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Transcript
```Magnetism and You
Fields
– Areas in which a force acts
– Lines represent the direction in which the
force acts on a body
– Fields for every force: gravity, electricity,
magnetism, etc.
– Sometimes multiple forces act in the same
area—this creates complicated field
situations.
1
Gravitational Field
–
Direction in which the force acts is toward the center
of the earth
Field lines point toward the center of the earth
–
–
The lines may represent the strength of the field also
•
•
Stronger as you get closer
Lines in a changing field are called gradients – they
represent the way the field changes in addition to its
direction (lots of information in one little picture)
Electric Fields
–
–
–
–
•
•
We create them by putting charges on things
+ or -, and the size varies.
They only affect charged objects (We could stand in
an electric field and not notice it)
Apply forces to charged particles just as gravity
applies forces to masses:
F=qE (q = charge, E = electric field strength)
F=mg (m = mass, g = gravitational field
strength)
–
strong electric field will accelerate a particle in the
direction of the field that opposes its charge (+
charges go toward the - side of the field)
2
Magnetic Fields
• Given the symbol B (?)
• Are created two different ways:
– Alignment of electron spins in substances
(ferromagnetism if natural; paramagnetism if created
by another B field)
– Charges moving in an electric field
• The relationship between electricity and
magnetism has been known for 150 years or so
– All electric devices create magnetic fields
– Moving magnets are used to generate electricity
(dynamo)
What does a B field look like?
• Shape varies, and can only be
demonstrated by something subject to B
• Can be modeled because the theory is so
well-known
– simulation
3
Magnetic AND electrical fieldstogether at last!
• The two are related:
electric and magnetic
fields are always
perpendicular
• Magnetic fields are
created by moving
charges…
• Right Hand Rule!
Thumb points in the direction of
the moving charge (or current
of the magnetic field!
Works in reverse too; thumb
towards B field, fingers show
direction of motion of a charged
particle!
Charged Particles in E/B Fields
• Cyclotron motion: when a charged particle
has velocity and enters a magnetic field
– Images
• Particles in the Earth’s magnetic field are
subject to cyclotron motion as well
4
Van Allen Belts
• Prof. James Van Allen
discovered these from
first satellites (Explorer I)
• Three major belts
– Proton belt (500-13,000km)
– Low-energy electron belt
(overlaps proton belt)
– High-energy electron belt
(farther out)
Simulated belts
lab. See the lab
guy?
Van Allens = Cyclotron motion?
• Yes.
Particles bounce around and follow the field lines. When they reach the end of a
field line, they reverse direction (the flux is so great they cannot proceed through
it)
5
Aurorae
Note:
Aurorae
• 2, Aurora Borealis (north); Aurora Australis
(south)
• Both explained (sort of) the same way:
– Charged particles trapped in the Earth’s B field are
accelerated toward the poles
– They collide with gases in the upper atmosphere,
causing the gas molecules to emit light
•
•
•
•
Red = O at 200 miles
Green = O at 60 miles
Blue = N* at high altitudes
Crimson/purple = N at lower altitudes
6
Aurora photographed from ISS – 200 mi. above earth
Crater
UV photo of proton aurora – IMAGE satellite
7
Aurora “Sounds”
• Moving charges create EMF signals, often
in the radio part of the spectrum.
• Radio signals can be turned into sounds
(they have the same wavelengths as
sound waves)
B Fields – Protecting the Earth
8
Bow Shock
• Charged particles of the solar wind strike
the earth’s field lines and are deflected
around the earth like wind around a car
• The majority of particles pass right by the
earth
Summary
• Charged particles follow circular paths around
field lines in magnetic fields (cyclotron motion –
right hand rule!)
• Planetary magnetic fields are the result of
circulating liquid metal in the planet’s core
• Planetary magnetic fields shield planetary
surfaces from damaging/lethal charged particle
radiation (Van Allen belts on Earth)
• Interaction between PMF and charged particles