Transcript Magnetism - San Francisco State University

Magnetism
Magnetism
Earliest ideas
• Associated with naturally occurring magnetic materials
(lodestone, magnetite)
• Characterized by “poles” - “north seeking” and “south
seeking”
• Other magnetic materials - iron, cobalt, nickel
(ferromagnetic)
Modern view
• Associated with magnetic fields
• Field lines go from north to south poles
Magnetic poles and fields
• Magnetic fields and poles
inseparable
• Poles always come in
north/south pairs
• Field lines go from north
pole to south pole
• Like magnetic poles
repel; unlike poles attract
Electric currents and magnetism
• Moving charges
(currents) produce
magnetic fields
• Shape of field
determined by
geometry of current
– Straight wire
– Current loops
– Solenoid
Earth’s magnetic field
• Shaped and oriented as if
huge bar magnet were inside
– South pole of magnet near
geographic north pole
• Geographic North Pole and
north magnetic pole different
– Magnetic declination = offset
Electromagnetism
Electromagnet
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Loops of wire formed into cylindrical coil (solenoid)
Current run through coil produces a magnetic field
Can be turned on/off by turning current on or off
Strength depends on size of current and number of loops
Widely used electromagnetic device
Solenoid switches
• Moveable spring-loaded iron core responds to solenoid field
• Water valves, auto starters, VCR switches, activation of bells and
buzzers
Galvanometer
• Measures size of current
from size of its magnetic
field
• Coil of wire wrapped
around an iron core
becomes an
electromagnet that
rotates in field of a
permanent magnet
• This rotation moves a
pointer on a scale
Electrical Motor
Electromagnetic induction
Causes:
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Relative motion between magnetic
fields and conductors
Changing magnetic fields near
conductors
– Does not matter which one moves or
changes
Effect:
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Induced voltages and currents
Size of induced voltage depends on:
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Number of loops
Strength of magnetic field
Rate of magnetic field change
Direction of current depends on
direction of motion
Generators
• Device that converts mechanical energy
into electrical energy
Structure
• Axle with many loops in a wire coil
• Coil rotates in a magnetic field
– Turned mechanically to produce electrical
energy
Transformers
• Steps AC voltage up or down
• Two parts
– Primary (input) coil
– Secondary (output) coil
• AC current flows through primary coil,
magnetic field grows to maximum
size, collapses to zero then grows to
maximum size with opposite polarity
• Growing and collapsing magnetic field
moves across wires in secondary coil,
inducing voltage
• Size of induced voltage proportional to
number of wire loops in each coil
– More loops in secondary coil – higher
voltage output (step-up transformer)
– Fewer loops in secondary coil – lower
voltage output (step-down transformer)
Self-Induction