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 induction

• • Causes: 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