Transcript Physical Science Insight

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