# The Newtonian Revolution: The discovery of natural law

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"The Newtonian Revolution: The discovery of natural law"

```					   Chapter 3 – Light, Matter
• The Atom and it’s particles
• The electromagnetic field
• Light – traveling electromagnetic waves, quantized
as “photons”
• A changing electric field generates… a magnetic
field. And, vice versa!
• Accelerate an electric charge, generates changing
E field and hence changing M field… = light!
• Two ways to create photons.
• Atoms and light; spectra. Emission and absorption
lines and how they tell us what stars and planets
Mass and Charge
• Mass (and equivalent energy, in General
Relativity) has an associated force: Gravity
• Another quality possessed by elementary
particles is charge. Comes in two flavors;
positive and negative. Likes repel, and
opposites attract. The associated force is
called Electromagnetism
The Three Stable Particles
Making up Most Ordinary
Matter
• The Proton: Mass = 1 U, net charge= +1
• The Neutron: Mass = 1 U, net charge = 0
• The Electron: Mass = 1/1860 U, net charge = -1
Only Charged Particles Feel the
Electromagnetic Force

• Likes repel: proton repels proton. Electron
repels electron
• Opposites Attract: Proton attracts Electron
A Caveat….

• Protons and neutrons are both examples of a type of
elementary particle called a Baryon. Baryons are made up
of more elementary particles called quarks.
• Quarks have charge; either - 1/3 or +2/3. The combination
of 3 of these will have either +1, or 0 net charge. However,
because the quarks are not all in exactly the same location,
there is a charge distribution, and therefore the neutron
does have a small but detectable interaction with the
electromagnetic field. This charge distribution is called -
its magnetic moment.
• Neutrons have net spin and this can interact with the EM
field
• Having said all this, you can forget it as far as any quiz
questions are concerned!
An Atom…
• A nucleus of protons (+ charge) and
neutrons (zero charge).
• A “cloud” of electrons surrounding the
nucleus; as many electrons as there are
protons.
• Electron energies are quantized – they can
only take on certain discrete values. This is
the realm of Quantum Mechanics, and your
intuition will need a heavy dose of new
thinking! (see chapter S4 if you dare…)
Different electron number
means different chemistry, so
we give different names…
• Hydrogen – 1 proton. ~90% of all atoms are
hydrogen
• Helium – 2 protons – most other atoms are helium
• Lithium – 3 protons
• Beryllium – 4 protons
• Boron – 5 protons
• Carbon – 6 protons
• Etc, to Uranium – 92 protons
isotopes
The 4 Phases of Matter
1. Solid

• Atoms are “elbow-to-elbow” and keep
their relative positions, but can still
vibrate. The material is
incompressible.
2. Liquid

• Atoms are still “elbow-to-elbow”, but
now there’s enough energy to keep
the atoms from locking together, and
they mill around square-dance
fashion. The material is still
incompressible, but now it can flow.
3. Gas

• Atoms now have enough energy to keep
from “sticking” together at all. They
richochette off each other violently, with
empty space around each atom. The atoms
(or molecules) now are caroming off each
other like balls on a pool table. With
empty space around each atom, the
material is now compressible.
4. Plasma = Ionized Gas
• There is so much energy that the atoms hit each other
so violently they knock electrons off and keep them
knocked off. Each such atom now called an “ion” and is
positively charged. Negatively charged electrons also
bouncing around.
• Charged – so feels the EM force, and in an EM field,
behaves in a complex way compared to an ordinary
(neutral) gas (more on EM fields later).
• Otherwise, it still is like a gas (compressible, empty
space around each ion)
• A gas may be “partially ionized” (only some atoms have
electrons knocked off), or “fully ionized”. Because the
ionized particles are knocking against the not-ionized
particles and thus influence their movement as well,
even a partially ionized gas has much in common with a
fully ionized gas.
Now Let’s See How Light Is
Produced
• To do that, we first need to get a feel for
how charges “feel” each other, and the
nature of the electromagnetic field
• Old “classical” view of electromagnetic
force, and the more modern “field”
“Action at a Distance” vs. field
• Original Newtonian paradigm: charges act
on other charges even if separated by
large distances – “action at a distance”.
• Modern paradigm: A charge doesn’t really
pull on a distant charge, instead, it sets up
an electromagnetic field around itself and
permeating all of space, and this field acts
directly on other objects.
• It is the FIELD which has reality, has
energy, and yet it itself has no mass.
