Original Message
From: John Hoaglund [mailto:hoaglund@essc.psu.edu]
Sent:
Tuesday, September 07, 2004 5:24 PM
Subject: Energy: a physics primer for a beginning
student
Eric,
I found some time
over the
Labor Day weekend to write up some answers to Ryan's questions (we had
lousy
weather). This primer is in words only
(mostly)! I wrote a physics and
chemistry review for my freshman science classes at the University of
Michigan that
has more details:
http://www.emsei.psu.edu/~hoaglund/physchem/egeocrsp2.html
but he can get as
much and
more from a decent physics or chemistry textbook. I
wrote it though to provide some examples of
science in more day to day stuff, often lacking in books that need to
be
condensed.
Here's my outline
for my
discussion.
I. Intro comments:
My advice
and opinionated comments on science ed.
II. Where to start. The four forces, mass, and charge.
III.
Thermodynamics: Energy
and available energy
IV. Energy supplies
V. Ryan's questions
1. How is
electricity generated?
2. How do solar
cells work? (More on electromagnetism and some modern physics)
3. What atoms are
fused in fusion? How?
I.
Intro comments: My advice and opinionated
comments on science education.
To answer Ryan’s questions, I need to lay a little ground
work. Before that, some advice (you
asked). Definitely get him involved in
the extra curricular (university) stuff for kids, or science camps. As a parent, obviously support him, but also
brace him for the "scientist = nerd" assault he'll endure from our
society. It'll start with his friends
teasing him when he tells them he's going to physics camp rather than
basketball camp, or because he likes his physics class and asks
questions. Also, when he gets to high
school and
college, or even before, have him STUDY PHYSICS BEFORE CHEMISTRY, and
STUDY
CHEMISTRY BEFORE BIOLOGY and STUDY ALL OF THE ABOVE BEFORE GEOLOGY. You are going to find stubborn resistance to
this because it is the complete inverse of the usual progression taught
in U.S.
high schools. This idea was championed
by Leon Lederman, the previous (or still current?) director of Fermi
Lab, in an
editorial in the New York Times. Though
one might argue that as a physicist [and a brilliant one] he's
presenting a
physics bias in the teaching debate, as a geologist, I couldn't agree
with him
more. How can you understand the energy
and forces involved in chemical bonds if you don't know what energy is
(other
than in fluffy non-technical terms) because you haven't studied physics
yet? The argument against doing this is
that the
kids don't have the math background early in their high school career
to study
physics. That depends upon how it is
presented and whether or not the teacher is willing to teach the needed
math as
she/he goes. Also, you're going against
the tide and may encounter a few problems.
For example, the physics teacher may assume the kids have a full
understanding of what atoms and elements are (from the assumption they
had
chemistry first) when she/he launches into a discussion of heat energy
as atoms
vibrating in their chemical bonds. These
problems are surmountable.
Sounds like Ryan has the usual, "yeah, but where
does that come from?" questions, and wants to know where to start. Kids ask the most challenging questions, and
it's hard to give easy, yet satisfying, answers to these [so don't
conclude
that you are "science challenged" for not answering clearly because
you're not]. I can't remember who, but
some famous scientist (it may have been Einstein) said that if you
can't
explain what you're doing in science to a 6 year old, you're a
charlatan. Ryan's a little older than
that, so I'll try
to target his age rather than expose myself as a charlatan.
Following Lederman's advice, I suggest he start with
physics. At the end of the 19th century,
it was thought that everything could be explained in terms of Newton's
laws
(forces in general), thermodynamics, and electromagnetism, what is
known as
"classical physics." That may
seem quaint, but in actual fact, I dare say 75% of working scientist's
research
is on, or within topics that can be explained by classical physics,
with
perhaps some instruments that use modern physics for measurement
purposes. One last piece of advice is that
a science
student should become very comfortable and fluid in being able to
convert
measurement units. So often, a concept
may be understood, but a calculation may be off, adding frustration to
the
study, when in fact the student has the "right" answer in the
"wrong" units. Practice
converting "furlongs per fortnight" to "miles per hour" to
"meters per second" for velocity, or "cubic furlongs" to
"cubic feet" to "gallons" to "liters" for
volume. We lost the Mars Rover to unit
screw ups.
II.
Where to start. The four forces,
mass, and charge.
My advice on where to start, especially for someone
interested in physics, is to start where ALL science starts: the 4 basic forces. I
wish someone had outlined these for me
before I started my science training. I
kept asking myself the, "yeah, but where did that come from?" questions. It took my college level physics course
before I learned that everything is derivable (at least with a hell of
a lot of
math) from the 4 fundamental forces of the universe and their
interactions with
matter: gravity, electro-magnetism,
strong nuclear force, and weak nuclear force.
