Phys104 - How Things Work
University of Maryland, College Park
Fall 2008, Professor: Ted Jacobson

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Tuesday 12/09


- discussed beta decay

- disussed isotopes of Americium. There are 14 isotopes, all unstable, and artificially produced.
 Americium 241 decays by alpha decay and gamma rays, with a half-life of about 433 years.

- showed cloud chamber tracks from decay of Americium 241; even showed that a strong magnet bends the alpha particle tracks!

- next covered the material from the textbook section 16.2; additionally:

- explained why atoms with higher nuclear charge are better absorbers of high energy X-rays: there is resonance between the wiggling frequency of the electric field of the X-ray and the orbital motion of the electrons, which move faster close to a nucleus of higher charge.

- mentioned that trillions of neutrinos coming from nuclear reactions in the sun go through your body per second, and virtually all of them go through the entire earth.

- a picture of an open CT (Computed Tomorgraphy - I incorrectly called it "computerized")  scan device can be seen at the Wikipedia: article: http://en.wikipedia.org/wiki/Computed_tomography

- note: the range of electromagnetic waves/photons called X-rays overlaps the range called gamma rays. In the overlapping region,  the name depends on how they are generated:  X-rays from electron acceleration, and gamma rays from nuclear transitions or particle transmutations (e.g. matter-antimatter annihilation).

- A good description of MRI and discussion of various ways of improving it are discussed in this article from the Berkeley Science review. One thing the book did not explain about MRI is how contrast is achieved between different types of tissue.  It's true there are slight variations in the density of hydrogen atoms, but these variations are not sufficient. Rather what is exploited is the fact that in different chemical environments, the proton spin directions get randomized at different rates after being (partially) oriented by an external magnetic field and radio frequency electromagnetic waves. In effect, the chemical environment produces a contribution to the magnetic field (coming from electron motion and spin) that the proton experiences.

- You can see videos of what happens when ferromagnetic material gets near an MRI machine here
http://www.youtube.com/watch?v=V9UJ6JAeVuE
http://www.youtube.com/watch?v=4uzJPpC4Wuk
and in other youtube videos...

Demo shown:  P4-32 CLOUD CHAMBER - TV

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Thursday 12/04

Discussed material from section 16.1, plus some other stuff:

- How does the sun shine? It is a "burning" that involves fusion of protons into helium.
This involves so-called strong and weak forces that we haven't yet discussed, since
everything else we talked about just involves gravity and electromagnetism.

-  What's a proton? Explained that its got stuff in it (quarks and gluons), unlike say
electrons which are point particles, and while small, has a finite size: If the diameter
of an atom were the width of the lecture hall, 10 meters, a proton in the nucleus would
be about 1/100,000 as big, i.e 1/10 of a millimeter. The mass of a proton is roughly
2000 times the mass of an electron.

- Two protons repel each other electrically, but if slammed together hard enough
they can nevertheless feel the short-ranged nuclear attraction, and snap together,
but only when  in the process of snapping together the proton transforms into a neutron,
a positron (the anti-particle of an electron), and a neutrino. This transformation of the
proton takes place via what's called the "weak interaction". What's left, then, is a
bound state of a proton and a neutron, called a deuteron, or deuterium, plus the
positron and the neutrino, and all of these particles have significant kinetic energy,
obtained from the energy of the initial state.

- A student asked where did the energy come from that ends up being released in
this reaction? That's a really deep question. To answer it you have to ask where
did the protons come from in the first place? Why didn't the universe start out in
its ground state? We actually know something about this: the universe is expanding,
and it used to be expanding faster. As it expands it cools, and so it used to be
much hotter, in fact so hot that no nuclear compounds at all could exist without
being broken apart. All nuclear material was protons and neutrons. Energy of the
"big bang" that produced this hot soup and these protons in the first place is what
later provides the energy released in proton fusion to make deuterium. As the
universe cooled, a few light nuclei were formed in collisions, but the only significant
amounts were 25% (by weight) helium-4 nuclei, and the rest protons (hydrogen nuclei).
 I didn't go into the next part in class, but I'll just say here that we do have a tentative
account of this energy that traces it back even further, but the story gets fuzzy and
is not at all entirely tested yet. (It goes under the name of "inflationary cosmology".)

