Magnetism Chapter 19

Introduction Applications involving magnetism Loudspeakers Meters Electromagnets Recording devices Tapes Computer disks MRI

Important Concepts All magnetic fields are produced by charges in motion. Magnetic fields will affect charges moving through them.

Magnetic Poles Magnets have poles. North South 191

The Law of Poles Like poles repel, unlike poles attract.

Can a magnet have only one pole? No Can a magnet have more than two poles? Yes

Effects of Cutting? If a magnet is cut in half, lengthwise, will the two halves attract or repel? Repel If a magnet is cut in half, across the width, will the two halves attract or repel? Attract 36-2

Magnetic Induction Magnetism may be induced in iron and other materials. Stroking with a permanent magnet Heating and then cooling in a strong magnetic field

Natural Magnets Magnetite (lodestone) A naturally occurring magnetic material Produced by heating and then cooling in the earth’s magnetic field

Soft Magnetic Materials Iron Easily magnetized Weakens quickly

Hard Magnetic Materials Cobalt and nickel Difficult to magnetize Stays strong longer

Magnetic Fields All charges are surrounded by an electric field. Charges in motion are surrounded by a magnetic field.

Magnet Fields Magnitude (strength) Direction Field descriptions must include: Magnitude (strength) Direction The direction that an isolated north pole would move when placed in the field 19.2, 192

Magnetic Field Lines Magnetic field lines can be shown by using small iron filings 79

Magnetic Field Of The Earth North-seeking and south seeking poles Compasses Geographic poles vs. magnetic poles Location (north magnetic is in Canada) Dip angle Magnetic declination The angle between true north and magnetic north 19.4

What is believed to be the source of the earth’s magnetic field? The earth’s magnetic field reverses every few million years Evidence?

Animals and Magnetism Some animals use the earth’s magnetic field for navigation. Anaerobic bacteria Lobsters Birds

Magnetic Forces A charge experiences a force when moving through a magnetic field. Maximum force occurs when the motion is perpendicular to the field. Zero force occurs when the motion is parallel or antiparallel to the field. 19.5

Factors Influencing Magnetic Force Which factors will determine the magnitude of the force on a charge that is moving through a magnetic field? Magnetic field strength Quantity of charge Velocity of the charge Direction of the velocity

Magnetic Symbols New Symbols Formula Magnetic force (F) Magnetic field strength (B) Angle (q) (measured between v and B) Formula

Magnetic Field Strength Magnetic field strength units SI Tesla (T) or (1 Wb/m2)

Magnetic Force Formula Right Hand Rule 19.6/19.7, 198

Magnetic Force On A Current Carrying-Conductor A current-carrying wire will experience a force when placed in a magnetic field. Symbols for current direction Head Tail 26

Magnetic Field Around a Conductor

Magnetic Force On A Straight Wire Formula There is no force when the current in the wire is in the same or opposite direction of the field.

Magnetic Force Applications Stereo speaker Voice coil Electromagnetic pump No moving parts Possible heart replacement Magnetohydrodynamic propulsion (MHD) 199

Torque On A Current Loop A torque is exerted on a current-carrying loop when it is placed in a magnetic field Applications Galvanometers Motors Generators 19.15, 19.17, 200, 201

Torque formula Shape of coil is not important Angle (q) is measured between B and the axis of the coil Number of Turns (N)

Motion Of A Charged Particle In A Magnetic Field If the velocity is perpendicular to the field, the magnetic force is always toward the center of the circular path. Centripetal acceleration No change in speed Direction change 195, 80, 19.19

Curved Motion in a Magnetic Field Radius of curvature formula: 197

QUESTIONS 1 – 8 Pg. 654

Electricity and Magnetism Oersted lecture An electric field deflected a nearby compass needle This was the first realization of a link between electricity and magnetism

Magnetic Field Of A Long Straight Wire Oersted demonstration Compasses surrounding a current-carrying wire The magnetic field lines form circles around the wire. 19.23a,b

Another Right-Hand Rule The Right-Hand-Rule for a current Indicates the directions for current, magnetic field , and force 19.23a

Magnetic Field Strength Formula for the magnetic field strength due to a current in a long straight wire mo is the permeability of free space mo = 4p x 10-7 T.m/A

Magnetic Force Between Two Parallel Conductors Two parallel current-carrying conductors will exert a force on each other. (very important) 19.28

Defining The Ampere Definition of the Ampere (1 A) Two long parallel wires One meter apart Same current in each Force/length = 2 x 10-7 N/m Remember: 1 Ampere = 1 Coulomb/second

Magnetic Field Of A Current Loop A current loop can increase the strength of the magnetic field. Current loops have poles. 203, 19.28, 19.31

Magnetic Field Of A Solenoid A solenoid is made up of several closely spaced loops. Electromagnets are solenoids Applications Solenoids have poles Field lines are nearly parallel within 204, 19.33, 19.32, 158, 159, 207

Formula for the magnetic field of a solenoid: n = N/L

Causes of Magnetism Orbital motion produces magnetic fields. These fields tend to cancel out.

Electron spin produces magnetic fields In most substances, electron pairs have opposite spins which cancel. Not in iron, nickel and cobalt

Magnetic Domains Orientation Alignment Growth

Ferromagnetic Materials Ferromagnetic materials are able to be magnetized. (permanent magnets) Magnetic cores are used in electromagnets. 206

Questions 10 - 12, 14 - 17 Pg. 655