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Chapter 14
MagnetismMagnetism
Properties of Magnets
You can investigate the properties of You can investigate the properties of magnets by bringing two magnets magnets by bringing two magnets together.together.
The ends of the magnets attract each The ends of the magnets attract each other and stick together.other and stick together.
The ends of the magnets repel each The ends of the magnets repel each other and the magnets move apart.other and the magnets move apart.
Magnetic Poles Magnetic poles: One of the two ends of a Magnetic poles: One of the two ends of a
magnet where magnetic force is strongest.magnet where magnetic force is strongest. When a magnet spins freely it always points When a magnet spins freely it always points
the same directionthe same direction The end of a magnet that points north is The end of a magnet that points north is
called the north pole, and the end pointing called the north pole, and the end pointing south is the south pole.south is the south pole.
All magnets have a north and a south pole.All magnets have a north and a south pole.
When two north poles or two south poles When two north poles or two south poles are brought near each other, they repel.are brought near each other, they repel.
But if the north and south magnetic poles But if the north and south magnetic poles are brought near each other they will are brought near each other they will attract. attract.
Magnetic poles can’t be separated from Magnetic poles can’t be separated from each other.each other.
If you break a magnet in half each piece is If you break a magnet in half each piece is still a magnet with a north and south pole.still a magnet with a north and south pole.
Magnetic Field: The area of magnetic force The area of magnetic force
surrounding a magnet.surrounding a magnet. The magnetic fields is strongest at The magnetic fields is strongest at
the poles of a magnet, but exists the poles of a magnet, but exists around the entire magnet.around the entire magnet.
Magnetic field lines exists from one Magnetic field lines exists from one pole to the other.pole to the other.
The number of field lines in any given The number of field lines in any given region indicates the relative strength of the region indicates the relative strength of the fieldfield
Although the magnetic field is invisible you Although the magnetic field is invisible you can see its effect around a magnet by can see its effect around a magnet by placing a piece of paper on top of a magnet placing a piece of paper on top of a magnet and then sprinkling iron fillings over the and then sprinkling iron fillings over the paperpaper
If you were to place a magnetic material, If you were to place a magnetic material, such as iron, near the magnet it would be such as iron, near the magnet it would be most attracted to either the north or south most attracted to either the north or south polepole
Iron would also be attracted by the Iron would also be attracted by the magnetic field around the magnetmagnetic field around the magnet
What do you think happens if you What do you think happens if you place the entire magnet in a dish of place the entire magnet in a dish of iron filingsiron filings
Magnetic fields
Magnetic Materials The electrons of all atoms spin as they The electrons of all atoms spin as they
move about the nucleusmove about the nucleus A spinning electron produces a magnetic A spinning electron produces a magnetic
field with both a north and south polefield with both a north and south pole In most materials, the magnetic fields of In most materials, the magnetic fields of
individual atoms cancel each other, so individual atoms cancel each other, so the materials aren’t magneticthe materials aren’t magnetic
In certain materials this isn’t the caseIn certain materials this isn’t the case
The poles line up in the same direction in The poles line up in the same direction in microscopic magnetic regions, called microscopic magnetic regions, called magnetic domainsmagnetic domains
When all the domains are arranged with When all the domains are arranged with their poles in the same direction, the iron their poles in the same direction, the iron bar becomes a permanent magnetbar becomes a permanent magnet
When the domains are arranged When the domains are arranged randomly, the iron bar is not magnetized randomly, the iron bar is not magnetized
Earth As A Magnet If you hang a magnet by a string, the north If you hang a magnet by a string, the north
seeking pole will always point north because seeking pole will always point north because the earth itself is a huge magnetthe earth itself is a huge magnet
An instrument that takes advantage of the An instrument that takes advantage of the earths magnetic field is the compassearths magnetic field is the compass
A compass has a magnetized needle in it that A compass has a magnetized needle in it that turns freelyturns freely
The north and south pole of the earth’s axis are The north and south pole of the earth’s axis are referred to as geographic north pole sometimes referred to as geographic north pole sometimes called true northcalled true north
Evidence suggests that the earths Evidence suggests that the earths magnetic field is caused by the movement magnetic field is caused by the movement of molten metals near the earths coreof molten metals near the earths core
Measurements show that the earths Measurements show that the earths magnetic poles change position over timemagnetic poles change position over time
Changes in the flow of the molten metals Changes in the flow of the molten metals inside the earth may cause the magnetic inside the earth may cause the magnetic poles to movepoles to move
Magnetic Effects The most visible effect of the earths The most visible effect of the earths
magnetic field is a colorful light display, magnetic field is a colorful light display, called an auroracalled an aurora
An aurora hangs like a curtain of light An aurora hangs like a curtain of light stretching over the polar regions of the stretching over the polar regions of the earthearth
Collisions between the charged Collisions between the charged particles and other particles in the upper particles and other particles in the upper atmosphere create glowing lightsatmosphere create glowing lights
Aurora
Aurora
Aurora
The color of aurora depends on the kind The color of aurora depends on the kind of atoms in the atmosphereof atoms in the atmosphere
Magnetic storms interfere with compass Magnetic storms interfere with compass needles and radio and television waves.needles and radio and television waves.
Magnetic storms occur when solar Magnetic storms occur when solar flares produce charged particles that flares produce charged particles that become trapped in the earth’s magnetic become trapped in the earth’s magnetic field.field.
Earth’s magnetic field affects living Earth’s magnetic field affects living things.things.
They have magnetic particles They have magnetic particles inside their bodiesinside their bodies
These particles help organisms These particles help organisms using the magnetic field to find their using the magnetic field to find their way.way.