- In the sun, the p + p --> pn + positron + neutrino process is followed by other processes.
For one, the positron finds an  electron in the sun and annihilates with it, yielding a pair
of gamma ray photons. As for the deuterons (pn), collision of a deuteron with another
proton, pn + p, yields ppn + gamma  ray photon.  The ppn bound state is also known
as a helium-3 nucleus. Next, ppn + ppn becomes ppnn + p + p. The ppnn bound state
is a helium-4 nucleus.   This sequence of proton fusion reactions is illustrated here.

- In fact, all the stuff we are made of besides the hydrogen and some of the helium was
forged in nuclear burning in stars, as well as in supernovae --- explosions of stars that
have run out of fuel, imploded gravitationally, and partially bounced back outwards,
flinging into space the new types of nuclei. Clouds of this material formed dust, held
together by chemical bonds made possible by the more interesting chemistry of atoms
with many electrons balancing the many protons in their complex nuclei.  These clouds
condensed gravitationally,  and eventually made new stars, sometimes with rocky planets
going around them, like earth. Then sunlight flowing out of nuclear burning in these
next generation stars bathed their planets with a flux of energy, and flow of this energy
(as well as energy from radioactive decays inside the planet and gravitational potential energy
from settling of heavier materials to the planet's core) through the atmosphere and oceans
of these planets enabled articulated patterns of chemical reactions to take place. On at least
one of these planets, this gave rise to life...

- The nuclear burning in stars made lots of types of nuclei, some stable, and some not.
I showed a web page with a chart of all the nuclides. "Nuclide" is the name for a bound state
of protons and neutrons when it is not being thought of as something that sits in the center
of an atom. What it means to be unstable is that the nuclide will be in a lower energy state
if it breaks apart. But it may be temporarily stuck togeher, because there is an energy barrier
to be overcome before it can break up. The barrier could be overcome by adding some energy,
but even when no energy is added, it can spontaneously decay. This is due to the fact that in
quantum physics, particles can't have  definite locations. I explained this in class by reference
to the Heisenberg uncertainty principle, that we previously discussed in explaining why the
electrons in atoms don't just collapse onto the nucleus.  But now that I think of it, I'm not so
happy with the explanation as I gave it. I'd rather say that the individual particles in the nucleus
simply are not confined to the nucleus with 100% certainty. There is some probability that they
can "leak" out. This leaking is called radioactive decay, and happens with a certain probability
each second. This leads to the notion of half-life, as explained in the textbook.

- I noted that even a single neutron by itself is unstable: it decays to a proton, and electron and
a neutrino, with a ten minute half-life. So why don't the neutrons inside nuclei decay?! 
For example why can't the neutron in a deuteron pn decay? The answer is that the system
doesn't have enough energy. If the neutron became a proton (plus electron and neutrino),
then there would be two protons sitting side by side, with lots of electric potential energy.
Similarly, the neutrons in other stable nuclei can't decay. But this process actually does
happen in some nuclei, and when it does the charge of the nucleus goes up by one,
and an electron and a neutrino are spit out. The electron is called a "beta particle" in this context.

- Other nuclei decay differently, for example by emitting an entire helium-4 nucleus (ppnn),
called in this context an "alpha particle". Sometimes gamma rays (high frequency photons)
come out. Or some combination of these.  I showed with a geiger counter the distinction
between alpha particles, which are stopped easily by paper, and beta particle and gamma rays.
I also showed a smoke alarm and explained how it works: there is a pair of conducting plates
with a voltage drop across them, so an electric field between them. A little bit of americanum-241,
a man-made unstable nucleus, spits out alpha particles, which ionizes air molecules, allowing
a small current to flow across the gap between the plates. If a smoke particle enters the gap,
it can bind with some of the ionized air molecules, causing a dip in the electric current.
This dip is sensed by an electric circuit, which turns on the alarm.

- Radiation damage to biological chemistry: the high energy particles when colliding with
molecules can mess up biochemical structure needed for healthy life to function, or cause
genetic mutations. I talked about the infamous case of polonium-210 "poisoning" of russian
dissident and ex-KGB agent Litvinenko a couple of years ago in London. ("Polonium" was
named by Marie Curie while working in Paris, after her home country, Poland.) Traces of the 
polonium were easily found in the hotel, on the teapot, in the plane  that the  guy who brought
the polonium from Russia flew on... It is very dangerous to handle polonium-210, and is only
available to very few parties with advanced nuclear technology. Why was Litvinenko killed
in this way, making conspicuous use of this substance?

- In 1938 physicists in Germany and in Austria doing research on fundamental physics found
(following on earlier work) that when a high energy neutron  slams into a radioactive nucleus,
that can supply the energy required to overcome the energy barrier that keeps the nucleus
from decaying quickly, thus splitting the nucleus. It was immediately clear to physicists around
the world that this meant that a chain reaction might be possible.
This applet illustrates the concept of a chain reaction.

