent

Paramagnetic material Domains can align with an external magnetic field The domains do not persist when the external magnetic field is removed Also called “soft” magnets

Nonmagnetic material Atom has no net magnetic field so there can be no domain Exhibits no magnetic effects Example: copper, gold and silver

Magnets: Overview Magnetism can be destroyed by heat and restored again by cooling.

Magnetic Earth The earth acts as a giant magnet. Geographic north is a south magnetic pole Freely 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

Magnetic Declination

20.1 Magnets and Magnetic Fields Magnetic fields can be visualized using magnetic field lines, which are always closed loops.

20.1 Magnets and Magnetic Fields A uniform magnetic field is constant in magnitude and direction. The field between these two wide poles is nearly uniform.

Magnetic Forces Hans Christian Ørsted 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 Particle A charged particle (or current) experiences a magnetic force when it is moving through, but not parallel with, a magnetic field.

Magnetic Force The 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 Rule Point your fingers in direction of velocity Rotate wrist so you can sweep your fingers in the direction of the magnetic field without moving your palm Thumb points in the direction of the Force.

Your turn A positive charge is moving south in a magnetic field that points west. What is the direction of the force on the charge? North South East West Up Down There is no force Correct Answer: 6

Your turn A positive charge is moving upwards in a magnetic field that points south. What is the direction of the force on the charge? North South East West Up Down There is no force Correct Answer: 3

Your turn A negative charge is moving west in a magnetic field that points north. What is the direction of the force on the charge? North South East West Up Down There is no force Correct Answer: 5

Your turn A positive charge is moving east in a magnetic field and the force is downwards. What is the direction of the field? North South East West Up Down There is no force Correct Answer: 2

Your turn A 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 -y There is no force Correct Answer: 7

Your turn An 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 -y There is no force Correct Answer: 6

Magnetic Field Defined The magnetic field strength is defined in terms of the magnitude of the force that is produced on a charged object.

Magnetic Field Defined

Magnetic Fields Source Field (Gauss) Earth 0.5 Appliance 10 Bar magnet 100 Human limit 2000 electro-magnet 50,000

Example 1 A 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 Conductors A 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 B Arrows are used to represent the magnetic field.

Force on a wire If the current is not perpendicular to the field B, then the force will be

Aurora Borealis

Application The speaker

Motion of Charged Particle Since F is always perpendicular to v, the motion is circular

Application Mass Spectrometer

Applications Animals and Magnetism: http://videos.howstuffworks.com/hsw/9511-understanding-magnetism-animals-and-magnetism-video.htm Other topics: http://videos.howstuffworks.com/search.php?terms=magnetism&x=0&y=0&media=video