Electricity and Magnetism Chapter 2+3 Notes

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Oct 18, 2013 (3 years and 9 months ago)

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Electricity and Magnetism Chapter 2+3 Notes


Magnetism and Electromagnetism



The Nature of Magnets



Earth As a Magnet



Magnetism in Action



Magnetism From Electricity



Electricity From Magnetism


The Nature of Magnets



The Greeks first discovered magnetism.



Magn
etism is the force of attraction or repulsion caused by magnetic
materials.



Every magnet has two poles: North and South.



Opposite poles attract and like poles repel.



The area over which magnetic force is applied is called the magnetic field.



Magnets can be

temporary or permanent.



Many permanent magnets are created from mixtures of aluminum, nickel,
cobalt, and iron (Alnico).



The atomic structure of groups of atoms determines their magnetic properties.



When atoms line up similarly they create a magnetic doma
in.



When groups of domains line up, the object becomes magnetic.



Striking or heating magnets can mix up their domains, causing them to lose
their magnetic properties.



Similarly, exposing substances to magnets can causes their domains to line up
and give th
em magnetic properties.


The Earth As a Magnet



The earth exerts magnetic forces which are strongest near its poles.



The magnetism is believed to be caused by the motion of iron and nickel in
the earth’s core.



Scientists have also been able to tell by exami
ning rocks that the earth’s
magnetic field has reversed many times in it’s history (every 500,000 years).



Compasses are small magnets which align themselves with the earth’s
magnetic field and are used to help find direction.



The north pole of a magnet poi
nts in the northward direction, meaning that
the geographic North Pole is actually a magnetic south pole.



In addition, the magnetic and geographic poles do not line up.



They are actually 1500 kilometers from each other.



The angular difference of the poles
is called magnetic variation or declination.



This must be taken into account when using a compass.



Generally, the closer a compass is to the equator, the more accurate it is.



Other planets and even our sun have magnetic fields.



Our sun’s magnetic field is
thought to play a role in the sunspot cycle.


Magnetism in Action



The earth’s magnetic field deflects charged particles radiated from the sun.



The region of the earth’s atmosphere where the magnetic field lines run is
called the magnetosphere.



The interact
ions can cause the northern and southern lights (aurora borealis
and australis).



These same types of interactions allow astronomers to use radio waves when
studying stars.



Additionally, magnetism is used to confine hot plasmas used in nuclear fusion
(an en
ergy source scientists are trying to perfect).


Magnetism From Electricity



In the 1800’s Hans Oersted discovered that a current carrying wire would
deflect the needle of a compass.



He inferred that an electric current would induce a magnetic field with
dir
ection dependent upon that of the current.



In playing with his new discovery, Oersted found that twisting the wire into
loops (a solenoid) would create a strong magnetic force.



By placing an iron core inside the solenoid, an even stronger magnetic force
ca
n be generated.



This combination is called an electromagnet.



Electromagnets are strong temporary magnets that can be turned on or off.



Since forces always occur in pairs, the wire not only exerts a force on the
magnet, but magnets exert a force on the curr
ent carrying wire.



There are many practical applications of Oersted’s discovery.



Electric motors use electromagnets that are free to rotate called armatures.



The armature sits in between the pole of a permanent magnet and spins by
being attracted and repel
led by constantly changing its poles due to an AC
circuit.



DC circuits can also be used in electric motors when a device called a
commutator is incorporated in the electromagnet to continually switch the
direction of the current.



A galvanometer is another
device that relies on electromagenetism.



Galvanometers measure current strength and direction.



Other uses of electromagnetism include doorbells, car starters, telephones,
and telegraphs.


Electricity From Magnetism



After scientists learned of Oersted’s di
scovery, many wondered if electricity
could be produced from magnetism.



In 1831 two scientists, working independently, found it to be possible: Michael
Faraday and Joseph Henry.



Faraday found that a changing magnetic field would induce an electric current
in a wire through a process called electromagnetic induction.



Electric generators, the type used at power plants to create our electricity, use
electromagnetic induction to produce electricity.



When a power source spins a turbine, electromagnets are spun b
etween coils
of wire creating an electric current in the wire.



The resulting current produced is alternating current.



It is carefully setup so that the current changes direction 120 times/sec or at
60 Hz (cycles/sec).



Transformers are devices that increase

or decrease voltage in a wire.



Two insulated wires are wound around the same iron core the alternating
current in one induces a current in the other.



If the induced wire has fewer coils (step down transformer), the voltage is
decreased.



If the induced wir
e has greater coils (step up transformer), the voltage is
increased.



Power companies use these transformers to transmit high voltage electricity to
your street and then step it down before being transferred to your house.