Loose ends and Magnetic Fields in Astronomy - Exam tips – come to class! - Course Evaluations –...
Transcript of Loose ends and Magnetic Fields in Astronomy - Exam tips – come to class! - Course Evaluations –...
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Loose ends and Loose ends and Magnetic Fields in AstronomyMagnetic Fields in Astronomy
- Exam tips – come to class!- Course Evaluations – come to class!
- energy in fields- magnetism in matter- astronomical applications
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Energy Density in EM Waves
• The energy density, u, is the energy per unit volume
• For the electric field, uE= ½ εoE2
• For the magnetic field, uB = ½ μoB2
• Since B = E/c and 1 o oc μ ε
221
2 2B E oo
Bu u ε E
μ
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Energy Density
• The instantaneous energy density associated with the magnetic field of an EM wave equals the instantaneous energy density associated with the electric field
– In a given volume, the energy is shared equally by the two fields
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Energy Density
• The total instantaneous energy density is the sum of the energy densities associated with each field– u =uE + uB = εoE2 = B2 / μo
• When this is averaged over one or more cycles, the total average becomes – uav = εo(E2)av = ½ εoE2
max = B2max / 2μo
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Where does magnetism come from ????
Recall these equations:
I=Q/t
T=2πr/v
L=mvr
μ=IA (sub in the above, and A=πr2)
Imagine a little circuit with an electron moving at a distancer from the nucleus of an atom. Combining these, we can show that: L
m
e
2 This is the magnetic moment of an
electron
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Where does magnetism come from ????
Now, because the angular momentum L for electrons isquantized, L=
The smallest possible non-zero value of the magnetic moment is :
m
e
2
Q: So, if all electrons have this unit of magnetic moment, why are not all substances magnetic ???
....3,2,,0
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EM in Astronomy
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Magnetic fields ‘rotate’ an electromagnetic wave. This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.
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This is the spiral
galaxy M51 and its
magnetic field.
Studying the fields
helps us understand
how galaxies form and
evolve.
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This is the jet of M87,
evidence for a super
massive black hole at
the center of this
galaxy.
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Sunspots are caused by
strong magnetic fields on
the surface of the sun.
We can detect them on other
stars currently!
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A supercomputer
simulation of the
reversal of earth’s
magnetic field
The earth’s magnetic field.
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Planetary formation needs to incorporate magnetic fields in the ‘protoplanetary’ disc.
These are called MHD (magneto-hydro-dynamics simulations.
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Redshift of EM waves
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Galaxy distribution
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Large Scale GalaxyDistribution
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Computer simulations of large scale structure formation
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HDF and earlygalaxy formation