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Transcript
objectives
1.
2.
3.
4.
5.
Understand the motion of charges relative to each
other produces a magnetic force.
For given situations, predict whether magnets will repel
or attract each other.
Describe the magnetic field around a permanent
magnet.
Describe the orientation of Earth’s magnetic field.
Understand the relative motion between a conductor
and a magnetic field may produce a potential difference
in the conductor.
The Earth’s Shield
4.3.1 Magnetic Fields and
Electromagnetic Induction
Magnetism
• Magnetism refers to physical phenomena arising from the force
between magnets, objects that produce fields that attract or repel
other objects.
• A magnet is any piece of material that has the property of
attracting iron (or steel).
• Magnetism may be naturally present in a material or the
material may be artificially magnetized by various methods.
• Magnets may be permanent or temporary.
• Materials which can be magnetized are called ferromagnetic
materials.
What is a magnetic field and how is it created?
• The cause of magnetism is from a property of the atoms.
• Atoms have a positively charged center called the nucleus and is
orbited by one or more negatively charged particles called
electrons.
• The electrons spin as they travel around the nucleus (which
contain protons and neutrons) much like the earth spins as it
orbits the sun.
• As the electrons spin and orbit the nucleus, they produce a
magnetic field.
Moving electrons creates magnetic fields.
Not all atoms have magnetic fields
• All the electrons produce a magnetic field as they spin and orbit
the nucleus; however, in some atoms, two electrons spinning and
orbiting in opposite directions pair up and the net magnetic field
of the atom is zero.
• Materials with one or more unpaired electrons are magnetic.
Materials with a small attraction to a magnet are called
paramagnetic materials, and those with a strong attraction are
called ferromagnetic materials. Iron, cobalt, and nickel are
examples of ferromagnetic materials.
•Not all the fields are aligned, but
when canceling spins are accounted
for, a net magnetic field remains.
MAGNETIC DOMAIN
• A magnetic domain is a region in which the magnetic fields of
atoms are grouped together and aligned.
• You can think of magnetic domains as miniature magnets within
a material.
One way to magnetize a metal
is to rub a magnet over the
surface of the metal.
MAGNETIC PROPERTIES
1. Attracts iron containing objects.
2. It has two ends called poles: north pole and south
pole. North pole points to North of Earth and south
pole points to South of Earth.
3. No matter how many times a magnet is broken,
each piece always has a north pole and a south
pole.
4. Like poles repel each other, unlike poles attract
each other.
Compass
• The needle of a compass is a small MAGNET
• The north pole of a compass needle
– … is marked with a small “N” or a prominent color
– … points toward the geographic north direction,
but magnetic SOUTH poles.
The geographic
NORTH pole of the
Earth acts like the
SOUTH pole of a
magnet!
Magnetic fields
• The region where magnetic force exists around a magnet is
called its magnetic field
• Magnetic field allows magnets interact without touching.
Magnetic force is a non-contact force.
• A magnetic field exerts a force on any moving charge and can
be measured and detected by this effect.
Magnetic Field Lines
• Rules for field line directions
1. The direction is defined as the direction the N-pole of a compass
would point in the field.
• Outside magnet  NORTH TO SOUTH
• Inside magnet  SOUTH TO NORTH
2. Magnetic lines
never intersect
3. Magnetic field is
strongest where the
lines are closest.
Conventions for the direction of a
magnetic field
In the plane of the page
Into the page
Out of the page
x
Magnetic Field Maps
N
S
N
S
Magnetic Field Maps
S
N
N
S
Magnetic Field Maps
S
N
Example #1
• Determine the direction of the magnetic field lines at
each point. Where is the field strongest?
Field is strongest
at point C.
A
B
N
S
C
D
Example #2
• Determine the direction of the magnetic field lines at
each point. Where is the field strongest?
H
E
S
N
N
F
Field is strongest
at point F.
G
S
Example #3
• Determine the direction in which the compass will
point.
S
N
S
N
Example #4
• The diagram below represents the magnetic
field near point P. If a compass is placed at point
P in the same plane as the magnetic field, which
arrow represents the direction the north end of
the compass needle will point?
S
A
N
B
C
D
Current produce magnetic field
• Moving charges produce magnetc field. An electric current
are moving charges. An electric current can cause a magnetic
field around it just like a magnet causes a magnetic field.
Magnetic field lines
ELECTROMAGNETS
• A conductive wire with a current flowing through it creates a
magnetic field. However, the magnetic field of one wire is small
and does not have much strength
• If we take a wire and coil it several times to form a long coiled
piece of electrical wire, we would have a magnetic field much
bigger and stronger when we turn on the current.
If an iron bar is placed through the
center of the coiled wire, it would
become a temporary magnet, called
an electromagnet, as long as the
electric current is flowing through
the wire.
Electromagnetic induction
• ..\..\labs\phet labs\generator_en.jar
• Just as an electrical current induces a magnetic field, when a
magnet is moved in and out of coils of wire or when an
electrical wire cuts across magnetic lines of force, a magnetic
force acts on the electrons and producing an induced potential
difference. As a result, current is generated. This process is
called electromagnetic induction.
• It does not matter if the magnet is moved or if the coils of wire
are moved. The important thing is that there is motion within
the magnetic field, and that the magnetic lines of force are cut.
Generator
• The electromagnetic induction is the principle by
which electric generators can make electricity.
Through the use of magnets, a generator can convert
mechanical energy to electrical energy.
• Inside a generator is a magnet, some electrical wire,
and a source of mechanical energy. The mechanical
energy moves the wire into the magnetic field of the
magnet so that the wire cuts through the magnetic
lines of force. As a result, electric current is produced.
• ..\..\RealPlayer Downloads\How generator works by
Khurram Tanvir.flv
• ..\..\RealPlayer Downloads\Magnetism- Motors and
Generators.flv
example
•
1.
2.
3.
4.
An accelerating particle that does not
generate electromagnetic waves could
be
a proton
a neutron
an electron
an alpha particle
Example
•
Which procedure will produce the greatest induced
potential difference in the conductor?
1. holding the conductor stationary between the poles
2. moving the conductor out of the screen
3. moving the conductor toward the right side of the
screen
4. moving the conductor toward the N-pole