Download MAGNETIC ATTRACTION

Magnetic Attraction
and
Electromagnetism
Spring 2011
The Nature of Magnetism
• Magnets are found everywhere…doorbells,
TV’s, computers…
• Magnets were discovered in a region in Greece
called….you guessed it…Magnesia!!!
• They discovered the unusual rock about 2,000
years ago.
The Nature of Magnetism…
• The rock had strange properties…
• It attracted anything that contained iron. This rock is
the mineral we call magnetite.
• Magnetism is defined as the attraction of a magnet to
another object.
• About 1,000 years ago…people discovered that
magnets have another unusual property…
The Nature of Magnetism
• If you swing a rock freely from a
string…one part of the rock always points
in the same direction…toward a specific
star…northern star (also called the
lodestar).
• For this reason…magnetic rocks are
sometimes called lodestones.
Magnetic Poles
• Any magnet has 2 ends – each is a magnetic pole.
• This is the area where the magnetic effect is the
strongest…just like one end of a magnetite rock always
points on direction…
• One pole will always point north and therefore it is called the
north pole. The other is, of course, the south pole.
• The north and south pole are direct opposites.
Attraction...
• What happens when you bring 2 magnets together?
• If you bring the 2 north poles together – they repel or push
away from each other. The same is true for 2 south poles.
• If you bring one north and one south pole together – they
are attracted to each other.
• So…alike poles repel…unlike poles attract.
• Any material that exerts a magnetic attraction is considered
a magnet.
So What’s the Big Deal??
• The Maglev
Train…magnetically
levitating train…
• Runs solely on
magnetism…
• Fast, efficient, and does
NOT harm the
environment…
What happens if???
• What happens if you break a magnet?
• Do you get one south pole and one north pole magnet?
• No…actually… when a magnet is broken in will develop
another pole on that end…so it will always have a north
and a south end.
• If you break them again…the same process would happen.
MAGNETO…
• The character Magneto
from the “X-Men”
comic series has the
ability to create
magnetic fields and use
magnetic force to control
objects.
So, What are Magnetic Fields???
• Magnetic forces are exerted all
around a magnet, but is
strongest at the poles.
• The region of magnetic force
around an object is called the
magnetic field.
• A magnetic field allows
magnets to interact without
actually touching.
Magnetic Fields…
• Magnetic field lines map
out the magnetic field
around an object. These
lines spread out from pole
to pole and curve around
the magnet to return to the
other pole. See the diagram
at left.
Magnetic Properties…
• Depend on the structure of the atoms making up the object.
• A spinning electron (negatively charged) creates a magnetic
field.
• A bar magnet has what is called a magnetic domain. It has
a strong representation and therefore a stronger attraction.
• Materials that have a very strong attraction to a magnet are
said to be ferromagnetic material. These are items made
from iron, nickel, cobalt, samarium, etc.
Creating Magnets…
• You can make a magnet by taking ferromagnetic material
and place it in a magnetic field or rub a magnet against it.
The field has to be relatively strong for this to occur.
• Once the domains line up – your magnet is created.
• This is why you can magnetize a paperclip to pick up other
paperclips.
• Unfortunately, most of theses are only temporary.
Permanent magnets are much more difficult to make and
hold their magnetism indefinitely
Destroying Magnets…
• You can destroy a magnet field by striking the magnet very
hard or dropping it as well.
• This knocks the domains out of alignment.
• Heating a magnet will also destroy the magnetism. Above a
certain temperature, materials lose their ferromagnetism
properties altogether.
Compass…
• Defined as a device that has a magnetized needle than can
spin freely.
• The needle usually points north.
• This occurs because the Earth acts like a giant magnet.
Earth has an enormous magnetic field surrounding it – just
like a huge bar magnet.
• A scientist named Gilbert believed the center of the Earth
contained magnetic rocks…unfortunately, he was wrong,
but it was a great start.
More on Compasses…
• Scientists now know that the
magnetism is partially caused by the
circulation of molten material made
from iron and nickel.
• The simple fact that Earth has a
magnetic field explains why a
compass actually works.
• The magnetized needle aligns its
domain with that of the Earth –
therefore pointing North.
Magnetosphere…
• Earth’s magnetic field extends
into space.
• Electrically charged particles
affect Earth’s magnetic field.
• The Van Allen Belts are
located about 1,000-25,000
km above the Earth’s surface.
• In this particular region,
electrons and protons move at
very high speeds.
Magnetosphere…
• Solar Winds from the sun also
carry electrically charged particles to
Earth.
• The region where Solar Winds
shape Earth’s magnetic fields is
called the magnetosphere.
• This is a continual process as Earth
rotates on it’s axis.