Also remember the measurement unit, Newtons, which is a kilogram
of mass
multiplied by an acceleration in meters per square seconds. Currently there are 4 forces in the universe
and
they just ARE with no reason. Oh sure,
you'll come across a paper now and then with someone claiming they
found a 5th
force (anti gravity comes to mind), but the forces are where you start. If you're still asking, "yeah, but why
the fundamental 4?" you're getting into philosophy (metaphysics to be
exact). There is a branch of physics
trying to relate (i.e. derive) all the 4 forces from some general
principles (a
"unified field theory"), but where to start there gets into
philosophy too: you have to assume
certain things to get started (like particles have fields of energy and
momentum that interact, etc.). Take the
4 forces for granted, start there, and move on.
99% of what he'll
study in
science is derivable from gravity and/or electro-magnetism and their
interactions with matter, so let's start there.
There is a beautiful symmetry in these forces, a "makes you
wonder." In fact it makes many a cosmologist wonder about the unity
(hence
unified field theory) of the forces.
Gravity:
F = ( G m1
m2 ) / R2 that is,
force = gravity constant
(the universal one, NOT Earth's value of constant acceleration due to
its
gravity) times the mass of the first object times the mass of the
second object
divided by the square of the distance between them [this is known as
Newton's
law of gravity]. As for the gravity
constant, it's another "just is" that doesn't vary but is now
measured and known to the 20th decimal place [originally measured by
Cavendish
in a really cool experiment]. Any two
objects with mass have a force of attraction between them governed by
that
law. People are attracted to each other
in more ways than one! There is a gravitational force between two
people, but
it is SO weak compared to their attraction to the Earth (due to the
Earth's relatively
HUGE mass) that the inter-person force isn't noticed.
This underscores that gravity is the weakest
of the 4 forces, and yet gravity is what keeps the planets in their
orbits [in
fact Newton proved his theory of gravity by showing he could explain
Kepler's
planetary laws with it].
Electro-magnetism:
F
= ( k q1 q2) / R2
that is, force = Coulomb constant times the charge of the first object
times
the charge of the second object divided by the square of the distance
between
them [this is known as Coulomb's law].
As for the Coulomb constant, it's another "just is" that
doesn't vary but is now measured and known to the 20th decimal place. Any two objects with charge have a force
between them governed by that law, attractive if the charges are
opposite,
repulsive if the same.
OK, I lied. That's just the electrostatic force. The other half, magnetism, is different, but
thanks to pioneering work in the late 1800's by James Clerk Maxwell, it
is
known to be related to the first half, electrostatic [some say Maxwell
was the
first to "unify" two forces].
Faraday discovered that a moving charge (electric current)
induces a
magnetic field, and/or a changing magnetic field induces a moving
charge (see
your electricity generating question below).
But magnetism also has an "inverse square law" like these
other forces, called the Biot Savart law, where the magnetic field, B,
induced
(created) by a moving current (i.e. stream of "moving charges" called
electrons) is equal to the magnetic constant times the current divided
by the
square of the distance from the wire (i.e. the current).
As for the magnetic constant, it's another
"just is" that doesn't vary but is now measured and known to the 20th
decimal place (all zeros once written in scientific notation because
the value
is exactly 107 Newtons per square Amps for the purpose of
defining
Amps). The magnetic field then induces a
force proportional to the magnetic field (so that force decays with the
inverse
square law along with the decay of the field) on any other magnets or
charges. The magnetic field exerts a 1)
force on a magnet according to F = q'B where q' is the "pole
strength" of the magnet (a property of the magnet itself), aligning the
magnet with its north pole in the direction of the applied magnetic
field. Magnetic field lines come out of
the north
pole and into the south pole of a magnet, so if the applied magnetic
field
comes from another magnet, this force wants to align the north pole of
the
original magnet toward the south pole of the other magnet, which is in
the
direction of the magnetic field lines.
The "pole strength" property of a material, and why certain
materials are magnetic (or magnetizable), is a long topic.
Suffice it to say that magnetism of materials
comes from the quantum electron configuations around the atoms in the
magnet,
specifically unpaired electron "spins" which induce magnetic
fields. That is something that is
covered a little more thoroughly in introductory chemistry than in
introductory
physics. The magnetic field exerts a 2)
force on a charged particle according to F=qv x B ( a charge times its
velocity
times the magnetic field; the "x" is a special type of multiplication
you do with vectors that will be explained in his math / physics
courses).
Strong nuclear
force:
The strong nuclear
force
holds all those positively charge protons together in the nucleus
(against that
charge force that wants to repel them) and glues some neutrons in there
as
well. These forces are not readily
described by single formulas. The sub
atomic
experiments haven't yielded a unique or simple formula.
