Transcript Magnet

Chapter 21
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
► Ancient
Greeks observed that magnetite, or
lodestone, attracts iron.
► By 1150 AD, Chinese navigators used
compasses with magnetized iron needles.
► In 1600, Gilbert published an explanation of
the properties of magnets.
► (Magnet is derived from Magnesia region of
ancient Greece, known for magnetite mines)
21.1 Magnets and Magnetic Fields
Magnetism
Magnetic Forces
► Magnetic
force is the force a magnet exerts
 on another magnet,
 on iron or similar metals,
 or on moving charges.
► Magnetic forces act over a distance.
► Magnetic forces vary with distance.
21.1 Magnets and Magnetic Fields
► All
Magnetism
magnets have two magnetic poles,
regions where the magnet’s force is
strongest.
► One end of a magnet is the north pole; the
other is the south pole.
► Like magnetic poles repel one other;
opposite magnetic poles attract one another.
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
Magnetic Fields
►A
magnetic field surrounds a magnet and
can exert magnetic forces.
► A magnetic field, which is strongest near the
poles, will either attract or repel another
magnet that enters the field.
► Where lines are close together, the field is
strong.
► Where lines are more spread out, the field is
weak.
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
Magnetic Field Around Earth
► Earth
is like a giant magnet surrounded by a
magnetic field.
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
► The
area surrounding Earth that is
influenced by this field is the
magnetosphere.
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
► Earth’s
Magnetism
magnetic poles are not at the
geographical poles.
► The magnetic south pole is at about 81o N
latitude.
► The angle between true north and magnetic
north is called declination.
► Magnetic declination varies with location.
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
Magnetic Materials
► Within
the atom, electrons move around the
nucleus.
► This movement and electron spin cause
electrons to act like tiny magnets.
► In many materials, electron pairs spin in
opposite directions and cancel out the
magnetic effects.
► Most materials have weak magnetic fields.
21.1 Magnets and Magnetic Fields
► Unpaired
Magnetism
electrons produce magnetic fields.
► Usually the fields do not combine because
of the arrangement of the atoms.
► In a few materials (Fe, Ni, Co) the unpaired
electrons make a strong magnetic field.
► The fields combine to form magnetic
domains.
21.1 Magnets and Magnetic Fields
► Magnetic
Magnetism
domains are regions that have a
very large number of atoms with aligned
magnetic fields.
► A ferromagnetic material can be magnetized
because it contains magnetic domains.
► When a material is magnetized, most of its
magnetic domains are aligned.
21.1 Magnets and Magnetic Fields
Magnetism
Nonmagnetized Materials
► The
fact that a material is ferromagnetic
does not mean it is a magnet.
► If the domains are aligned randomly, the
magnetization is cancelled and the material
is not a magnet.
21.1 Magnets and Magnetic Fields
Magnetism
21.1 Magnets and Magnetic Fields
Magnetism
Magnetized Materials
►A
nonmagnetized ferromagnetic material
can be magnetized by placing it in a
magnetic field.
► The applied magnetic field causes magnetic
domains aligned with the field to grow
larger.
► Such magnetization can be temporary.
21.1 Magnets and Magnetic Fields
► Removal
Magnetism
of the material from the magnetic
field causes the domains to become
randomly oriented again.
► In some materials, called permanent
magnets, the domains stay aligned for a
long time.
► They are not permanent because heat or
jarring can realign the domains.
21.1 Magnets and Magnetic Fields
► If
Magnetism
a magnet is cut in half, each half will have
its own north pole and south pole because
the domains will still be aligned.
► No matter how many times a magnet is cut,
each piece will have two different poles.
► A magnet can never have just a north pole
or a south pole.
21.1 Magnets and Magnetic Fields
Magnetism
21.2 Electromagnetism
Magnetism
21.2 Electromagnetism
► The
connection between electricity and
magnetism was discovered accidentally by
Oersted in 1820.
► During a demonstration using electricity, he
noticed a nearby compass needle move.
► When he turned off the current, the needle
moved back to its original position.
21.2 Electromagnetism
Magnetism
Electricity and Magnetism
► Electricity
and magnetism are different
aspects of the electromagnetic force.
► The electric force results from charged
particles.
► The magnetic force usually results from the
movement of electrons in an atom.
► Both forces are caused by electric charges.
21.2 Electromagnetism
Magnetism
Magnetic Fields Around Moving
Charges
► Moving
electric charges create a magnetic
field.
