Download magnetism

objectives
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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.
Magnetism
• Magnetism is a FIELD FORCE of attraction
or repulsion in and around a material
– Acts over a DISTANCE without CONTACT
• Magnets produce FIELDS around them that
influence some types of metal
– IRON, NICKEL, and COBALT
• Closely related to ELECTRICITY
What is a magnet?
• 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.
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.
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. A nucleus contains one or more protons and
neutrons 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.
•In an unmagnetized
object, all the magnetic
domains are pointing in
different directions.
When the metal became
magnetized, all like
magnetic poles lined up
and pointed in the same
direction.
• When we rubbed the magnet over the surface of the metal,
some of the magnetic domains aligned and the metal became
partially magnetized. The more we rub the magnet over the
metal, the stronger the magnetic domains became aligned and
the metal became a stronger magnet. You can turn a paper clip
into a magnet this way.
THE TWO ENDS OF A MAGNET
• One end of any bar magnet will always want to point
north if it is freely suspended. This is called the
north-seeking pole of the magnet, or simply the
north pole. The opposite end is called the south pole.
• The needle of a compass is itself a magnet, and thus
the north pole of the magnet always points north.
• However, when you bring the compass near a strong
bar magnet, the needle of the compass (north pole)
points in the direction of the south pole of the bar
magnet.
• We can conclude that the north end of a compass is
attracted to the south end of a magnet.
• ..\..\labs\phet labs\magnets-and-electromagnets_en.jar
• Since the north seeking pole of
the compass needle is always
attracted to the north, then the
earth must be like a huge magnet
with a magnetic pole at each end.
• This can be a little confusing
since it would seem that what we
call the North Pole of the Earth
is actually its magnetically
south pole.
• This is exactly the case but
magnetic north is slightly
different from the north axis of
rotation of the earth. Scientists
believe that the movement of the
Earth's liquid iron core and other
things are responsible for the
magnetic field around the earth.
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 noncontact force.
• A magnetic field exerts a force on any
moving charge and can be measured
and detected by this effect.
Magnetic Field Lines
Magnetic
Magnetic
fields
fields
lines
can
travel
be represented
from NORTH
using
poles
FIELD
to LINES
SOUTHdrawn
poles OUTSIDE
around magnetic
magnets
objects
Field lines DO NOT CROSS one another
Magnetic flux lines
(field lines)
1. Magnetic flux lines never intersect and are
closed – north to south
2. The direction is defined as the direction the Npole of a compass would point in the field.
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 strength
• The number of magnetic lines of
flux per unit area passing through a
plane perpendicular to the direction
of the lines is called the magnetic
field strength, B, or flux density.
• Magnetic field strength is a vector
quantity
• Weber, or Wb, is a derived SI unit
for measuring the number of lines of
flux
• Tesla, T, is the derived SI unit of flux
density or magnetic field strength 1
T = 1 Wb/1 m2
example
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In the diagram, a steel paper clip is attached to a string,
which is attached to a table. The clip remains
suspended beneath a magnet. As the magnet is lifted
the paper clip begins to fall as a result of
an increase in the potential energy of the clip
an increase in the gravitational field strength near the
magnet
a decrease in the magnetic properties of the clip
a decrease in the magnetic field strength near the clip
example
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The diagram shows the magnetic field that
results when a piece of iron is placed between
unlike magnetic poles. At which point is the
magnetic field strength greatest?
A
B
C
D
example
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The diagram below shows a bar magnet.
Which arrow best represents the
direction of the needle of a compass
placed at point A?
↑
↓
→
←
example
• Which diagram best represents the
magnetic field between two magnetic north
poles?
A
B
C
D
example
• 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
ELECTROMAGNETS
• Moving electrons produce magnetic
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.
• When the compass is put near the
electrical wire with current flowing
through it, the compass needle
pointed in the direction of the
current's magnetic field.
• Moving CHARGES makes a MAGNETIC
FIELD
Magnetic field around a current carrying wire
Magnetic field of a current carrying wire
• Two current carrying wires exert a force on each
other through their magnetic fields.
• The magnetic field produced by one wire exerts a
force on the charges in the other wire.
Same direction of current,
wires attracts each other
opposite direction of current,
wires repel each other
• 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.
• An iron bar placed through the center of the coiled
wire would become a temporary magnet, called an
electromagnet, as long as the electric current is
flowing through the wire.
• ..\..\labs\phet labs\magnets-and-electromagnets_en.jar
Electromagnetic induction
• ..\..\labs\phet labs\generator_en.jar
• http://www.vjc.moe.edu.sg/academics/dept/physics_d
ept/applet/fara/faraday_demo.htm
• 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 in the
conductor causing a difference of the amount of negative
charge at each end of the conductor 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
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An accelerating particle that does not
generate electromagnetic waves could
be
a proton
a neutron
an electron
an alpha particle
Example
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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
Electromagnetic radiation
• Electromagnetic radiation is the changing
electric and magnetic fields that radiate outward
into the surrounding space in the form of waves.
It is also called electromagnetic waves.
• Electromagnetic radiation is produced by
oscillating or accelerating electric charges.
• http://www.phys.hawaii.edu/~teb/java/ntnujava/e
mWave/emWave.html