• Things can physically EXIST and yet have
no MASS!
The Electromagnetic Field
• Charge sets up around itself an Electric
Field
• The force felt at a location has an amount,
and a direction, so it’s a vector field
• So, imagining a charge and the field it sets
up around itself, it is the FIELD which acts
on other charges. It is not the charges
which act on each other at a distance
A changing Electric field
creates an Magnetic field…
• Accelerate a charge, you wiggle the field,
and this wiggle moves outward at the speed
of light. 300,000 km/sec
• A changing electric field creates a
magnetic field, and a changing magnetic
field creates an electric field
• And a moving electromagnetic field
is… light!
EM Field Waves are Quantized

• These field changes are not quite like
water waves; they’re quantized into
individual little bundles of energy
possessing wave-like and particle-like
characteristics.
• They’re… photons!
• How to picture a photon?....
EM wave
Two Ways to Produce Photons…
• 1. Accelerate a charge, as we just
showed.
• 2. Transitions between allowed energy
levels (or “orbitals”, although this
isn’t literally a proper term) in an
atom
How do transitions work?
• Absorption – an electron can be bumped to
a higher orbit if a photon hits the atom and
it has exactly the same energy as the
energy difference between the two
orbitals.
• Emission – an electron will fall down to a
lower orbital if it is available, giving off
the energy difference between the
orbitals as a photon
H levels
What is a spectrum?
• Light (photons) emerge from objects
usually with a wide range of
wavelengths
• A graph of how much energy is
emitted at each wavelength is called
the objects “spectrum”.
Types of Spectra…
• Thermal spectrum – a spectrum produced by an
object purely because of its temperature. Must be
denser than typical interstellar gas in order to be
a thermal radiator. Examples: stars, planets,
asteroids
• Emission spectrum – a spectrum dominated by
emission lines. Clouds of gas with hot stars shining
on them produce this kind of spectrum
• Absorption spectrum – a smooth continuous range
of wavelengths, but certain wavelengths have less
energy than surrounding wavelengths and so
appear dark by contrast. Stars usually have this
kind of spectrum
H spectrum
Kirchoff diagram
Emission Absorption gas
Absorption by Molecules
• Molecules have additional degrees of
freedom which can be excited
• Collisions or photons can excite these and
cause absorption, or it’s time inverse;
emission
• These degrees of freedom are…
Absorption Bands for Molecules

• These additional ways of absorbing or
emitting photons tend to be effective over a
wider range of wavelength. So we talk not
of “absorption lines” but “absorption
bands”.
• This is one reason why the Greenhouse
Effect is as effective as it is – absorption
bands in molecules! As we shall see later.
• Atoms vibrating against each other deform their
electron clouds, a kind of “acceleration of a
charge” and so this produces photons of light
• This light bouncing around will exchange energy
with the particles so that the particles and
photons end up on average with the same
average energy: Thermal energy = average
kinetic energy
• Emerging light will have a “grade curve”
distribution of photon energy. This distribution
we call a Thermal spectrum.
The Two Laws of Thermal
• Wien’s Law: the wavelength of the
maximum intensity (per unit wavelength) is
inversely proportional to the temperature
• Stefan-Boltzmann Law: The luminosity
per unit area from a thermal radiator is
proportional to the temperature to the 4th
power
• Here’s what they mean…
•   Black body thermal emission intensity as a function of wavelength for various absolute
temperatures. Wien's law is not obvious in the picture, because the total emission includes
a geometrical factor of 1/λ2 which counts the number of fourier modes of wavelength λ,
and a second factor of 1/λ2 to convert intensities per-unit-frequency to intensities per-unit-
wavelength
The Stefan-Boltzmann Law
• At higher temperatures, the particles are
banging against each other more often, and
with more deformation, and both produce
more photons which are each more
energetic, on average. The net result is a lot
more energy for a thermal radiator which is
at higher temperature
Blackbody curves
BB curves, linear
The Doppler Effect

• The change in the observed energy (and therefore
frequency and wavelength) of a wave, caused by
motion between the source and the observer.
• Not just light, but sound, or surface waves, or in
fact any wave.
• Source, observer approach each other blueshifts
the light, i.e. photon wavelengths are shorter.
• Source, observer moving apart redshifts the
light; i.e. photon wavelengths are longer.
Doppler shifted spectra
Spectral analysis

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 views: 0 posted: 10/3/2012 language: English pages: 45