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11/24/2014
Magnetic interaction
Magnetism
Ch 17
• If you bring the like poles
of two magnets near each
other, they repel each
other.
• If you bring opposite poles
near each other, they
attract each other.
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Magnetic Field
Magnetic interaction
• Symbol: B (it is a vector)
• Unit: Tesla (a derived unit)
More about this unit later with we have an equation.
• Testing Tool: A Compass
The north pole of a compass points toward geographic north (Canada); the
south pole points toward geographic south (Mexico).
• Earth acts as a giant magnet,
with its magnetic south pole
close to its geographic north
pole and its magnetic north pole
close to its geographic south
pole.
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Representing the magnetic field: Field lines
Magnetic field
Forces
• Magnets interact without
contact; we introduce the
magnetic field as the
mechanism behind
magnetic interactions.
• We can assume that a
magnet produces a
magnetic field—a magnetic
disturbance with which
other objects with
magnetic properties (e.g.,
another magnet, anything
made of iron) interact.
Non-Contact Contact
Gravitational Normal
Electric
Friction
Magnetic
Tension
Buoyancy
Forces and Fields
Force Field Interacting Particle
Gravitational
W, Fg
g
Mass
Electric
FE
E
Charge
Magnetic
FB
B
magnet
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Magnetic field produced by a current
Direction of the
magnetic field
• Charged objects in motion
produce a magnetic field;
stationary charged objects
do not.
• The method for
determining the shape of
the B field produced by the
electric current in a wire is
called the right-hand rule.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Right-hand rule for the B field
Observational
experiment
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© 2014 Pearson Education, Inc.
Magnetic fields
produced by
different
shapes of
currentcarrying wires
Current loops and bar magnets
• The B field produced by the current in a
loop or a coil and that produced by a bar
magnet are very similar.
=
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• Wire coils with current are known as
electromagnets.
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Magnetic permeability
Magnetic Fields
Are Caused By:
• The constant μo is known as the magnetic permeability. It is used
when calculating the magnetic field in a vacuum, although the value
is approximately the same for air.
• μ is the magnetic permeability of a substance and replaces μo if the
magnetic field is being calculated inside a material.
Magnets
Show direction by using a
compass
Current Carrying Wires
Straight wire
Loop
Coil
Show direction with a RHR
Show direction with a RHR
Show direction with a RHR
Magnitude by the equation:
Magnitude by the equation:
Magnitude by the
equation:
• μ for iron is approximately 1000 times larger than μo.
=
=
=
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Magnetic force exerted on a current
Right-hand rule for the magnetic force
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Magnitude of the force that a magnetic field
exerts on a moving charged particle
Magnitude of the force that a magnetic field
exerts on a moving charged particle
• We use the equation for the force a magnetic field
exerts on a current-carrying wire to determine the
force exerted on an individual charge:
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© 2014 Pearson Education, Inc.
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Vector Multiplication
Ampere
Dot Product (Results in a SCALAR)
Cross Product (Results in a VECTOR)
Work (ch6)
Torque (ch7)
=
∥
·∆
Flux (ch18)
=
×
Force on charge in B field (ch17)
Φ=
∥
·
=
×
=
×
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The direct current electric motor
Quantitative Exercise 17.3
• Each of the lettered dots
shown in the figure
represents a small object
with an electric charge
+2.0 x 10–6 C moving at a
speed 3.0 x 107 m/s in the
directions shown.
Determine the magnetic
force (magnitude and
direction) that a 0.10-T
magnetic field exerts on
each object.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Circular motion in a magnetic field
Mass spectrometer
• The force exerted by the
magnetic field always
points perpendicular to the
particle's velocity, toward
the center of the particle's
path.
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• A mass spectrometer helps determine the
masses of ions, molecules, and even
elementary particles such as protons and
electrons.
• It can also determine the relative
concentrations of atoms of the same
chemical element that have slightly
different masses.
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Intensity-modulated radiation therapy (IMRT)
• An IMRT machine
accelerates electrons to the
desired kinetic energy, then
uses a magnetic field to
bend them into a target,
resulting in the production
of X-rays. Movable metal
leaves then shape the X-ray
beam to match the shape
of the tumor.
Ions moving through a perpendicular
magnetic field and electric field
• This device separates positively and negatively
charged particles.
• When the electric and magnetic forces exerted on the
moving charged particles balance, the ions travel with
constant velocity downward despite the presence of
both a magnetic field and an electric field.
© 2014 Pearson Education, Inc.
© 2014 Pearson Education, Inc.
Magnetic properties of
atoms
Diamagnetic materials
• Each electron has a magnetic
dipole moment (an electron
orbital magnetic moment).
• The electron itself acts like a
tiny bar magnet, which also
contributes to the total
magnetic moment produced
by the atom.
• In diamagnetic materials, the magnetic moments produced by
individual electrons in the atoms cancel each other, making the total
field produced by the atom zero.
• In the presence of an external magnetic field, the motion of the
electrons in the individual atoms changes slightly, and the net
magnetic field in the material is no longer zero, causing the
diamagnetic object to be repelled by the magnet.
• Electrons are paired in the orbital
• Examples: water, most body chemicals
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© 2014 Pearson Education, Inc.
Paramagnetic materials
Ferromagnetic materials
• If the orbital magnetic moments of
the electrons don't cancel, an
atom will have a magnetic
moment similar to that of a small
bar magnet.
• When a paramagnetic material is
placed in an external magnetic
field, the atoms behave like tiny
compasses and tend to align with
that external magnetic field.
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• Ferromagnetic materials have
individual atoms with magnetic
moments, just like paramagnetic
materials.
• The "magnetization" effect in an
external magnetic field is thousands
of times stronger in ferromagnetic
materials than in paramagnetic
materials.
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