Descriptions of the strong force include that
it decays even faster than an "inverse square law" with distance, and
that it changes from attractive (come closer) to repulsive (but not too
close)
as that distance closes. It is really
important though because it is basically the glue of matter. When two bodies interact (a rock falls and
hits another rock), the gravitational force pulling the two bodies
together
meets the electrostatic force bonding the nuclei together, and the
strong
nuclear glue force holding each nucleus together. The
solid bodies may smash into pieces
(broken chemical bonds), but generally the atoms don't...except
subatomic
interactions like when a neutron hits a uranium atom causing it to
break up and
release more neutrons.
Weak nuclear
force:
The weak nuclear
force is
responsible for certain instabilities in the nucleus.
Again, there are not simple formulations on
it, but it determines why some nuclear configuations are stable (stable
isotopes) and others are not (radioactive isotopes), and it's
responsible for
one of the three types of radioactivity: beta radiation, which is
essentially a
high energy electron that emanates from the nucleus (as opposed to the
orbiting
electron shells) turning one of the nucleus' neutrons into a proton
[beta
radiation is one of two radiations that transmutes (i.e.
changes) one element into another (the
transmutation of the elements), the other being alpha decay, which is
essentially the loss of a charged helium atom from the nucleus].
Mass and charge:
Now
when you get into these definitions of
force, you're left with, "yeah, but what is mass, and what is
charge?" If you look up mass, you
get, "a quantity of matter measured in relation to its inertia, the
ratio
of it's weight (a force) to the acceleration of gravity," and when you
look up inertia, you get, "the property of matter to stay at rest if at
rest or to stay in motion if in motion, unless objected upon by a
force." You're left with, "wait, you just
used
force to define mass, and you used mass to define one of the 4 forces
(gravity). What gives?" And
with charge, you get, "the departure
from electrical neutrality at a point (or region) as by the
accumulation or
deficit of electrical particles." So
you want to ask, "what are these electrical particles?" and in
response you're probably going to get something like, "matter that
moves
in response to the electrical force." Again, you're left with, "wait,
you just used force to define charge, and you used charge to define the
force." It gets worse.
A little pea brain named Albert Einstein
showed us that electromagnetic radiation (energy from electromagnetic
waves,
including radio, heat, light, uv, x-rays, gamma rays, etc.; see energy
below)
is quantized (in steps), for which he won the Nobel prize in 1905, and
that
matter and energy are interchangeable, for which he is most famous. He and other pea brains have shown that matter
and energy have particle-like and wave-like properties, and at the
atomic level
the energy is "quantized" (i.e.
released or absorbed in steps).
Hey, you're there baby! First
principles from basic observations.
Matter stays moving if it's moving or stopped if it's stopped,
unless
acted upon by a force, and it's only going to get acted upon by gravity
if it
has mass, and the electromagnetic force if it has charge (and whatever
for the
strong and weak forces). And those come
from the very root of what we can observe in this material world, stuff
moving
or not moving, and changing its "moving". If
it's changing its relative motion, it is
being acted upon by a force. If it's not
attributable to the 4 known, congratulations, you just discovered
another force
and will probably win a Nobel prize.
Suffice it to say that whenever a physicist discovers a new
particle,
the two fundamental questions they want answered are, "does it have
mass?" and "does it have charge?"
III Energy and
available
energy
Energy follows from force. "Work"
is a force acting through a
distance. You apply a force to a rock
and lift it through a distance (against gravity), performing "work"
equal to the force (minimally the rock's weight) times the distance,
hence the
measurement unit is a Newton meter, also known as a Joule.
Energy is the capacity to do work and
overcome resistance, and it has many forms: heat,
kinetic (the energy of motion),
gravitational potential, chemical potential, electrical (i.e charge
related
energy), electromagnetic waves, mechanical (e.g. sound,
water, earthquake...) waves, nuclear,
to name a few. They all have the same
measure as work, the Joule. The rock may
have been moved by you (chemical potential energy in your food
converted to
electrical energy in your muscles, converted to kinetic energy in your
arm,
transferred to kinetic energy in the rock), or another rock
(gravitational
potential energy of the other rock converted to kinetic energy of the
other
rock transferred, upon collision, to kinetic energy in the first rock),
etc. The neat thing about energy is that
it is
conserved (the first law of thermodynamics), but the not so neat thing
is that
you gotta get it from somewhere (the second law of thermodynamics). Remember all that jazz about entropy in
Rifkin's book Hydrogen Economy? Entropy
is a measure of the amount of energy UNavailable for doing work in a
changing
system. Another way of saying that is
that energy is expended from "available" to "unavailable." The sad news is that (universal) entropy, the
amount of energy NOT available, is always either zero or increasing,
and is
zero only for completely reversible processes (in which case you put as
much
energy back into it as you got out). [I
say "universal" entropy because between two systems, one system's
entropy may be negative and the other positive, but the net (universal)
is
always 0 or greater.]
