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
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)