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Electromagnetism What is a Magnet? • The earliest magnets were found naturally in the mineral magnetite which is abundant the rock-type lodestone. These magnets were used by the ancient peoples as compasses to guide sailing vessels. • Magnetism is the force of attraction or repulsion of a magnetic material due to the arrangement of its atoms, particularly its electrons. What is a Magnet? • Magnets have two ends or poles: North & South • Unlike poles of magnets attract each other and like poles of magnets repel. • No Monopoles: If you cut a magnet in half, you get two poles on each Magnetic Domains • Atoms have magnetic properties due to electron spin (more about this in a minute!) • Small areas where the groups (billions) of atoms are aligned are called Domains • Domains align in the same direction when placed in a magnetic field. Magnetic Domains • Ferromagnetic materials: Iron, Nickel, Cobalt • Often magnets are made of alloys; e.g. AlNiCo, Steel • To create a temporary magnet: heat it or beat it in presence of a magnetic field • Even ferromagnets can lose magnetism Magnetic Fields • Iron filings will align around a magnet • Long-Range force: Field Force • These “lines of force” are called Magnetic field lines • Magnetic field lines go from North to South… • And make a complete loop back around! • Lines closest together (e.g. at poles) field is strongest Earth: A Giant Magnet • Magnets & compasses always orient themselves in a NorthSouth direction in Earth’s magnetic field • But… Opposite poles attract! • So the North pole of a compass magnet is attracted to Earth’s magnetic South, and the compasses’ South pole is attracted to Earth’s geographic North pole. Earth: A Giant Magnet • The magnetic field of the Earth is very weak compared with the strength classroom ceramic magnets. • Both the strength of the Earth’s magnetic field and the location of the north and south magnetic poles can switch places. • Today, the Earth’s magnetic field is losing approximately 7 percent of its strength every 100 years. Earth: A Giant Magnet • The magnetic field of the Earth is affected by solar wind (charged particles) • Most are deflected by magnetosphere • Some get in near the magnetic poles and interact with atmosphere Aurora • Borealis: Near North Geographic Pole • Austrialis: Near South Geographic Pole Electromagnetism • 1820: Hans Christian Oersted laid a compass under a wire, expecting the needle to point toward the wire or in the same direction as the current in the wire • Instead, the needle rotated until it pointed perpendicular to the wire Electromagnetism • Reversing the current caused the needle to reverse directions. • A magnetic field (full loops!) is set up around any current-carrying wire • Magnetic Field is perpendicular to current • Right Hand Rule: Wrap hand around wire, thumb with current Electromagnetism • All magnetic fields originate from moving electric charges. • Electricity and Magnetism are interchangeable: Moving charges create a magnetic field, changing magnetic fields cause charges to move Electromagnets • When current is passed through a coil, the magnetic field loops through (and around) • Adding an iron core strengthens the field • Electromagnets are very strong and can lift a lot of heavy metal! Electromagnets • Electromagnets are also used to make speakers • Varying electric currents in the wire changes the magnet field of an electromagnetic coil • The changing field exerts forces on the permanent magnet • The moving permanent magnet creates vibrations in the diaphragm (cone) • The vibrating air pressure is detected as sound waves. Galvanometer • An electromagnet that interacts with a permanent magnet: The stronger the electric current passing through the electromagnet, the more is interacts with the permanent magnet. • Galvanometers are used as gauges in cars and many other applications. Magnetic Force on Current • If a current-carrying wire is placed in a magnetic field, a perpendicular force is exerted on it • Right Hand Rule again: Point fingers in direction of Magnetic Field, curl in direction of current, thumb points in direction of Force Electric Motors • An electric motor is a device which changes electrical energy into mechanical energy. • As current flows through the loop of wire, the magnetic field exerts an upward force on one side and downward force on the other side causing it to rotate Electromagnetic Induction • So if moving charges through a magnetic field can cause a loop of wire to move… • Then moving a magnet through a loop of wire can also cause charges to move! • 1831: Michael Faraday found that if there is relative motion between a magnet and a coil of wire, a current is “induced” in the wire • Electromagnetic Induction Electromagnetic Induction • So if moving charges through a magnetic field can cause a loop of wire to move (motor) … • Then moving a magnet through a loop of wire can also cause charges to move! • For current to be induced, there must be a change in magnetic “flux” (or a change in the # of field lines going through the coil) Generators • Heinrich Emil Lenz determined that the current induced produces a field that tries to “restore” the field or counteract the change (Lenz’s Law) • As the flux increases, current induced produces a magnetic field opposing the change • This follows the law of conservation of energy! Generators • So if the flux increases, current goes one direction; as flux decreases, current goes other direction • So if a loop of wire is rotated in a magnetic field, the flux will alternate • This sets up and alternating current: AC! Motors vs. Generators • Motors use electrical energy, change it into mechanical energy (kinetic) • Generators use mechanical energy (kinetic) and change it into electrical energy Transformers • Two unconnected coils wrapped around a soft iron core • AC is sent through the first coil • This induces a changing magnetic field in the core • The changing magnetic field in the core induces an alternating current in the second wire • Depending on the number of coils in each wire, a transformer will “step up” or “step down” the voltage Transformers • To save on power loss, voltage is “stepped up” (to reduce current) at the plant for transmission over long distances • (Secondary coil has more turns) • Where power will be used, it must then be “stepped down” to a usable voltage and higher current • (Secondary coil now has fewer turns)