- After this I described some of what the book discusses regarding the design of nuclear fission
weapons. The separation of uranium-235 and 238, required to make a bomb,
cannot be done chemically, since these isotopes differ only in mass, and only slightly (1.5%),
so their chemical properties are essentially identical. To separate them you have to do
something to them that depends on their mass difference. One way to do this is with centrifuges.
A centrifuge is a rapidly spinning tube. Other things being equal, more massive particles, having
more inertia, will "sink" to the outermost part of the tube. (It takes more centripetal force to produce
their centripetal acceleration than it takes for a less massive particle.) It takes many cycles of the
process and many (thousands) of centrifuges to separate a significant amount. Iran is currently
doing just this... By the way, although it's often called an "atom" bomb, it's really a nuclear fission
bomb. A "dirty bomb" would be the result of a fission chain reaction that turns off before a large
fraction of the nuclei are split, but nevertheless scatters much radioactive fallout and can thus
make a city uninhabitable for thousands (?) of years... [Perhaps the term "dirty bomb" could also
apply to a conventional bomb that disperses radioactive waste material from a reactor, for example...]

- After the physicists in the Manhattan project at Los Alamos tested one of their two bomb designs,
and had one more of each type of bomb already manufactured, President Truman decided to use
the two bombs to end the war with Japan. Why didn't he just demonstrate the bomb to the Japanese
military and Emperor, causing them to surrender, rather than dropping them on cities? I don't know.
Possible reason's I've read are that they had only two bombs, and that they had not tested the design
on one of them, and that they knew the bomb might fizzle. Also, I suppose it may have alerted Japan
to defend their airspace more carefully. It's not at all clear to me that these are good reasons. 
Whatever it was, the result was horrific...

- Physicists quickly began working on a fusion bomb, with a fission bomb as trigger, the so-called
"H-bomb", H being for hydrogen, since hydrogen nuclei (in the form of deuterium and tritium) are fused. 
Some protested and argued this bomb should not be developed. Others said that our enemies would
do it anyway, so it must be done. Some, I imagine, were motivated in part by the challenge and
fascination of it. The fact that the human race came to this point is somehow both ethically abysmal
and intellectually wonderful at the same time, in my view. I showed a video showing an H-bomb test,
which was apparently the first such test. I don't think you can really get a sense of the scale from the
video, but it's nevertheless overwhelming to watch... the test codename was Ivy Mike (link to
Wikipedia article, which also contains a link to the original, one hour classified movie about the test).

Demos shown:

P4-01 GEIGER COUNTER

P4-08 IONIZATION SMOKE ALARM

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Tuesday 12/02

Discussed some of the material in section 15.2. In addition to what's in the book:

- explained binary numbers

- briefly discussed error correcting encoding

- said a bit about writable CD's: they have a light sensitive dye that becomes permanently absorbing when exposed to high intensity laser light. The absorbing spots play the role of the "pits". Erasable CD's have a material that can be in either crystalline (transparent) and amorphous (light absorbing) phases. Laser light can "melt" the crystal, making it amorphous, and if it then cools quickly it remains amorphous. To erase, the material is heated up past the crystal melting temperature, then cooled slowly, upon which it recrystallizes.

- demonstrated diffraction with a laser and difraction gratings: closer line spacing makes for wider spaced diffraction patterns. Shing the laser on a vinyl LP, a CD, and a DVD we could "see" the difference in the line spacings on those surfaces.

- demonstrated the optical system of a CD or player using the demo L7-13 (see below).

- demonstrated total internal reflection with the M7-11 set-up -- also took the opportunity to demonstrate the polarization of reflected light.


Demos shown:

M1-22 LASER DIFFRACTION - GRATINGS

M7-11 OPTICAL BOARD - BREWSTER’S ANGLE

L7-13 OPTICAL BOARD - GALILEAN TELESCOPE

L5-24 FIBER OPTICS - COMMUNICATION LINE

VINYL PHONO RECORD, CD, SMALL DVD

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Tuesday 11/25

Started out with Karo syrup between crossed polarizers. It produces beautiful colors that change when one polarizer is rotated. The reason is the sugar molecules are twisted one way and not the other [like DNA and other biomolecules...which by the way is a bit of a puzzle: what is the origin of this asymmetry in nature?]. When light passes through the syrup, its polarization is rotated, but by a different amount for different wavelengths of light. Thus, the intensity of the light that makes it through the second polarizer depends on the wavelength, so what starts out as white light comes out as colored.