• When the particles move too close to
Earth’s surface, we see the Aurora
Borealis or Northern Lights in the
sky. (called the Southern Lights in
the Southern hemisphere…)
Earth’s Magnetic Field…
• Earth’s magnetic directionality has changed throughout
history.
• From time to time…the poles will switch due to the magnetic
field created by the Earth.
• The last time the poles switched was about 780,000 years
ago.
• Scientists are perplexed as to why this occurs, but believe it
has something to do with the change in flow of the magnetic
molten material at Earth’s core.
Electric Charge
• Charges
Exert Force Atoms are
composed of particles with electric
charge. The law of electric charges
states that like charges repel and
opposite charges attract.
Electric Charge, continued
• The Force Between Protons and Electrons
Because protons and electrons have opposite
charges, they are attracted to each other.
• The Electric Force and the Electric Field
The force between charged objects is an
electric force. An electric field is the region
around a charged object in which an electric
force is exerted on another charged object.
Charge It!
• Friction Charging by friction happens
when electrons are “wiped” from one object
onto another.
• Conduction Charging by conduction
happens when electrons move from one
object to another by direct contact.
• Induction Charging by induction happens
when charges in an uncharged metal object
are rearranged without direct contact with a
charged object.
Charge It!, continued
Charge It!, continued
• Conservation of Charge
When you charge
something by any method,
no charges are created or
destroyed. The numbers of
electrons and protons stay
the same.
• Detecting Charge You
can use a device called an
electroscope to see if
something is charged.
Static Electricity
• Static electricity is the electric charge at
rest on an object.
• Electric Discharge The loss of static
electricity as charges move off an object is
called electric discharge. One of the most
dramatic examples of electric discharge is
lightning. The next slide shows how
lightning is formed.
Static Electricity, continued
• Lightning Dangers It is particularly
dangerous to be at the beach or on a golf
course during a lightning storm. Even
standing under a tree during a storm is
dangerous.
• Lightning Rods A lightning rod is a
pointed rod connected to the ground by a
wire. Objects that are joined to Earth by a
conductor, such as a wire, are grounded.
Electric Current
• AC and DC There are two kinds of electric
current—direct current (DC) and alternating current
(AC).
Voltage
• Voltage is the potential difference between
two points in a circuit. Voltage is expressed
in volts (V).
• Voltage and Energy Voltage is a measure
of how much work is needed to move a
charge between two points. You can think of
voltage as the amount of energy released as a
charge moves between two points in the path
of a current.
Voltage, continued
• Voltage and Electric
Current As long as there is a
voltage between two points
on a wire, charges will flow
in the wire. The size of the
current depends on the
voltage.
• Varying Nature of Voltage
Different devices need
different levels of voltage.
Resistance
• Resistance is the opposition to the flow of electric
charge.
• Resistance and Material Good conductors, such as
copper, have low resistance. Poor conductors, such as
iron, have higher resistance.
• Resistance, Thickness, and Length Thick, short
wires have less resistance than thin, long wires.
Resistance, continued
• Resistance and Temperature In general, the
resistance of metals increases as temperature rises.
Connecting Current, Voltage, and Resistance
• Georg Ohm (1789 –1854) studied the resistances of
materials. He measured the current that resulted
from different voltages applied to a piece of metal
wire.
• Ohm’s Law Ohm found that the ratio of voltage to
current is a constant for each material. This ratio is
the resistance of the material.
V
R=
, or V= I  R
I
Electric Power
• What Is Electric Power? The rate at which electrical
energy is changed into other forms of energy is
electric power. Electric power is calculated using the
following equation:
power = voltage  current, or P = V  I
• Watt: The Unit of Power The unit for power is
the watt (W). Another common unit of power is the
kilowatt (kW). One kilowatt is equal to 1,000 W.
Electric Power, continued
• Power Ratings When you
read the wattage label on a
light bulb, you are reading the
bulb’s power rating. The power
rating describes the rate at
which an electric device uses
electrical energy.
Parts of an Electric Circuit
• Forming a Loop An electric circuit is a complete,
closed path through which electric charges flow.
• All circuits need three basic parts: an energy
source, wires, and a load.
Parts of an Electric Circuit, continued
• A Switch to Control a Circuit Sometimes, a circuit
also contains a switch. A switch is used to open and
close a circuit.
Types of Circuits
• Series Circuits A series circuit is a circuit in which all
parts are connected in a single loop.
• Uses for Series Circuits Series circuits are useful in
wiring burglar alarms. If any part of the circuit fails,
there will be no current in the system and the alarm
will sound.
Types of Circuits, continued
• Parallel Circuits A parallel circuit is a circuit in
which loads are connected side by side.
