Transcript Magnetism - UCF Physics

PHY 2054 Magnetism - I
An Attractive New Topic
This Magnetic Week
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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.
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Wednesday – We will continue with
magnetism – assuming that you have
viewed the Monday Lecture.
Quiz next Friday
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EXAMS WILL EVENTUALLY BE RETURNED
Maybe
WELCOME BACK
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How Did You Do??
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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?
A.
B.
C.
D.
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A Lot
A Little
Not really
No
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Magnetism was known
long ago.
Refrigerator
Magnetics
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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• 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.
Compare to Electrostatics
N
Magnet
What Happens??
S
Pivot
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• Notice the general behavior trends of attraction and repulsion, dipole or monopole.
Magnets
S N
Shaded End is NORTH Pole
Shaded End of a compass points
to the NORTH.
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• Like Poles Repel
• Opposite Poles
Attract
• Magnetic Poles are
only found in pairs.
– No magnetic
monopoles have
ever been
observed.
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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
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

B
N
S
The magnetic Field B goes from North to South.
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Introduce Another Permanent Magnet

B
N
N
S
pivot
S
The bar magnet (a magnetic dipole) wants to align with the B-field.
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Field of a Permanent Magnet

B
N
N
S
S
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.
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field attracts and exerts a torque on the small magnet.
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Field of a Permanent Magnet

B
N
N
S
S
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
Examples of Creating Magnetic fields
• Fields are created by electric currents in a variety of
ways and observed in a variety of places.
Convention For Magnetic Fields
X
Field INTO Paper
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B

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.
B
Right hand RuleThumb 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
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– 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
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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.
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!
Compass
Direction
Magnetic
DIRECTION
S
N
And it REVERSES from time to time.
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A Look at the Physics

B
q

v

q B
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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.
• If the charge is moving in a
different direction, there
is a force on the charge,
perpendicular to both v and
B.
F=q vB
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Nicer Picture
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Another Picture
The Vector Cross Product
F  qv  B
F  qvB sin(  )
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Practice
Which way is the Force???
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Units
F  Bqv Sin(θ )
Units :

F
N
N
B


qv Cm / s Amp  m
1 tesla  1 T  1 N/(A - m)
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teslas are
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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:
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
F  qE
F  qvB sin(  )
(Use “Right-Hand” Rule to
determine direction of F)
And --- the charge must be moving !!
• The effect of an
existing magnetic
field on a charge
depends on the
charges direction
of motion relative
to the field.
The Right Hand Rule
The effect of the sign of a moving
Positive and negative
charges will feel
opposite effects
from a magnetic
field.
Mass Spectrometer
Smaller Mass
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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
2mK
B
2 2
ed
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Problem Continued
r
From Before
mv
r
qB
1 2
2K
K  mv so v 
2
m
m 2K
2mK
r

d
2 2
eB m
e B
Solve for B :
2mK
B
e2d 2
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Let’s 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 ?.
x x x B
E
x x x
v
q• m
FE = q E and FB = q v B
If FE = FB the particle will move
following a straight line trajectory
qE=qvB
v=E/B
FB FE
•
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