If the energy is conserved (1st law), where is it
expended (2nd law)? Whenever energy
changes form, there are "losses" which means some is
unavailable. Where does the unavailable
energy go? The "losses" you
usually hear about are heat losses that are "dissipated."
See, heat is one of those forms of energy
that is useless to us unless we can move it from one system (hot) to
another
(cold) and extract work in the process.
You can transform all of heat energy into work, but not
cyclically. For example, you can blow off
a bomb
converting all of the chemical energy into heat energy, and all of the
heat
energy into gas expansion, but that's it, all over, you're done, it's
not cyclic. In something cyclic, like an
engine, you can
use the heat to expand a piston, but to get the piston "unexpanded"
so you can do it again (i.e. cycle it), you either have to cool it
(contracting
the gas), phase change the gas into liquid), or put some of the work
you got
out back in to compress it. What
actually happens is a little of both, you put some of the work you got
out with
expansion back in with compression (an expanding piston rotates a crank
shaft
that is helping to re-compress expanded pistons, but the crank shaft is
also
moving the drive train, for a net gain of work out), and you cool it
(with the
radiator/cooling system and by "liberating" the heat in exhaust
gases), passing the "waste heat" somewhere else. That
heat is dissipated, not useful for the
original process. You may be able to use
it in some other process (like heating the car), but again, only if
some of it
is ultimately dissipated.
[For example, I've
always
thought that "waste heat" from a power plant, a major pollutant in
the environment, should be used to heat homes.
The "waste heat" is hot water at 100 °C left over from when
steam at 100 °C transforms into water at 100 °C and releases the latent
heat of
the phase change. After steam expansion
to turn the turbine, the turbine “exhaust” is still steam.
Steam heating is very common, but this steam
has to be condensed to liquid and piped back to complete the
power-cycle. Water is used to cool the
condenser and take
the heat away. Still, hot water is hot
water, right? And there's a lot of it
near power plants, which is a problem environmentally.
But it takes a lot more work to pump water
around than it does steam, and it's amazing how quickly the water loses
its
heat when piped. Steam
"stores" heat energy as latent heat that is only released with the
phase change to water. That is why the
university's heating system uses steam for cross-campus heating. Piping hot water to homes right near the
power plant might work for heating, but the loss of energy pumping it
around
and from heat dissipation generally makes this uneconomical (i.e. no net energy). Not
many people choose to live near power
plants, but if so, a home, rather than a cold river, is receiving the
dissipated heat of the original power generating process, it's just
that the
"cold" system that you're moving the heat to is your home.]
Rifkin
sums it up best with these quotes:
"While
energy cannot be created or destroyed,
it is continually changing in form, but always in one direction, from
available
to unavailable."
(page 44), and:
"energy is always
transformed in one direction: from hot
to cold, concentrated to dispersed, or ordered to disordered." (page
45).
IV Energy
supplies
Right now, roughly
85% of our
"available" energy comes from chemical potential energy, i.e. something
is burned: wood, animal dung, coal, oil, and natural gas.
Basically a little energy called ignition
[often kinetic energy converted into frictional heat (the striking of a
match),
or an electric spark (from a battery or car generator) ], is added to
some
relatively "unstable" chemical compounds (a substance made of two or
more elements), breaking the chemical bonds (overcoming the electrical
forces
of attraction) between them and forming more "stable" chemical
compounds and releasing a lot of energy (more than enough to make up
for the
initial ignition "investment").
The "unstable" compounds require more energy to hold
themselves together than the "stable" compounds do, so when the
"stable" compounds form from the "unstable" ones, some
energy is left over and is released. Or
if you'd rather (and I prefer to think of it this way), the more
"stable" compounds require more energy to bust them apart than the
"unstable" ones do. The
"unstable" compounds are wood, animal dung, coal, oil, and natural
gas, and the "stable" compounds are CO2 and water (both
are greenhouse gases). About 5% of our
energy comes from nuclear, and another 5% from hydro, with less than 5%
coming
from renewables (solar, wind, tidal, etc.).
Kerry's pushing for 20% renewables, but you can see, even that
does not
solve the problem, and demand is increasing.
V.
Ryan's questions:
These are a little
outside my
discipline, but I'll try to give my best shot:
“Ryan is really
interested in
science, chemistry and physics in particular.
He asks me questions like the following:”
1.
How is electricity generated? [His dad tells
him generally how turbines work, but he wants to know how the electrons
move
down the wire.]
The rotating turbine is kinetic energy (the energy of
motion). The kinetic energy comes from
either falling water, or pressurized stream (from a heat source which
is itself
either chemical potential energy from burning something, or heat from
nuclear
fission, or solar, and hopefully someday controlled fusion). Falling water transfers gravitational
potential energy into the kinetic energy of the rotating turbine in a
fairly
intuitive process. Alternatively,
converting pressurized steam to kinetic energy is not so
straightforward. It is done usually with
water/steam in closed
loop, in what is known as a Rankine cycle.
