Magnetic Fields Jolyon Johnson Dartmoor School Ancient Uses

Magnetic Fields Jolyon Johnson Dartmoor School

Ancient Uses of Magnets • Thales of Miletus, who also studied static electricity, observed the magnetic effect of lodestones (now called magnetite). • By the 12 th Century, Chinese navigators made compasses out of lodestones. • In 1600, William Gilbert published his observation that magnetism has two poles and is a separate force from static electricity. William Gilbert 1544 -1603

Magnetic Field • Like electricity, magnetism has two “charges” called poles: north and south. • Charles Coulomb discovered in 1785 that unlike electric fields, magnetic fields always have both poles. • Magnetic field lines always point away from north and toward south.

Connecting Electricity and Magnetism • In 1819, Hans Christian Oersted demonstrated that an electric field produces a magnetic field encircling it. - + N Hans Christian Oersted 1777 -1851

Left vs Right Hand Rules • In this course, we will talk about current in terms of moving electrons so we must use the left hand rules. • “Conventional current” is moving positive charges, which require right hand rules. • Electrons flow from negative to positive. • Looped magnetic field line arrows point to north.

First Left Hand Rule

Electromagnets • Ampere showed that magnets could be created by loops of electricity. • These are called electromagnets. • An iron core amplifies the effect.

Second Left Hand Rule + Pinky to negative/south N Thumb to positive/north -

Induced Magnetism • A metal paper clip sticks to a magnet even though the paper clip is not a magnet. • The paper clip is given a magnetic field by the magnet and temporarily acts like a magnet.

Electrons and Magnetism • Spinning electrons produce magnetic fields. • The resulting magnetic field follows the same left hand rule as a coil of wire. • An nonmagnetic substance has randomly oriented electrons. • A permanent magnet has all electrons oriented the same direction. Spin down Spin up

Magnetic Induction • Nikola Tesla 1856 -1943

Magnetic Flux •

Example Magnetic Strengths • Surface if a neutron star 108 T • Strong electromagnet 10 T • Small bar magnet 0. 01 T • Earth’s magnetic field 5 x 10 -5 T

Magnetic Force •

Third Left Hand Rule

Electric Motor Note that the arrows for current are backwards!

Lorentz Force • Hendrik Antoon Lorentz 1853 -1928

Earth’s Magnetic Field • Notice the sun rises in the east and sets in the west. • Which way is Earth’s magnetic north pole?

Plate Tectonics • Every million years or so, Earth’s outer core reverses its flow. • This flip is recorded in magnetite formed at spreading centers.

Migration • Birds, fish, and some insects have cells containing magnetite. • This magnetite aligns with Earth’s magnetic poles. • Thus, some animals have a sense that we do not have.

Uses of Motors