PHY 2054 Magnetism - I

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PHY 2054 Magnetism - I

• An Attractive New Topic

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This Magnetic Week

• Today we begin chapter 20 Magnetism
• There will be NO CLASS on MONDAY
• There will be a PowerPoint presentation posted.
  Run it as a presentation (click the screen icon
  bottom right) and it will play as a recording.
  Print it as usual.
• Wednesday We will continue with magnetism
  assuming that you have viewed the Monday Lecture.
• Quiz next Friday

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WELCOME BACK

• EXAMS WILL EVENTUALLY BE RETURNED
  Maybe

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How Did You Do??

• A. 80-100
• B. 60-79
• C. 40-59
• D. 20-39
• E. 0-19
• F. Less than 0

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Did the Card Help?

1. A Lot
2. A Little
3. Not really
4. No

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Magnetism was known long ago.
Refrigerator Magnetics
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Lodestone (Mineral)

• Lodestones attracted iron filings.
• Lodestones seemed to attract each other.
• Lodestone is a natural magnet.

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New Concept

• The Magnetic Field
• We give it the symbol B.
• A compass will line up with it.
• It has Magnitude and direction so it is a VECTOR.
• There are some similarities with the Electric
  Field but also some significant differences.

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Magnetism

• Refrigerators are attracted to magnets!

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Where is Magnetism Used??

• Motors
• Navigation Compass
• Magnetic Tapes
• Music, Data
• Older Television Tubes Oscilloscopes
• Beam deflection Coil
• Magnetic Resonance Imaging

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And in magnets!!
Although the magnet on the left is an
electromagnet/huge and the one on the right is a
permanent magnet/small, the idea is the same.
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Compare to Electrostatics
N S
Magnet
What Happens??
Pivot
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• Notice the general behavior trends of
  attraction and repulsion, dipole or monopole.

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Magnets

• Like Poles Repel
• Opposite Poles Attract
• Magnetic Poles are only found in pairs.
• No magnetic monopoles have ever been observed.

S N
Shaded End is NORTH Pole Shaded End of a compass
points to the NORTH.
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Observations

• Bring a magnet to an electrically charged object
  and the observed attraction will be a result of
  charge induction or polarization.
• Magnetic poles do not interact with stationary
  electric charges.
• Bring a magnet near some metals (Co, Fe, Ni )
  and it will be attracted to the magnet.
• The metal will be attracted to both the N and S
  poles independently.
• Some metals are not attracted at all. (Al, Cu,
  Ag, Au)
• Wood is NOT attracted to a magnet.
• Neither is water.
• A magnet will force a compass needle to align
  with it. (No big Surprise.)

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Magnets
Magnetic Field
N S
N S
Cutting a bar magnet in half produces TWO bar
magnets, each with N and S poles.
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Consider a Permanent Magnet
The magnetic Field B goes from North to South.
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Introduce Another Permanent Magnet
pivot
The bar magnet (a magnetic dipole) wants to align
with the B-field.
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Field of a Permanent Magnet
The south pole of the small bar magnet is
attracted towards the north pole of the big
magnet. The North pole of the small magnet is
repelled by the north pole of the large magnet.
The South pole of the large magnet creates a
smaller force on the small magnet than does the
North pole. DISTANCE effect. The field attracts
and exerts a torque on the small magnet.
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Field of a Permanent Magnet
The bar magnet (a magnetic dipole) aligns with
the B-field. It is now happy!
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Electric field of an electric dipole
Electric Field
Magnetic Field
The magnet behaves just like the Electric dipole
and aligns itself with A MAGNETIC
field. Similarities will continue.
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Iron filings will align as a compass does

• Each small filing lines up tangent to the field
  lines allowing a visual demonstration

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Examples of Creating Magnetic fields

• Fields are created by electric currents in a
  variety of ways and observed in a variety of
  places.

