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 North- South 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 inter- changeable: 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

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)