► The moving charges may be the vibrating
charges that produce an electromagnetic
wave.
► They may also be the moving charges in a
wire.
21.2 Electromagnetism
► Figure
Magnetism
7 shows how to remember the
direction of the magnetic field that is
produced.
► Point the thumb of your right hand in the
direction of the current (positive charge
flow), your fingers curve in the direction of
the magnetic field.
► The magnetic field lines form circles around
a wire carrying a current.
21.2 Electromagnetism
Magnetism
21.2 Electromagnetism
Magnetism
Forces Acting on Moving Charges
► An
electric field exerts a force on an electric
charge.
► The force is either in the same direction as
the electric field or in the opposite direction,
depending on whether it is a positive or
negative charge.
21.2 Electromagnetism
►A
Magnetism
charge moving in a magnetic field will be
deflected in a direction perpendicular to
both the magnetic field and the velocity of
the charge.
► If a current-carrying wire is in a magnetic
field, the wire will be pushed in a direction
perpendicular to both the field and the
direction of the current.
21.2 Electromagnetism
► Reversing
Magnetism
the direction of the current will
still cause the wire to be deflected, but in
the opposite direction.
► If the current is parallel to the magnetic
field, the force is zero, and there is no
deflection.
21.2 Electromagnetism
Magnetism
Solenoids and Electromagnets
► In
Figure 9B, the magnetic fields of the
loops combine so that the coiled wire acts
like a bar magnet.
► The field through the center of the coil is
the sum of the fields from each turn of the
wire.
► A coil of current-carrying wire that produces
a magnetic field is a solenoid.
21.2 Electromagnetism
Magnetism
21.2 Electromagnetism
► Placing
Magnetism
a ferromagnetic material inside the
coil of the solenoid, increases the strength
of the magnetic field.
► The magnetic field produced by the current
causes the material inside the coil to
become an electromagnet.
► Changing the current in an electromagnet
controls the strength and direction of its
magnetic field.
21.2 Electromagnetism
► The
Magnetism
strength of an electromagnet depends
on the current in the solenoid, the number
of loops in the coil in the solenoid, and the
type of ferromagnetic core.
21.2 Electromagnetism
Magnetism
Electromagnetic Devices
► Electromagnetic
devices can convert
electrical energy into motion that can do
work.
► Electromagnetic devices such as
galvanometers, electric motors, and
loudspeakers change electric energy into
mechanical energy.
21.2 Electromagnetism
Magnetism
Galvanometers
►A
galvanometer uses a solenoid to measure
small amounts of current.
► A solenoid is attached to a spring and is free
to rotate about an iron core.
► The solenoid is placed between the poles of
two permanent magnets.
21.2 Electromagnetism
► When
Magnetism
there is a current in the solenoid’s
coils, the resulting magnetic field attempts
to align with the field of the permanent
magnets.
► The greater the current, the more the
solenoid rotates as shown by a pointer on
the scale.
21.2 Electromagnetism
Magnetism
21.2 Electromagnetism
Magnetism
Galvanometer
21.2 Electromagnetism
Magnetism
Electric Motors
► An
electric motor is a device that uses an
electromagnet to turn an axle.
► When current flows through a loop of wire,
one side of the loop is pushed by the field
of the permanent magnet.
► The other side of the loop is pulled.
► These forces rotate the loop.
21.2 Electromagnetism
► As
Magnetism
the loop rotates, the commutator
connects with a different brush, reversing
the current.
► As long as current flows, rotation continues.
21.2 Electromagnetism
An Electric Motor
Magnetism
21.2 Electromagnetism
Magnetism
Loudspeakers
►A
loudspeaker contains a solenoid placed
around one pole of a permanent magnet.
► The changing current produces a changing
magnetic field in the solenoid coil.
► The magnetic force exerted by the
permanent magnet moves the coil back and
forth.
21.2 Electromagnetism
► As
Magnetism
the coil moves, it causes a thin
membrane to vibrate, producing sound
waves that match the original sound.
21.2 Electromagnetism
Loudspeaker
Magnetism
Microphone
21.3 Electrical Energy Generation and Transmission
Magnetism
21.3 Electrical Energy Generation
and Transmission
► All
of the electrical energy used comes from
the two aspects of the EM force.
► A magnetic field can be used to produce an
electric current.
► Electromagnetic induction is the process of
generating a current by moving an electrical
conductor relative to a magnetic field.
