Basic Physics Concepts That Underlie Space Science

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Basic Physics Concepts That Underlie Space Science

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Title: Basic Physics Concepts That Underlie Space Science

1
Basic Physics Concepts That Underlie Space Science
Ruth Skoug LASSO Summer 2008 Workshop July 14-25,
2008
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Outline

Atoms and Atomic Structure
Forces
Electromagnetic Waves
Phases of Matter
Particle Distributions
Plasmas
Gravity/Orbits/Propulsion
Distance Scales
Time Scales

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Atom Smallest unit of matter that can be
identified as a given element Central nucleus
includes positively charged particles
protons electrically neutral particles
neutrons Negatively charged particles (electrons
) orbit the nucleus Number of protons
identifies the element

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Atom Smallest unit of matter that can be
identified as a given element Central nucleus
includes positively charged particles
protons electrically neutral particles
neutrons Negatively charged particles (electrons
) orbit the nucleus Number of protons
identifies the element

H or 1H
He or 4He
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Protons positive electrical charge mass
1.67x10-27 kg size 10-15 m
(0.000000000000001 m) Neutrons no electrical
charge mass 1.67x10-27 kg (approx. same as
proton) size 10-15 m (approx. same as
proton) Electrons negative electrical
charge mass 9.11x10-31 kg proton mass /
1800 size 10-18 m proton size / 1000
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
H (or H0)
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
H (or H0)
H
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
H (or H0)
H
He
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
H (or H0)
H
He
He
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Ions Charged atoms In space physics, ion is
used only to refer to positively charged atoms.
H (or H0)
H
He
He
He
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Isotopes Same element, different number of
neutrons
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Isotopes Same element, different number of
neutrons
1H (or H) (or even 1H0)
3H
2H
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Isotopes Same element, different number of
neutrons
1H (or H) (or even 1H0)
3H
2H
4He
3He
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Ions Charged atoms
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Ions Charged atoms
16O5
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Evolution of the Atomic Model

Plum pudding model
negatively charged electron raisins floating in
a positively charged pudding

1897 (Thomson)
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Rutherford Scattering
Expected scattering at small angles found
large-angle scattering Implies a heavy
nucleus Disproves Thomsons model
20
Evolution of the Atomic Model
1909 (Rutherford)
1897 (Thomson)

Scattering implies heavy nucleus
Electrons orbit around nucleus

21
Atomic/Molecular/Nuclear Transitions
Atoms/molecules/nucleii have a characteristic
lowest energy state (ground state) The addition
of energy raises the energy level by a quantized
amount To get back to the ground state, this
excess energy is released as an EM wave The
wavelength (color) of this light is a
characteristic of the material
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Evolution of the Atomic Model
1909 (Rutherford)
1897 (Thomson)

Characteristic colors for each atom imply
electrons can only be in special orbits
Electrons can jump from one orbit to another, but
no partial jumps
Energy can only change in small, discrete jumps
-- quanta

1913 (Bohr)
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Two-Slit Experiment
Shine light through two slits -- see an
interference pattern The same pattern is seen
when particles pass through two slits! Implies
wave-particle duality (particles can act like
waves)
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Evolution of the Atomic Model
1909 (Rutherford)
1897 (Thomson)

Wave/particle duality
Only know probability of an electron being in a
given location

1913 (Bohr)
Present (Schrodinger)
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Forces

Force mass acceleration (F m A)
Forces
Gravity
Electric force
Magnetic force
Strong and weak nuclear forces (not discussed
here)
Sources of magnetic fields (electricity
magnetism)
Dipole magnetic fields
Charged-particle motion in a dipole magnetic field

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Forces

Force mass acceleration (F m A)
Force is a push or pull on an object
Weight is a force how hard the gravity of the
Earth is pulling on something
Force is a vector -- both magnitude and direction
are important
A force can change the speed of an object
A force can change the direction of an object

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Particle motions are affected by forces Gravity
attraction between any two things which have
mass very weak force F Gm1m2/r2
r distance between objects M, m mass of
objects
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Particle motions are affected by
forces Electrical force acts between any two
charged particles opposite charges attract
(positive negative) like charges repel (both
or both ) neutral particles not affected
F kq1q2/r2
r distance between particles q charge of
particle
Electric field E Electric force per unit charge
F q E
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Magnetism

