Download What is a magnet?

Entry quiz
1. What is the element for today?
2. What properties do we expect?
3. What is atomic number?
9th element
Fluorine
Atomic symbol F
Atomic number 9
Atomic weight 18.998
Gas, nonmetal
Gas: very pale yellow
Liquid: bright yellow
Solid: transparent, opaque
Foundations of Magnetism
Unit : Electricity and Magnetism
Chapter. Electric Charges and Forces
 1 Properties of Magnets
 2 Magnetic Properties of Materials
 3 The Magnetic Field of the Earth
Chapter Objectives
1. Describe the forces between two permanent
magnets.
2. Sketch the magnetic field of a single permanent
magnet.
3. Predict the direction of the force on a magnet placed
in a given magnetic field.
4. Explain why ferromagnetic materials always attract
magnets of either pole.
5. Describe the theory behind why a compass works.
6. Use a compass to find the direction of true north.
Chapter Vocabulary Terms









magnet
north pole
south pole
magnetization
demagnetization
magnetic field
compass magnetic
field lines
diamagnetic








paramagnetic
ferromagnetic
gauss
soft magnet
magnetic declination
magnetic domain
hard magnet
permanent magnet
1 Properties of Magnets
Key Question:
How do magnets interact
with each other?
What is a magnet?
 If a material is magnetic, it has the ability to exert
forces on magnets or other magnetic materials.
 A permanent magnet is a material that keeps its
magnetic properties even when it is NOT close to
other magnets.
Properties of Magnets
 Magnets have two
opposite poles.
— north
— south
 Magnets exert forces on
each other.
 The forces depend on the
alignment of the poles.
Properties of Magnets
 Plastics, wood, and most insulating materials are
virtually transparent to magnetic forces.
 Conducting metals, like aluminum, also allow
magnetic forces to pass through, but may change
the forces.
The force between two magnets
 The strength of the force
between magnets
depends on the distance
between them.
 The magnetic force
decreases with distance
much faster than does
either gravity or the
electric force.
The force between two magnets
 Two magnets near each other
often feel a twisting force, or
torque.
 This is a result of having two
poles.
 The combination of attractive
and repulsive forces on the
same magnet creates a
torque.
The magnetic field
 All magnets create a
magnetic field in the
space around them, and
the magnetic field
creates forces on other
magnets.
The magnetic field
 The number of field lines in a
certain area indicates the relative
strength of the magnetic field in
that area.
 The arrows on the field lines
indicate the direction of the force.
 The closer the lines are together,
the stronger the field.
 Magnetic field lines always point
away from a magnet’s north pole
and toward its south pole.
Magnetic Properties of Materials
Key Question:
How do magnets interact
with different materials?
Magnetic Properties of Materials
 The sources of nearly all
magnetic effects in matter
are the electrons in atoms.
 There are two ways in which
electrons create magnetism:
1. Electrons around the
nucleus and their motion
makes the entire atom a
small magnet.
2. Electrons themselves act
as though they were
magnets.
Magnetic Properties of Materials
 All atoms have electrons, so you might think that all
materials should be magnetic, but there is great
variability in the magnetic properties of materials.
 The electrons in some atoms align to cancel out
one another’s magnetic influence.
 While all materials show some kind of magnetic
effect, the magnetism in most materials is too weak
to detect without highly sensitive instruments.
Magnetic Properties of Materials
 In diamagnetic materials,
the electrons are oriented
so their individual magnetic
fields cancel each other out.
 Individual atoms in
paramagnetic materials are
magnetic but the atoms
themselves are randomly
arranged so the overall
magnetism of a sample is
zero.
When paramagnetic materials
are placed in a magnetic field,
the atoms align so that the
material is weakly magnetic.
Magnetic Properties of Materials
 A small group of metals have
very strong magnetic
properties, including iron,
nickel, and cobalt.
 These metals are the best
known examples of
ferromagnetic materials.
 Atoms with similar magnetic
orientations line up with
neighboring atoms in groups
called magnetic domains.
Magnetic Properties of Materials
 Magnetic domains in a ferromagnetic material will
always orient themselves to attract a permanent
magnet.
— If a north pole approaches, domains grow that have south
poles facing out.
— If a south pole approaches, domains grow that have north
poles facing out.
Magnetic lab activity 4
1.
2.
3.
4.
Draw a map of what you see
Find the N and S direction
Identify the direction I tell you
Plan to get to that place and then come back to the initial point using
compass for both ways
5. Record how many steps you did. Can you tell approximately the distance
you walked?
6. Record everything and make necessary conclusions
Properties of magnets
 Materials that make good
permanent magnets are
called hard magnets.
 Steel, which contains iron
and carbon, is a common
and inexpensive material
used to create hard
magnets.
 Materials that lose their
magnetism quickly are
called soft magnets.
The Magnetic Field of the Earth
Key Question:
How do we use Earth’s
magnetic field to tell
direction?
The Magnetic Field of the Earth
 As early as 500 B.C. people
discovered that some naturally
occurring materials— such as
lodestone and magnetite—have
magnetic properties.
 By 1200, explorers from Italy
were using a compass to guide
ocean voyages beyond the sight
of land.
The Magnetic Field of the Earth
 When you use a compass, the
north-pointing end of the needle
points toward a spot near (but
not exactly at) the Earth’s
geographic north pole.
 The Earth’s magnetic poles are
defined by the planet’s
magnetic field.
 That means the south magnetic
pole of the planet is near the
north geographic pole.
The Magnetic Field of the Earth
 The gauss is a unit used to measure the strength of a
magnetic field.
 The magnetic field of the Earth is very weak (0.5
gauss) compared with the strength of the field on the
surface of the classroom ceramic magnets (1000
gauss).
 Historical data shows that both the strength of the
Earth’s magnetic field and the location of the north and
south magnetic poles can switch places.
 Today, the Earth’s magnetic field is losing
approximately 7 percent of its strength every 100
years.
The Magnetic Field of the Earth
 Depending on where you are, a compass will point
slightly east or west of true north.
 The difference between the direction a compass points
and the direction of true north is called magnetic
declination.
After correcting for the
declination, you rotate the
whole compass until the
north-pointing end of the
needle lines up with zero
degrees on the ring.
The large arrow points in the
direction you want to go.
Application: Magnetic Resonance
Imaging
EXIT QUIZ
 1. what is 10th element?
 2. what is 6th element?
 3. what is 4th element?
 4. what are inert gases?
 5. what is another name for them?