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)