PHY 2054 Magnetism - I An Attractive New Topic This Magnetic Week Today we begin chapter 20 –...
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Transcript of PHY 2054 Magnetism - I An Attractive New Topic This Magnetic Week Today we begin chapter 20 –...
PHY 2054 Magnetism - IPHY 2054 Magnetism - I
An Attractive New TopicAn Attractive New Topic
This Magnetic WeekThis Magnetic Week
Today we begin chapter 20 – MagnetismToday we begin chapter 20 – Magnetism There will be NO CLASS on MONDAYThere will be NO CLASS on MONDAY
• There will be a PowerPoint presentation posted. There will be a PowerPoint presentation posted. Run it as a presentation (click the screen icon Run it as a presentation (click the screen icon bottom right) and it will play as a recording. bottom right) and it will play as a recording. Print it as usual.Print it as usual.
Wednesday – We will continue with Wednesday – We will continue with magnetism – assuming that you have magnetism – assuming that you have viewed the Monday Lecture.viewed the Monday Lecture.
Quiz next FridayQuiz next FridayMagnetismMagnetism 22
WELCOME BACKWELCOME BACKEXAMS WILL EVENTUALLY BE RETURNED MaybeEXAMS WILL EVENTUALLY BE RETURNED Maybe
MagnetismMagnetism 33
How Did You Do??How Did You Do?? A. 80-100A. 80-100 B. 60-79B. 60-79 C. 40-59C. 40-59 D. 20-39D. 20-39 E. 0-19E. 0-19 F. Less than 0F. Less than 0
MagnetismMagnetism 44
Did the Card Help?Did the Card Help?
A.A. A LotA Lot
B.B. A LittleA Little
C.C. Not reallyNot really
D.D. NoNo
MagnetismMagnetism 55
Magnetism was known Magnetism was known long ago.long ago.
RefrigeratorMagnetics
Magnetism 7
Lodestone (Mineral)
• Lodestones attracted iron filings.
• Lodestones seemed to attract each other.
• Lodestone is a natural magnet.
Magnetism 8
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.
Magnetism 9
Magnetism
• Refrigerators are attracted to magnets!
Where is Magnetism Used??Where is Magnetism Used??
• Motors• Navigation – Compass• Magnetic Tapes
– Music, Data
• Older Television Tubes & Oscilloscopes– Beam deflection Coil
• Magnetic Resonance Imaging
Magnetism 10
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
Magnetism 12
N
S
Pivot
Magnet
What Happens??
• Notice the general behavior trends of attraction and repulsion, dipole or monopole.
Magnets
• Like Poles Repel• Opposite Poles
Attract• Magnetic Poles
are only found in pairs.– No magnetic
monopoles have ever been observed.
Magnetism 14
Shaded End is NORTH PoleShaded End of a compass points
to the NORTH.
S N
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.)
Magnetism 15
Magnets
Magnetism 16
Cutting a bar magnet in half produces TWO bar magnets, each with N and S poles.
Magnetic Field
N S N S
Consider a Permanent Magnet
Magnetism 17
N S
B
The magnetic Field B goes from North to South.
Introduce Another Permanent Magnet
Magnetism 18
N S
B
N
S
The bar magnet (a magnetic dipole) wants to align with the B-field.
pivot
Field of a Permanent Magnet
Magnetism 19
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.
N S
B
N
S
Field of a Permanent Magnet
Magnetism 20
N S
B
N S
The bar magnet (a magnetic dipole) aligns with the B-field.It is now happy!
Electric field of an electric dipole
Magnetism 21
Electric Field Magnetic Field
The magnet behaves just like theElectric dipole and aligns itself withA MAGNETIC field.
Similarities will continue.
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
Magnetism 24
X
Field INTO Paper Field OUT of Paper
B
Typical Representation
Magnetism 25
B
B is a vector!
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.
Magnetism 26
Current Carrying Wire
Magnetism 27
Current intothe page.
B
Right hand Rule-Thumb in direction of the currentFingers curl in the direction of B
Current Carrying WireCurrent 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.
Magnetism 28
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!
Magnetism 29
Planet Earth
Magnetism 30
Inside it all.
Magnetism 31
8000Miles
On the surface it looks like this..
Magnetism 32
Inside: Warmer than Floriduh
Magnetism 33
Much Warmer than Floriduh
Magnetism 34
Finally
Magnetism 35
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.
Magnetism 36
Magnetism 37
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.
Our Earth has a magnetic field.
Repeat
Magnetism 39
CompassDirection
NavigationDIRECTION
N
S
If N directionis pointed to bythe NORTH poleof the Compass Needle, then the
pole at the NORTHof our planet must
be a SOUTH MAGNETICPOLE!
MagneticDIRECTION
S
N
And it REVERSES from time to time.
A Look at the Physics
Magnetism 40
B
q
There is NO force ona charge placed into amagnetic field if thecharge is NOT moving.
Bq
v
• If the charge is moving in a different direction, thereis a force on the charge,perpendicular to both v and B. F = q v B
There is no force if the chargemoves parallel to the field.
Nicer Picture
Magnetism 41
Another Picture The Vector Cross Product
Magnetism 42
)sin(qvBF
q
BvF
Practice
Magnetism 43
Which way is the Force???
Units
Magnetism 44
m)-N/(A 1 T 1 tesla1
mAmp
N
/
:
)
sCm
N
qv
FB
Units
Bqv Sin(θF
teslas are
Magnetism 45
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:
And --- the charge must be moving !!
)sin(qvBF
F qE
(Use “Right-Hand” Rule to determine direction of F)
• 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
Magnetism 50
Smaller Mass
Magnetism 51
An Example
Magnetism 52
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
22
2
de
mKB
Problem Continued
Magnetism 53
r
22
22
2
e
2mKB
:Bfor Solvee
22
eB
m
2 so
2
1
qB
mvr
Before From
d
dB
mK
m
Kr
m
KvmvK
Magnetism 54
Let’s Look at the effect of crossedcrossed E and B Fields:
x
x xx
xx
•q , m
B
v
E
Magnetism 55
What is the relation between the intensities of the electric and magnetic fields for the particle to move in a straight line ?.
•FEFB
FE = q E and FB = q v B
If FE = FB the particle will movefollowing a straight line trajectory
q E = q v B
v = E / B
x
x xx
xx
•q m
B
v
EB
E