People have known for thousands of years that
certain stones attract iron
Magnets have north and south poles (dipoles)
Like poles repel
Opposite poles attract
Isolated magnetic poles are NOT observed

Compass needles (which are themselves magnets)
are attracted to magnets
North pole of compass attracted to south pole of
magnet

32
In 1600, William Gilbert determined that compass
needles point in specific directions because the
Earth is a permanent magnet
33

The Earths magnetic axis is currently 11 away
from the geographic spin axis
The Earths geographic North pole is a magnetic
South pole

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Magnetic field line patterns of a bar magnet
35

Near the Earth, the Earths field lines look like
those of a bar magnet
Particles can be trapped on these field lines
(Van Allen belts)
First discovery of the space age, 1958

Magnetism
There is a connection between electricity and
magnetism
1821 Oersted finds that an electric current
causes a compass needle to move
Upon learning this, Ampere finds
electric currents attract iron
parallel electric currents attract each other
proposes that electric currents are the source
of all magnetism
Magnetism is due to electric currents, that is to
moving electric charges

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Magnetism Magnetic force on charged particles
given by F q (v x B) q charge, v
velocity, B magnetic field force is
proportional to speed v force is always
perpendicular to the direction of motion force
is perpendicular to the magnetic field B
B
So, how do particles move in a magnetic field?
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Magnetism Magnetic force on charged particles
given by F q (v x B) force is
proportional to speed v force is always
perpendicular to the direction of motion force
is perpendicular to the magnetic field B

In circles around the field line
In a straight line along the field direction
Putting those together gives spirals

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Magnetism
So, back to magnetism in matter
Atoms contain electrons which
orbit the nucleus
spin on their own axis
These moving charges create magnetic fields
There are 3 types of magnetism
Paramagnetism permanent magnetic dipoles are
randomly oriented, dipoles can be aligned by an
external magnetic field
Ferromagnetism permanent magnetic dipoles are
aligned within domains, domains can be aligned by
an external magnetic field, and will stay aligned
after the field is removed
Diamagnetism no permanent magnetic dipoles, but
dipoles can be induced by an applied magnetic
field

41
Magnetic Domains
Unmagnetized material
Magnetized material
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Electromagnetic waves time-varying
combination of electric and magnetic fields
propagate in vacuum at the speed of light 3 x
108 m/s (300,000,000 m/s) carry energy
have no mass
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Electromagnetic Waves
Wavelength is the distance between wave crests or
troughs Period is how long (time) it takes to
cover one wavelength. Frequency 1 /
period Speed Distance / Time Wavelength /
Period Wavelength Frequency For
electromagnetic waves (in vacuum), speed is a
constant the speed of light (c 3x108 m/s)
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Electromagnetic Spectrum
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Electromagnetic Spectrum
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Electromagnetic Spectrum
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Radiation

Energy traveling through space
Waves (gamma rays, light, UV, microwaves, radio
waves, )
Particles (alphas (He), betas (electrons),
neutrons)
Some atoms and nuclei are unstable and give off
radiation when they decay (break apart)
Energy can also be released when atoms come
together (fusion)
Radioactive substances are those that decay
naturally

49
Phases of Matter

Temperature
Solids
Liquids
Gases
Plasmas

50
Energy

Energy is the ability to do work
Work is the transfer of energy
Work is moving an object by means of a force
Something moving has kinetic energy
E 1/2 m v2
m mass, v speed
Stored energy is called potential energy
Lift an object in a gravitational field
Compress a spring
Units of energy
Electron volt (eV)
Air molecules 1/40 eV
Atom at Suns surface 0.5 eV
Auroral particle 100010,000 eV (110 keV)
Ring current particle 50,000 eV (50 keV)
Radiation belt particles 10,000,000 eV (10 MeV)

