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# Loose ends and Magnetic Fields in Astronomy - PowerPoint PPT Presentation

Loose ends and Magnetic Fields in Astronomy. Exam tips – come to class! - Course Evaluations – come to class! - energy in fields - magnetism in matter - astronomical applications. Energy Density in EM Waves. The energy density, u , is the energy per unit volume

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Loose ends and Magnetic Fields in Astronomy

• Exam tips – come to class!- Course Evaluations – come to class!- energy in fields- magnetism in matter

• - astronomical applications

• 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

• 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

• 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 εoE2max = B2max / 2μo

Recall these equations:I=Q/t

T=2πr/vL=mvrμ=IA (sub in the above, and A=πr2)Imagine a little circuit with an electron moving at a distance

r from the nucleus of an atom. Combining these, we can

show that:

This is the magnetic moment of anelectron

Now, because the angular momentum L for electrons isquantized, L=The smallest possible non-zero value of the magnetic

moment is :

Q: So, if all electrons have this unit of magnetic moment,

why are not all substances magnetic ???

Magnetic fields ‘rotate’ an electromagnetic wave. This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.

This is the spiral This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.

galaxy M51 and its

magnetic field.

Studying the fields

helps us understand

how galaxies form and

evolve.

This is the jet of M87, This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.

evidence for a super

massive black hole at

the center of this

galaxy.

Sunspots are caused by This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.

strong magnetic fields on

the surface of the sun.

We can detect them on other

stars currently!

A supercomputer This is called Faraday Rotation, and it allows us to measure the magnetic field of interstellar space.

simulation of the

reversal of earth’s

magnetic field

The earth’s magnetic field.

Planetary formation needs to incorporate magnetic fields in the ‘protoplanetary’ disc.

These are called MHD (magneto-hydro-dynamics simulations.

Redshift of EM waves the ‘protoplanetary’ disc.

Galaxy distribution the ‘protoplanetary’ disc.

Large Scale Galaxy the ‘protoplanetary’ disc.

Distribution

HDF and early the ‘protoplanetary’ disc.

galaxy formation