21.3 Electrical Energy Generation and Transmission
► Faraday
Magnetism
discovered EM induction in 1831.
► According to Faraday’s law, a voltage is
induced in a conductor by a changing
magnetic field.
► Changing the magnetic field through a coil
of wire induces a voltage in the coil.
► A current is only produced if the coil is part
of a complete circuit.
21.3 Electrical Energy Generation and Transmission
► Moving
Magnetism
the magnet in and out of the coil
causes an electric current first in one
direction and then in the other.
► Similar alternating current occurs if you
move the coil and keep the magnet still.
21.3 Electrical Energy Generation and Transmission
Magnetism
Generators
► Most
of the electrical energy used is
produced at large power plants using
generators.
► A generator is a device that converts
mechanical energy into electrical energy by
rotating a coil of wire in a magnetic field.
► Electric current is generated by the relative
motion of a conducting coil in a magnetic
field.
21.3 Electrical Energy Generation and Transmission
Magnetism
AC Generators
► AC
generators produce alternating current,
in which charges flow in one direction and
then in the other.
► While a motor converts electrical energy
into mechanical energy, a generator does
the opposite. See figure 14.
► A wire coil in the generator is attached to
slip rings.
21.3 Electrical Energy Generation and Transmission
AC Generator
Magnetism
21.3 Electrical Energy Generation and Transmission
► Slip
Magnetism
rings are in contact with brushes that
are attached to a circuit.
► As a loop of wire is rotated, the magnetic
field induces a current in the wire.
► The current flows in one direction, and then
when the loop turns halfway, the current
reverses direction.
21.3 Electrical Energy Generation and Transmission
Magnetism
DC Generators
►A
DC generator produces a direct current.
► A commutator replaces the slip rings.
► As the loop rotates, one side of the
commutator contacts a brush.
► When the loop rotates, current is induced in
the other direction, but the other side of the
commutator contacts that brush, so current
only flows in one direction.
21.3 Electrical Energy Generation and Transmission
Magnetism
Transformers
►A
transformer is a device that increases or
decreases voltage and current of two linked
AC circuits.
► A transformer works only with AC because
the alternating current induces a constantly
changing magnetic field.
21.3 Electrical Energy Generation and Transmission
►A
Magnetism
transformer changes voltage and current
by inducing a changing magnetic field in
one coil.
► This changing field then induces an
alternating current in a nearby coil with a
different number of turns.
21.3 Electrical Energy Generation and Transmission
Magnetism
Why are Transformers Needed?
► Over
long distance, the resistance of the
wire causes large losses of power due to
heat.
► Power losses can be reduced by using lower
current transmitted at a higher voltage.
21.3 Electrical Energy Generation and Transmission
Magnetism
Changing Voltage and Current
► When
there is an alternating current in the
primary coil, the current creates a changing
magnetic field in the iron core.
► Because the iron core is also inside the
secondary coil, the changing field induces
an alternating current in the secondary coil.
21.3 Electrical Energy Generation and Transmission
► The
Magnetism
number of turns in the primary and
secondary coils determines the voltage and
current.
► To calculate the voltage, divide the number
of turns in the secondary coil by the number
of turns in the primary coil.
► The result is the ratio of the output voltage
to the input voltage.
21.3 Electrical Energy Generation and Transmission
Magnetism
Types of Transformers
►A
step-down transformer decreases voltage
and increases current.
► A step-up transformer increases voltage and
decreases current.
21.3 Electrical Energy Generation and TransmissionFigure 16
Magnetism
21.3 Electrical Energy Generation and TransmissionFigure 16
Magnetism
21.3 Electrical Energy Generation and Transmission
Magnetism
Electrical Energy for Your Home
► Most
of the electrical energy (52%)
generated in the US is produced using coal
as an energy source.
► Other sources include nuclear energy
(20%), natural gas (16%), hydroelectric
(7%), wind (2%), and petroleum (3%).
21.3 Electrical Energy Generation and Transmission
Magnetism
21.3 Electrical Energy Generation and Transmission
►A
Magnetism
turbine is a device with fanlike blades that
turn when pushed.
► Burning fossil fuels or nuclear reactions are
used to heat water to produce steam that
spins a turbine.
► To produce electrical energy, the turbine
turns the coils of a generator or spins
magnets around the coils of wire.
21.3 Electrical Energy Generation and Transmission
Magnetism