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Heat and Temperature Heat can be thought of as
the internal energy of the atoms and molecules
that make up a substance. Temperature is a
relative term, and refers to the average kinetic
energy (KE 1/2 m v2) of the atoms and
molecules in a substance. Three temperature
scales Farenheit, Celsius, Kelvin
F (9/5) x C 32 C (5/9) x (F 32) K
C 273
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Cold substances are solid
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When they warm up, they become liquid
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When they warm up further, they become a gas
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Further warming can ionize the particles, forming
a plasma
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Phases of Matter
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Plasma
Ionized gas, consisting of positively and
negatively charged particles (ions and electrons)
Fourth state of matter
99 of the known material in the Universe
consists of plasma

Plasma
Ionized gas, consisting of positively and
negatively charged particles (ions and electrons)
Fourth state of matter
99 of the known material in the Universe
consists of plasma
Examples
Sun, solar wind, magnetosphere, ionosphere,
lightning discharge
Fluorescent light bulb, neon sign
Formed in reactors for fusion research

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Particle Distributions

Velocity distributions
Moments
density
velocity
temperature

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Describing a Gas
v
Each particle is characterized by its speed and
its direction of motion, i.e., its vector
velocity, v Full description Number of
particles N within a given volume of space ?x
that are moving with velocities in a range ?v
about v
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Distribution Function
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Particle Distributions the bell-shaped curve
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Shorthand Description Average Properties

Density Average number of particles (all
energies) within a given volume of space
Flow Velocity Average vector velocity of all
the particles in a given volume of space
Temperature Average kinetic energy of the
random motion relative to the average flow

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Average Properties (Moments)
Density area under the curve Flow speed
location of peak Temperature width of peak
)
Flow
x,v
Speed
f(
Density
Temperature
v
0
x
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Particle Distributions Spectrogram Display
104
103
102
Energy (eV)
101
100
Time -gt
68
Plasmas

Examples
Collective behavior
Frozen-in magnetic field
Reconnection
Shock waves

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Plasma Distributions
fe(x,v) fi1(x,v) fi2(x,v)
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Plasma Distributions Multiple Species
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Plasma Behavior
Charged particles respond to electric and
magnetic fields F qE F qvxB Charged
particles create electric and magnetic fields E
q/r2 B qv/r
Therefore, each particle feels the combined
effects of all the others Collective motion!
The fields and particles evolve self-consistently.
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Plasmas Behave Like a Fluid
Magnetic field line
Magnetic flux tube
Magnetohydrodynamics Flux tube maintains its
integrity as the plasma flows, and two parcels of
plasma linked by a flux tube remain linked.

Implications
The magnetic field is frozen-in to the flow.
The flow carries the imbedded magnetic field
along.
Plasmas have a hard time mixing across magnetic
field lines.

73
Magnetic ReconnectionLocal Violation of
Frozen-In Flow
Plays a crucial role in many plasma systems!
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Waves in Plasmas
Waves are the means for transmitting information
from one point to another.
Example Sound waves launched from the nose of an
airplane propagate into the incoming air flow and
tell it to deflect around the airplane.
In neutral gases, there is only one wave mode
sound waves. In plasmas, there are several wave
modes, with different propagation speeds.
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Shock Waves
What if the oncoming flow speed is greater than
the speed of the sound waves? How can the flow
know that it needs to deflect around the airplane?
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Shock Waves
What if the oncoming flow speed is greater than
the speed of the sound waves? How can the flow
know that it needs to deflect around the airplane?
Shock Wave
Sonic Boom!
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Shock Waves in Plasmas
Same principle, different physics!

Examples of Shock Waves in Plasmas
Earths bow shock
Interplanetary shocks
Supernova shocks
Shocks in the solar atmosphere
High-altitude nuclear explosions

Shock waves in plasmas can accelerate some
particles to extremely high energies (e.g.,
cosmic rays)!
78
Gravity/Orbits/Propulsion
Acceleration of a body experiencing a force F
m a (or a F/m) Gravitational force exerted
by body of mass M on another body of mass m F
GMm/r2 Resulting acceleration A
GM/r2 Orbital radius of a body in a circular
orbit R GM/v2 (or v2 GM/r) Orbital
period T (4p2/GM)1/2 r3/2
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Gravity/Orbits/Propulsion
Escape velocity vesc (2GM/R)1/2 Amount of
energy needed to move a body from R to
infinity E GMm/R (from the Earth, E/m6 x 107
joules/kg)
80
Distance Scales