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