Next discussed atomic energy levels more, and in particular the Pauli exclusion principle, which accounts for why electrons don't all go into the lowest energy state in an atom.

Discussed how light interacts with atoms, according to quantum physics: the atom will not absorb a photon unless the photon energy is exactly what the atom needs to make a transition to another allowed energy level. (I made the analogy of someone who won't accept a gift if they don't have a space on their shelf for it.)

The energy levels of atoms are evident in the spectra we observed, showing light emitted at particular frequencies corresponding to differences between two energy levels in the atom.

To demonstrate the fact that atoms only accept photons when they have the energy level available, I showed the photoelectric effect demo (see below). We could see that a negatively charged plate can be discharged by UV photons, but not by visible light.  (A piece of glass blocked the UV but let the visible light through, enabling us to see this distinction.) For the same reason, you can't really get a sunburn through a glass window.

A material can absorb UV and emit at lower energy, visible. Demo with UV lamp on laundry soap shows this.

LASER: Light Amplification by Stimulated Emission of Radiation. Special properties of light from a laser: 1) one wavelength (nearly), 2) one direction (nearly), 3) in step (one "phase") nearly - this is also called "coherence".  Because of these properties, laser light can be controlled in terms of where it goes, and it can  carry  much information in a small space, so it can be used for surgery, information transfer (like in scanners), CD/DVD burning and reading, intense power delivery to small things, extremely accurate measurements, guided weapons, and many other applications. So far there is no optical computer however.

How lasers work: This is explained well in the book. Key concepts are stimulated emission of radiation, pumping the laser medium, and mirrors, one being semitransparent. Most lasers in everyday devices are laser diodes, which work, as do LED's ("light emitting diodes") by converting electrical potential energy to light by applying a voltage difference across a junction between two specially chosen materials. The lowest energy conduction electron states in the two materials are different, and when electrons flow from the material with higher energy to the one with lower energy conduction band, the electrons give up the energy difference as a photon. It's lind of like water flowing over a waterfall, giving up gravitational potential energy which turns into kinetic energy. In a laser diode, mirrors are placed on either end, and (I think) a flowing current supplies the power, so the medium is not really "pumped" as in other types of lasers.  Here is a brief expanation with diagrams: http://dictionary.zdnet.com/definition/laser+diode.html
Here's a rather complete discussion: http://en.wikipedia.org/wiki/Laser_diode


Demos shown:

M8-01 POLAROIDS AND KARO SYRUP

P2-02 PHOTOELECTRIC EFFECT IN ZINC - ARC LAMP

P3-53 ATOMIC ENERGY LEVEL MODEL

P3-67 FLUORESCENCE OF LAUNDRY SOAP

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Thursday 11/20

Started out by showing on a video camera how the infrared part of the spectrum shows up as bright white when using the "night vision" feature. A student mentioned that he heard this can be used to "see through" various things. I looked for this and quickly found it's true! All you have to do is filter out the visible light with exposed film, so all that enters the camera is the infrared. Then since clothing doesn't absorb much infrared radiation, the IR emitted behind it can be seen by the camera....ahem.

Next I showed how a polarizing filter can make the display on my wristwatch appear completely black...the reason is that at the top of the display is another polarizing filter, and if the directions of the two filters are perpendicular, no light gets through.

LCD - Liquid crystal display: In the set-up shown here, the two polarizers P1 and P2 are crossed. The liquid crystal medium LC is aligned with etchings on the plates E2 and E1, which are oriented perpendicularly to each other, making the LC twist as shown in the panel on the left. When light passes through, its polarization is also twisted, so it can pass the second polarizer. When the voltage V is turned on, as in the panel on the right, it creates an electric field that orients the LC molecules along the electric field, because they have electric dipoles (separated + and - charge). In this configuration the polarization of incoming light is no longer rotated, so the light is blocked by the second polarizer.

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The question of the green color of plants came up. A student in the class is taking a class on remote sensing, and he sent me some lecture slides from the class, about the absorption of visible and IR radiation by plants and other surface materials on earth. One slide contained the following graph showing the absorption of light by different clorophylls and carotinoids. A link to an explanation with some chemical details, and a similar graph was sent by another student, and can be found here: http://www.chm.bris.ac.uk/motm/chlorophyll/chlorophyll_h.htm. There must be a very good reason why plants can't just absorb all the radiation strongly...