The pressurized steam is allowed to expand in the turbine
without adding
or removing heat. The expansion of the
gas performs what is known as pressure-volume work on the turbine
blades. That work is the conversion to
kinetic
energy. Now you've got expanded gas that
you need to recompress, and this is the interesting part.
The gas is allowed to convert from gas to
liquid in the condensor, which liberates the latent heat of a phase
change into
heat energy. This is the heat release
part that is needed in any cyclic engine, and at least some of it is
dissipated
(lost). In fact, power plants cycle
coolant water to take this heat away, and it is the coolant water (not
the
water in the Rankine cycle) that is cooled in those familiar cooling
towers
and/or liberated to the environment. The
Rankine cycle water is then brought back to the boiler and re-converted
to
pressurized steam, completing the cycle.
Now the kinetic energy is converted into electrical
energy by rotating a magnet inside a coil of wire.
The rotation is constantly changing the
magnetic field through the coil (called a solenoid), which induces a
current to
flow in the coil wires by Faraday's law (see above).
Moving charges (in the magnetic field)
relative to other charges (outside of the magnetic field) causes a
change in
voltage, a.k.a. electric potential,
between the two sets of charges. You can
think of electric "potential" as the potential energy of a charge to
move toward (or away) from another. It
is a change in the electric potential from one point to another point
that
causes a charge to move (by the electric force) from the point of high
potential to the point of low potential.
Because your turbine (rotating magnet inducing charges to move)
is
moving a hell of a lot of charges, it is making a hell of a lot of
potential
difference between the outgoing wire and the incoming wire in a circuit. [The current is always moving in a circuit. When the circuit is cut, so is the flow of
electric current.]
Of course another source of potential difference other
than a power plant and rotating magnets is the chemical battery. In one cell is an anode made of a metal
dissolved in a solution of the metal (i.e. has the same metal in it,
except it
is dissolved). The solid metal of the
anode loses electrons (i.e. is oxidized) and therefore dissolves into
its
solution. The anode is thus a source
(donor) of electrons, the negative of a battery. In
another cell is a different metal, also
dissolved in a solution of itself. The
metal transmits the electrons into the solution of itself where the
dissolved
metal gains electrons (i.e. is reduced) and deposits on the solid metal. This acceptor of electrons is called the
cathode, the positive of a battery.
There is a "salt bridge" that, in the anode, accepts the
excess metal being dissolved, and contributes a salt, to charge balance
the
anode cell, and in the cathode, exchanges salts with the cathode cell
to keep
it charged balanced as well. The result
is that if you hook a wire between the anode and the cathode, electrons
will
flow from the anode to the cathode because of a change in electrical
potential
between the chemicals. Eventually too
much salt is exchanged for metal and the battery "dies."
Sometimes a little more life can be added to
the battery by removing the cathode and scraping off the metal
deposits, but
even that doesn't work for long.
Now Ryan is asking a great question about what causes the
electrons to move down the wire. The
change
in electric potential, which is the work needed to move charge toward
(or away
from) other charge, is like the change in gravitational potential,
which is the
work needed to move mass away from (or toward) other mass.
The electric force moves the charge from high
electric potential to low, just like the gravitational force moves the
mass
from high gravitational potential to low.
A change in the electric potential can also be thought of as the
sum of
all the charges accumulated at some distance from a given charge,
making the
charge want to move because of the electric force.
Now, how easily that charge moves down the wire is the
topic of conductivity. Actually
physicists and engineers express it in terms of the resistance to the
charge
moving down the wire. There is always
some resistance, except for superconductors.
How well something conducts or resists is a long topic. It mostly has to do with how satisfied the
electrons are in the material. If they
are ALL tightly bonded in a nice low energy arrangement, they're not
going to
want to move, and you are left with a highly resistant material. In fact it is such a struggle that there is a
lot of interaction between the electrons that want to move through and
the
electrons that make up the material, causing bounces and wiggles (a.k.a. heat; resistors get really hot).
Metals make good conductors because in
addition to the electrons bonding the metal together, there are a whole
bunch
of "delocalized" electrons, kinda like electrons without a home, that
are ready to move on. Superconducting
materials are rare, but they move electrons through them with no
resistance. So far the ones we know of
only superconduct at really low temperatures, and it takes a lot of
energy to
cool them to the point where the superconductivity (with no "heat"
losses like in resistors) can pay off against the energy invested to
cool
them. Another holy grail in science is
to find a "room temperature" superconductor, one that will
superconduct at room temperature, and that can be shaped into a wire,
or
something that can be readily engineered to transmit electric current. Superconductors are getting
"warmer," i.e. better and better materials are being found that
superconduct at higher and higher temperatures.
A breakthrough here happened not too long ago when a
superconductor was
found that could superconduct above the boiling point of liquid
nitrogen,
making it really cheap to cool. The
search (lab search that is) goes on.