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Convention For Magnetic Fields
B
X
? Field INTO Paper
Field OUT of Paper
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Typical Representation
B
B is a vector!
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Experiments with Magnets Show

• Current carrying wire produces a circular
  magnetic field around it.
• Force (actually torque) on a Compass Needle (or
  magnet) increases with current.

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Current Carrying Wire
Current into the page.
Right hand Rule- Thumb in direction of the
current Fingers curl in the direction of B
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Current Carrying Wire

• B field is created at ALL POINTS in space
  surrounding the wire.
• The B field has magnitude and direction.
• Force on a magnet increases with the current.
• Force is found to vary as (1/d) from the wire.

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Compass and B Field

• Observations
• North Pole of magnets tend to move toward the
  direction of B while S pole goes the other way.
• Field exerts a TORQUE on a compass needle.
• Compass needle is a magnetic dipole.
• North Pole of compass points toward the NORTH.
• The NORTH geographic pole of the planet is
  therefore a magnetic South pole!

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Planet Earth
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Inside it all.
8000 Miles
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On the surface it looks like this..
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Inside Warmer than Floriduh
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Much Warmer than Floriduh
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Finally
Hot Hot Hot
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In Between

• The molten iron core exists in a magnetic field
  that had been created from other sources (sun).
• The fluid is rotating in this field.
• This motion causes a current in the molten metal.
• The current causes a magnetic field.
• The process is self-sustaining.
• The driving force is the heat (energy) that is
  generated in the core of the planet.

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After molten lava emerges from a volcano, it
solidifies to a rock. In most cases it is a black
rock known as basalt, which is faintly magnetic,
like iron emerging from a melt. Its magnetization
is in the direction of the local magnetic force
at the time when it cools down. Instruments can
measure the magnetization of basalt. Therefore,
if a volcano has produced many lava flows over a
past period, scientists can analyze the
magnetizations of the various flows and from them
get an idea on how the direction of the local
Earth's field varied in the past. Surprisingly,
this procedure suggested that times existed when
the magnetization had the opposite direction from
today's. All sorts of explanation were proposed,
but in the end the only one which passed all
tests was that in the distant past, indeed, the
magnetic polarity of the Earth was sometimes
reversed.
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Our Earth has a magnetic field.
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Repeat
Navigation DIRECTION N S
If N direction is pointed to by the NORTH pole of
the Compass Needle, then the pole at the
NORTH of our planet must be a SOUTH MAGNETIC POLE!
And it REVERSES from time to time.
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A Look at the Physics
There is NO force on a charge placed into
a magnetic field if the charge is NOT moving.
There is no force if the charge moves parallel to
the field.
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Nicer Picture
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Another Picture The Vector Cross Product
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Practice
Which way is the Force???
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Units
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teslas are
HUGE!
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The Magnetic Force is Different From the
Electric Force.
Whereas the electric force acts in the same
direction as the field
The magnetic force acts in a direction orthogonal
to the field
(Use Right-Hand Rule to determine direction of
F)
And --- the charge must be moving !!
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• The effect of an existing magnetic field on a
  charge depends on the charges direction of motion
  relative to the field.

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The Right Hand Rule
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The effect of the sign of a moving
Positive and negative charges will feel opposite
effects from a magnetic field.
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Mass Spectrometer
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(No Transcript)
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An Example
A beam of electrons whose kinetic energy is K
emerges from a thin-foil window at the end of
an accelerator tube. There is a metal plate a
distance d from this window and perpendicular to
the direction of the emerging beam. Show that we
can prevent the beam from hitting the plate if we
apply a uniform magnetic field B  such that
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Problem Continued
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Lets Look at the effect of crossed E and B
Fields
x
x
x
B
E
x
x
x
v

q , m
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What is the relation between the intensities of
the electric and magnetic fields for the
particle to move in a straight line ?.
FE q E and FB q v B
If FE FB the particle will move following a
straight line trajectory
q E q v B
FE
FB