• Uses for Parallel Circuits Almost all appliances are
built with parallel circuits so that they will keep
working if part of the system fails.
Household Circuit Safety
• Circuit Failure Broken wires or water can cause a
short circuit. In a short circuit, charges do not go
through one or more loads in the circuit.
• Fuses A fuse has a thin strip of metal. Fuses keep
charges from flowing if the current is too high.
Household Circuit Safety, continued
• Circuit Breakers A circuit
breaker is a switch that
automatically opens if the current
is too high. Charges stop flowing.
• Electrical Safety Tips Do not
overload circuits by plugging in
too many electrical devices. Do
not use electrical devices near
water.
It’s Electric!
• Electrons carry a negative charge.
Protons carry a positive charge.
• When these charges flow through a
wire or other object the create an
electric current.
• Electric currents are defined as the
flow of a charge through a material.
• The amount of charge that passes
through the wire in a unit of time is
measured in amperes (an amp).
It’s Electric!
• An electric current produces a
magnetic field.
• The direction of the current
determines the direction of the
magnetic field.
• If the current reverses, so does
the field.
Electric Circuits…
• Electric currents do not
automatically flow through
wires.
• Current only flows through
electric circuits.
• An electric circuit is a complete
path through which electric
charges can flow.
• All electrical devices contain
electric currents.
Conductors…
• Electric currents do not flow through
all objects.
• Electric currents move freely
through objects called conductors.
• Examples of conductors: copper,
silver, iron, and aluminum are all
good conductors.
• In a conductor, some of the electrons
are only loosely bound to their
atoms. These electrons are able to
move around and generate an
electric current.
Light switch???
• Did you ever wonder why a
light comes on when you flip a
switch?
• Electrons are not created every
time you flip a switch – they
are always there in the
conductors of the circuit.
Insulators…
• Insulators are a different kind
of material where charges are
not allowed to move freely.
• The electrons are bound tightly
to their atoms and do not flow
easily.
• Examples of good insulators:
rubber, glass, sand, plastic, and
wood.
Electrical Resistance…
• A resistor uses electrical energy as it
interferes with, or resists, the flow of
charge.
• Resistance is the opposition to the
movement of charges flowing
through a material.
• Resistance depends on an objects
atomic structure. (Like trying to
cross a crowded room…the more
collisions…the more energy is
converted.)
Light Bulb…
• Edison used resistance when
developing his light bulb.
• He needed a material that conducted
electrical currents, but that would
offer enough resistance to make the
material heat up and glow.
• He tried a variety of items: copper
wires, silk fibers, shredded corn husks
to name a few.
• He eventually used the metal
tungsten for the purpose.
SUPERCONDUCTORS!!!
• A superconductor is a material that
has no electrical resistance.
• This normally occurs at very low
temperatures. (That’s the problem!)
• There is no loss of energy and
therefore the electrical device is much
more efficient.
• They also strongly repel magnets.
• A strong magnetic field would
destroy a superconductor.
Electric Motors
• Device that changes electrical energy
into mechanical energy.
• All electric motors have an armature –
a loop or coil of wire that can rotate.
• Armatures are mounted between the
poled of an electromagnet.
• A Commutator reverses the current.
Electricity from Magnetsim
• Galvanometers - device that measures
current
– Runs by using an electromagnet
• Hans Christian Oersted discovered that an
electric current could make a magnetic field.
• Michael Faraday – first to publish results in
great detail.
• Electromagnetic induction – electric current
is made by changing a magnetic field.
Electric Current continued…
• An electric current is created when a magnet moves in a coil
of wire or when a wire moves between the poles of a magnet.
• Electric Generators – uses electromagnetic induction to
change mechanical energy into electrical energy.
• Electric generators generate alternating currents.
• Transformers – increases or decreases voltage of an
alternating current.
Electromagnets…
• Solenoids is a current carrying wire
with many loops.
• The solenoid creates a magnetic field
at the center of the coil.
• The 2 ends act like poles and can be
turned off or on by a switch.
• If you place a ferromagnetic
material inside a solenoid – the
magnetic field is increased.
• A solenoid with a ferromagnetic core
is called an Electromagnet!
Electromagnets…
• Electromagnets are strong magnets
that can be turned off or on.
• These are ideal for lifting large pieces
of scrap metal like at a junk yard.
• When the switch is on – the magnet
is activated. When the switch is
turned off – the magnet is no longer
magnetized.
• You can increase and decrease the
power of the electromagnet by
increasing or decreasing the number
of coils of the solenoid or by using a
stronger ferromagnetic material.
FINITO!!
That’s All Folks!!