2.
How do solar cells work? Again, he wants the specifics.
The term solar cell usually implies photovoltaic (PV)
cells. Before discussing that, I should
say that there are three ways to harvest solar energy: photochemically,
photovoltaically, and
thermally (and combinations). I'm not
familiar with any commercial photochemical methods, but will just say
that
plants harvest solar energy photochemically, specifically with
chlorophyll in
photosynthesis. I will focus the
discussion on photovoltaic and thermal methods.
Photovoltaic
Solar
Photovoltaic (PV) cells produce power by photoconduction,
producing I, electric current (in amps), a stream of moving electrons,
and photovoltage,
producing V, voltage (in volts), an electric force per unit charge, or
in other
words the electric potential. Power in
Watts (energy delivered in a unit time, i.e.
Joule / second) is the combination (product) of the two, P = IV. Many people mistakenly say that the current
is produced photoelectrically (that is electrons are emitted off the
surface of
the cell), alluding to the photoelectric effect where electrons are
known to be
ejected off the surface of metals in the presence of light. While the concept of the photoelectric effect
is extremely important in understanding PV cells, the current produced
is not
from "emitted" or "ejected" electrons, but rather electrons
that are "excited" from what is known as the "valence band"
to the "conductance band" in a semi conductor solid.
The electrons are "excited" by
gaining energy from electromagnetic photons, a concept first proposed
by Albert
Einstein in 1905 in his study of the photoelectric effect.
Before 1905, electromagnetic (EM) radiation (e.g.
light) was thought to have only wave
properties, and those properties were completely described by Maxwell's
electromagnetic theory of light. As a
wave of vibrating electric and magnetic fields, the intensity
(brightness),
which is the energy delivered per unit time per unit area, is just a
constant
times the square of the electric field, with NO dependence on the
frequency (or
wavelength) of the light. This is
intuitive:
if you want to make a brighter light, add more electricity to increase
the E
field. We still use the EM concept of
intensity to describe the energy delivered by light (particularly to
describe
lighting, etc.), but the EM energy absorbed or emitted by matter is a
different
story. A phenomenon called the
photoelectric effect was discovered by Hertz: light
incident on certain metal surfaces in a
vacuum emits electrons. Lenard observed
that the intensity of the light does increase the number of electrons
emitted,
but not their energies. Einstein
proposed a radical idea to explain the observations of Hertz and Lenard. Everyone assumed that the energy of those
electrons (measured by the voltage needed to repel them) would be
related to
the intensity. Einstein proposed that
the energies of the electrons were equal to the energy of
electromagnetic
photons, an energy bearing particle, and that the energy absorbed by
the metal
and emitted by the electrons was related to the frequency (inversely
related to
the wavelength) of the light. This idea,
the fact that light has both particle and wave properties, and that the
energy
received by EM radiation is related to frequency, turned the physics
world on
its head. In 1911, Millikan set up an
experiment to disprove Einstein, and instead confirmed his hypothesis!
What the result means to solar cells and other phenomena
is that the energies absorbed by atomic particles from EM radiation
depend on
the frequency of the radiation. You may
have heard of ionizing radiation.
Ionizing radiation delivers enough energy to strip electrons
from a
substance, literally pulling an electron out of an atom's electron
orbitals
where they are used in bonding compounds.
The first ionizing radiation is ultra violet (uv), the component
of the
sun's rays that gives you a sunburn.
Ultra violet lamps are used in a lot of chemical detection
equipment,
like that used by the gas man who checks for gas leaks in a house. The uv-stripped electrons from the gas are
"countable," and the electron count is related to how much gas is
there. In a solar cell, we do not need
as much energy as it takes to ionize electrons, just enough to excite
them from
one part of the solid (the valence band) to another (the conductance
band). In a solid, bonding electrons
shared between
sets of atoms are in what is known as the valence band.
In a conductor (such as a metal) or in a semi
conductor, there is a conductance band where any electrons in the band
can move
because they are not involved in bonding the solid.
In a conductor, some of the valence band
electrons are also in the conductance band.
These electrons are "delocalized" and free to travel. In a semi conductor, the valence and
conductance band are separated by an energy barrier.
If an electron gets enough energy (the
"energy gap") it can jump from the valence to the conductance
band. Once an electron is in the
conductance band, it is free to travel (conduct on through). [In an insulator, there is also a conductance
band, but the energy required to get there is too extreme, so no
electrons
conduct. Under extreme conditions (like
extreme heating), electrons may get enough energy, and the insulator
may become
a semi conductor or conductor.]
