lecture 13 2 26 2004
TRANSCRIPT
Magnetism
• Similar to electric forces• Magnetic Poles • North and South Poles• Historic Designations• Like Poles Repel, Unlike Poles Attract• Due to alignment of atoms in material
Magnetic Fields
• Similar to electric and gravitational fields• Magnetic fields are produced by the
MOTION of electric charges
Magnetic Fields
• Revolving electron produces field• Spinning electron produces field• In most atoms, combination of spins and
orbits are in opposite directions• The fields cancel each other out• Some atoms, they don’t cancel• Iron, nickel, cobalt, aluminum
Magnetic Domains
• In iron, for example, field by an atom is so strong it causes neighboring atoms to arrange their spins and orbits to line up
• These clusters form magnetic domains• Domains are randomly oriented in ordinary
iron, nickel, etc.• Can force them to line up with external
field
Electric Currents & Magnetism
• A moving charge produces a magnetic field
Electric Currents & Magnetism
Electromagnetics
• Place a piece of unmagnetized iron in a coil of wire carrying a current
• Produce a strong magnetic field in the iron by lining up domains
• Makes the whole thing a very powerful magnet
Magnetic Forces and Charges
• If a charge isn’t moving, a magnetic field has no effect on the charge
• Reciprocal kind of idea• Moving charges produce magnetic fields• Magnetic fields apply a force to a moving
charge
Magnetic Forces and Charges
F qvB
Magnetic Forces and Charges
Magnetic Forces and Charges
Meters
• Use a compass needle (magnet)
Meters
Motors
Electromagnetic Induction
Electromagnetic Induction
Electromagnetic Induction
Electromagnetic Induction
Electromagnetic Induction
• Faraday’s Law• The induced voltage in a coil is
proportional to the number of loops, multiplied by the rate at which the magnetic field changes within those loops.
• The amount of current depends on the resistance of the wire
• Changing magnetic field in a loop is key!!
Electromagnetic Induction
• Traffic light control (buried loops)• Airport Security Systems• Mag strips on credit cards• Tape Recorders• Computer Hard Drives
Generators
Generators
Power Production
Transformers
Transformers
• The more turns, the more voltage induced• Step-up Transformer has more turns in
secondary than in primary• Step-down Transformer has more turns in
primary than in secondar
Transformers
• Energy Conservation• Power In = Power Out
Vin Iin Vout Iout
Field Induction
• An electric field is induced in any region of space in which a magnetic field is changing with time
• A magnetic field is induced in any region of space in which an electric field is changing with time
• “Let there be light!!!”