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