It
is not enough to just excite electrons into the conductance band. You also have to provide a voltage to move
them. The way this is done is to
"dope" semi conductors with "impurities" [elements that can
sub for the dominant "cation" (positively charge element) in the semi
conductor crystal] that leave charge imbalances in the structure. In a solar cell, a positively (p) charged
semi conductor is placed in contact with a negatively (n) charged semi
conductor
along what is known as a p-n junction. What
p-n junctions do is allow charge to move one way across the junction,
but not
the other way back again. Photons of
light, then, excite electrons across the junction, building up negative
charge
in the n-semi-conductor and positive charge in the p-semi-conductor
(building
up voltage). For a given frequency of
dominant electromagnetic radiation (in this case from the sun), there's
a
certain "energy gap" that balances the right amount of voltage build
up (photovoltage) with available electrons (photocurrent).
It turns out the dominant wavelength (inverse
of frequency) of our Sun is around 500 nanometers [calculated with
Wein's
displacement law for a Sun surface temperature of 5800K], green light,
with an
average wavelength of yellow light (575 nanometers).
The electromagnetic power (energy per time)
distribution of the Sun has about 47% in the visible spectrum, 45% in
the
infrared (heat), and 8% uv [these power distributions are given by the
integral
(summation) of Planck's law, with the total power output of the Sun
given by
the Stefan Boltzman law]. The ideal
photovoltage/photocurrent balance for this distribution corresponds to
an ideal
"energy gap" of 1.1 electron volts (another unit of energy used at
the atomic scale), with a theoretical efficiency of converting the
Sun's power
to electrical power of 44%. The 1.1
electron volt energy gap corresponds to silicon semi conductors, with a
practical efficiency of about 30%. So
crystalline silicon semi conductors are the best, but they're expensive
to make
(you have to grow crystals while doping them).
Some of the solar cell "breakthroughs" lately are in finding
cheaper (or cheaper to build) materials to make cells that have
efficiencies
close to the silicon champions. The
efficiency is critical: the Sun's power output, even at the Earth's
surface
after going through the atmosphere, is generous [1340 W/m2
total is
cut to about 1000 W/m2 on the Earth's surface], but the
translation
to electric power is cut to 2/3 by the efficiencies.
On my table on my energy page:
http://www.emsei.psu.edu/~hoaglund/energyslaves.html
I got the day night
world
averaged solar output of 238 W/m^2, which for an 80 square meter
rooftop (a
double wide trailer) corresponds to 460 kW-hrs of energy.
Multiply by ~ 1/3 (say 28%), you get 128 kW
hrs compared to an average home electric usage of 16 kW-hr. Clouds screw that up of course, but that's
why you store the energy in hydrogen.
Unfortunately 80 square meters of silicon would cost a mint, so
now you
see the economics driving the race for cheaper-and-as-efficient solar
cells.
Thermal
Solar
As the name
implies, you can convert a lot of the Sun's output power into heat. Most of the Sun's spectrum is above (more
energy intense) than infrared (heat), and as the higher energy uv waves
are
absorbed, they re-emit at lower energy infrared (heat) waves. This is the greenhouse effect: high energy
light and uv passes the glass, gets absorbed by the walls and plants,
and re-radiates
as heat which gets trapped, hence the heat builds up.
Your car is an oven on a summer (or even
winter) day, another example. Thermal
collectors are cheap to build (something like glass to transmit light
but trap
heat), backed with something "black" (as in "Stefan Boltzman
black body absorber"), and filled with a heat carrier like water. The efficiencies of the collectors are great,
as high as 75% or more, but once you heat something above the ambient
(surrounding) temperature, it starts to radiate heat.
You're also left with a medium you have to
pump around (water pumps and plumbing etc.), and to generate
electricity, you
have all the losses associated with the "cyclic engine" (see power
plant discussion above). Bottom line:
great for heating water for a house (and lots and lots of people do
just that),
but unless collected from reflectors at a large scale to flash steam,
not so
efficient for producing electric power.
Photovoltaic/Thermal
Solar
(PVT cells).
Remember the ideal
"energy gap" for the solar cell is around 1.1 eV? Well,
the sun's average photon energy is
around 1.9 eV (in the red part of the spectrum, it's a typical
"yellow" sun but the atmosphere plays tricks). That
leaves some extra energy in the cell,
which is absorbed as heat. A PVT is a
combo cell, taking the PV out in current, and circulating a medium
through it
to take the heat. I'm not sure what their
efficiencies are.
3. What
atoms are fused in fusion? How?
Fusion versus
Fission: We
should
start by defining each term and show the energies that are released. Fusion is the fusing of atoms producing
heavier atoms while fission is the breaking up of heavy atoms into
smaller
ones. Both processes are radioactive,
the difference is that fission leaves radioactive by products, things
that
continue to release radioactivity, whereas the fusion radioactivity
ceases with
the cessation of the reaction. The
byproduct
wastes remind me of Charles Schultz's Pig Pen from the Peanuts cartoon:
it keeps having this annoying stuff coming
off
of it. The radioactivity has 3 forms: alpha (a helium atom without its electrons),
beta (a high energy electron emitted from the nucleus, as opposed to an
orbital), and gamma (a high energy electromagnetic wave).
