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