Electromagnetism In 1820, Christian Oersted, a Danish In 1820, Christian Oersted, a Danish
physicist made an observation that physicist made an observation that when a compass was brought near when a compass was brought near electric current, the compass needle no electric current, the compass needle no longer pointed north. It turned 90 longer pointed north. It turned 90 degrees.degrees.
The compass needle turned in the The compass needle turned in the opposite direction when he reversed the opposite direction when he reversed the current.current.
He hypothesized that when an He hypothesized that when an electric current flowed through it, electric current flowed through it, the wire acted like a magnet. the wire acted like a magnet. Somehow electricity could produce Somehow electricity could produce magnetism.magnetism.
Electromagnets Oersted’s discovery is responsible for the Oersted’s discovery is responsible for the
invention of new tools based on the invention of new tools based on the principles of electromagnetism.principles of electromagnetism.
Electromagnet: a magnet made of a soft-Electromagnet: a magnet made of a soft-iron core surrounded by a coil of wire iron core surrounded by a coil of wire through which an electric current passed.through which an electric current passed.
The strength depends on the number of The strength depends on the number of turns in the coil, the amount of current, turns in the coil, the amount of current, and the size of the iron core.and the size of the iron core.
The greater number of turns a coil has, the The greater number of turns a coil has, the stronger the magnetic field can produce.stronger the magnetic field can produce.
The greater the size of the soft-iron core, The greater the size of the soft-iron core, the stronger the magnet is.the stronger the magnet is.
When a magnet is turned on an electric When a magnet is turned on an electric current flows through the wire coil, creating current flows through the wire coil, creating a magnetic field around the coil.a magnetic field around the coil.
The magnetic domains in the soft-iron core The magnetic domains in the soft-iron core align with the magnetic field of the coil.align with the magnetic field of the coil.
The soft-iron core becomes magnetized.The soft-iron core becomes magnetized. One end of the soft-iron core is a north One end of the soft-iron core is a north
pole, and the other end is the south pole.pole, and the other end is the south pole. The magnetic field of the magnetized The magnetic field of the magnetized
soft-iron core combines with the soft-iron core combines with the magnetic field of the wire coil. The magnetic field of the wire coil. The combined magnetic fields create a very combined magnetic fields create a very strong magnet.strong magnet.
Pure iron is referred to as soft iron.Pure iron is referred to as soft iron. An electromagnet exerts a An electromagnet exerts a
magnetic force that can make magnetic force that can make things move.things move.
Electric Motors An electromagnet, called an armature, is An electromagnet, called an armature, is
placed in the magnetic field of permanent placed in the magnetic field of permanent magnet.magnet.
When current flows through the When current flows through the electromagnet, its poles repel the like electromagnet, its poles repel the like poles of the permanent magnets.poles of the permanent magnets.
When the direction of the current changes, When the direction of the current changes, the poles on the electromagnet reverse, the poles on the electromagnet reverse, and the electromagnet spinsand the electromagnet spins
The commutator is split metal ring that The commutator is split metal ring that acts as as a switchacts as as a switch
The communicator reverses the current The communicator reverses the current in the electromagnet in the electromagnet
Electric current enters the Electric current enters the electromagnet through brushes that electromagnet through brushes that touch the spinning communtator ringstouch the spinning communtator rings
Current Meters The response of magnetic forces The response of magnetic forces
between an electromagnet and a between an electromagnet and a permanent magnet is used in permanent magnet is used in various kinds of metersvarious kinds of meters
Current Meters The two springs connected to the rod The two springs connected to the rod
through the electromagnet control the through the electromagnet control the pointer of the galvanometerpointer of the galvanometer
When an electric current passes the When an electric current passes the electromagnet, the poles of the electromagnet, the poles of the electromagnet respond to the poles of electromagnet respond to the poles of the permanent magnetthe permanent magnet
Electromagnetic induction: The process of inducing a current by The process of inducing a current by
moving a magnetic field through a wire coil moving a magnetic field through a wire coil without touching it.without touching it.
This occurs any time motion takes place This occurs any time motion takes place between the wire and the magnetic field.between the wire and the magnetic field.
A weak current is produced when the A weak current is produced when the movement of the wire is slow.movement of the wire is slow.
A strong current is produced when the A strong current is produced when the movement is fast.movement is fast.
Generators Devices for converting mechanical Devices for converting mechanical
energy to electrical energy.energy to electrical energy. Spin a coil of wire through a magnetic Spin a coil of wire through a magnetic
fieldfield Will make a current flow through wireWill make a current flow through wire Make alternating current as the go past Make alternating current as the go past
the different poles of the magnet.the different poles of the magnet.
Transformer Changes the voltage of alternating currentChanges the voltage of alternating current Power comes at high voltage because the Power comes at high voltage because the
power company loses less energy.power company loses less energy. A step-down transformer lowers the A step-down transformer lowers the
voltage to 120V or 240V for your housevoltage to 120V or 240V for your house Uses two coils of wire and a soft iron coreUses two coils of wire and a soft iron core Primary coil inPrimary coil in Secondary coil outSecondary coil out
Step-down Transformer Has more coils Has more coils
on primary than on primary than secondarysecondary
Decreases Decreases voltagevoltage
Step-Up Transformer Has more coils Has more coils
on secondary on secondary than primarythan primary
Increases Increases voltagevoltage
Transformers As the alternating current changes As the alternating current changes
directiondirection So does the magnetic fieldSo does the magnetic field Which makes the alternating Which makes the alternating
current in the other coilcurrent in the other coil
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