Gammas are the most lethal and require 6 feet
of concrete / lead to stop them. Of the
particulate radiation, alphas are the most lethal, but a piece of paper
stops
them. Betas can be stopped by a piece of
wood.
When comparing
fission and
fusion, you should look at the curve of binding energy.
http://hyperphysics.phy
astr.gsu.edu/hbase/nucene/nucbin.html#c2
The mass number is
the total
number of neutrons and protons in the nucleus, leftmost on the graph is
light
elements like hydrogen and rightmost are the heavy elements like
uranium. Fission works from right to left
(towards
iron at the peak). The binding energy is
the energy required to break an atom up into its constituents. If you form an element of a higher binding
energy from one with a lower, you release excess energy (that sounds
backwards
until you think about it). Compare the
fission (right to left) gradual slope versus the fusion (left to right)
steep
slope. You can see there is a huge
"bang for the buck" with fusion.
The ideal atoms fused in fusion are hydrogen,
specifically two isotopes of hydrogen.
Hydrogen has 1 proton. If it has
one neutron as well, it is called deuterium, a "stable" (i.e. not radioactive) isotope.
If it has two neutrons, it is called tritium,
which is a radioactive isotope (but the radioactivity goes away
quickly). Deuterium-tritium fusion results
in stable
helium (2 protons, 2 neutrons) plus an extra neutron.
The extra neutron goes into breeding tritium
from deuterium and/or deuterium from hydrogen.
These things only fuse at really high temperatures and pressures
because
you need tremendous energy (heat) to overcome the charge repulsion (two
plus
charges repel) and get the two positively charged hydrogen atoms close
enough
to become helium. The Sun does this with
gravity, but that's because it is so huge, there's enough mass to
compress the
hydrogen. Remember, the repulsion force
is related to the inverse of the square of the distance separating
them, and as
that distance gets smaller and smaller, that force approaches infinity
(almost
division by zero). Heating is done with
lasers, and pressure is produced with confinement (either magnetic or
inertial). It also takes energy to strip
electrons to create a plasma in the first place. A
plasma is all the hydrogen matter stripped
of its electrons. It is basically a 4th
state of matter and has its own properties, but being charged, it can
be
manipulated by magnets for containment and confinement.
Containment is critical because at the
temperatures of fusion, no vessel can hold the plasma.
Right now, the technical issue is getting
more energy out than what goes in. The
energies for containment, confinement, plasma generation, and heating
require
more energy than the limited fractions of a second the reaction can be
sustained. When you hear about the
occasional "breakthrough," it usually means some lab somewhere has
increased (set a new record for) the sustained reaction time (and got
more
energy out). I think the record is at
the Joint European Torus ( JET);
http://www.jet.efda.org/index.html
lab in the UK.
The helium has
slightly less
mass than the deuterium and tritium used to make it.
Where did that mass go? It is converted into
energy by Einstein's famous E = mc2, which is energy (out) =
mass
(lost) times the speed of light squared.
That's a hell of a lot of energy out.
One end note: it
takes less
input energy to fuse slightly heavier elements, and I read a few years
back
that they were having some success with lithium-lithium or
lithium-hydrogen
fusion, but it results in some radioactive byproducts which, in my
opinion,
defeats the purpose.
Another end note:
the
temperatures for fusion in a hydrogen bomb are created by a fission
bomb. It is the fission bomb that results
in most
of the radioactive by products, not the hydrogen, although fusion
itself does
release a considerable amount of gamma (electromagnetic wave) radiation
and
neutrons. I've heard that one of the
technical problems is that the neutrons make reactor equipment brittle
over
time.
I found this
summary of the
radioactivity associated with DT fusion:
http://www.lanl.gov/p/projects/pds_in15.shtml
"The 16.75 MeV
gamma
rays that accompany DT fusion provide a high bandwidth alternative to
14 MeV
fusion neutrons for DT burn history measurements. Fusion
of deuterium with tritium produces an
excited 5He nucleus, which then has several possible
de-excitation
modes. The most common mode emits a 14
MeV neutron and a 3.5 MeV alpha particle.
Much less frequent gamma ray modes emit gamma rays at 16.75 MeV
to the
ground state and possibly at ~ 12 MeV to a broad level near 4 MeV. Recent values for the DT branching ratio
(16.75MeV gamma rays per 14 MeV neutron) vary from 5x105 to
1x104."
John Hoaglund,
Research
Associate
The Pennsylvania
State
University
Earth and
Environmental
Systems Institute
2217 Earth
Engineering
Sciences
University Park, PA
16802
6813
(814) 865 4792
hoaglund@essc.psu.edu
http://www.emsei.psu.edu/~hoaglund/