magnetism. earliest ideas associated with naturally occurring magnetic materials (lodestone,...
TRANSCRIPT
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
• 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 inductionCauses:• Relative motion between magnetic
fields and conductors• Changing magnetic fields near
conductors – Does not matter which one moves or
changes
Effect:• Induced voltages and currents
Size of induced voltage depends on:• 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