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Today we also looked at spectral lines of particular atoms, using both a prism and some hand-held plastic sheet diffraction gratings to separate the radiation into different wavelengths. Regarding spectra, I made the following points: why don't the electrons in an atom simply radiate energy and crash into the nucleus, collapsing the atom? This can only be understood using quantum physics. The essential element is the Heisenberg uncertainty principle. The uncertainty in velocity is inversely proportional to the uncertainty in position. The electrons don't have enough energy to collapse into the nucleus! If they became too localized near the nucleus, their velocity and hence their kinetic energy would become very uncertain, in particular it could be very large. So if their energy is strictly limited, they cannot get too close. In the ground state of the atom, they are in their lowest energy configuration, as close as they can get. If an electron absorbs some energy by colliding with another electron, or absorbing some light, for example, then it has extra energy, and since it is accelerating can radiate, shaking off the extra energy and settling back into its lowest energy state in the atom. The ways the electron can move are like the normal modes of a string or a drum: particular patterns and particular frequencies. When an electron changes from one pattern to another of lower energy it gives off the energy difference in the form of a "packet" of light, called a photon. The energy of the photon is equal to the frequency of the photon times Planck's constant. Each kind of atom has its own characteristic patterns of electron vibrations, with corresponding energies and frequencies. Hence, each atom has its own characteristic frequencies of light it emits. We compared mercury and cadmium in class today. We also saw how the spectra of fluorescent lights have a few distinct frequencies. These come from different components of the coating of the tubes, which emit visible light when bombarded by ultraviolet photons from mecury vapor in the tube.

I also tried to explain briefly how the diffraction gratings work. The basic point is that light is a wave, and when incident waves reflecting from different places and then recombine, the combination will be bright er or darker  depending on whether the waves combine in step or out of step. This is called wave interference. This in turn will depend on the wavelength of the light, so at a given viewing angle, a given wavelength will combine in step and others will not. In that way the light in different wavelengths is separated into beams at different angles.

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Demos shown:

N2-21 PRISMATIC SPECTRUM OF MERCURY - SUPERPRESSURE LAMP

N2-02 DIFFRACTION SPECTRA - THREE SOURCES - EXPENDABLE GRATINGS

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Tuesday 11/18

The visible spectrum is a tiny slice of a huge expanse of frequencies/wavelengths (the question marks on the lower plot label should be minus signs, for negative powers of 10):


(Diagram from the Bloomfield textbook.)

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Why do we see this particular narrow range? Look how the visible spectrum exploits the dramatic, narrow dip in the absorption coefficient of electromagnetic radiation by liquid water. Note that red is absorbed more than blue.


(Diagram from http://www.lsbu.ac.uk/water/vibrat.html)

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The visible spectrum:


(Diagram from the Bloomfield textbook.)

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A good explanation of rainbows, including a very illuminating applet, is at this link.

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Demos shown:

N1-05 SPECTRA - VISIBLE AND INVISIBLE

N1-41 RAINDROP RAY MODEL

M7-05 ROPE AND COOKIE COOLERS

M7-34 ROTATION OF POLARIZATION - POLAROID AND WAX PAPER

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Thursday 11/13

Exam 2

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Thursday 11/11

- A question was asked about how a string could vibrate simultaneously in more than one normal mode. I explained that this is due to a special proprty of the string, that if you add together the displacements in two different vibrations, you wind up with a possible vibration. This is called the principle of superposition. I didn't explain this in class, but for the string it's true to a very good approaximation provided the displacements are small enough.

- The principle of superposition holds exactly for electromagnetic waves. That's why you can fill the "airwaves" with all kinds of signals, differing only in their frequencies, and then separate them out by frequency filters, without them having distorted each other in any way. In the case of EM waves, what you are adding is not "sideways displacements" like the string, but rather the electric field vectors of the different waves.

- Explained circular polarization, by adding together a horizontally polarized EM wave and a vertically polarized wave. If the two waves are exactly 1/4 cycle out of step, then where one has it's minimum ther other has it's maximum, and the result can be that the electric field is never zero, but rather just goes around in a circle as the wave passes a given point in space (see demo below).

- electromagnetic wave energy

- AM and FM - The book explains this well, but the animated gif in this Wikipedia article may be illuminating: http://en.wikipedia.org/wiki/Frequency_modulation#Applications_in_radio

- bandwidth

- speed = wavelength/period = wavelength x frequency

Demos shown:

M9-03 CIRCULAR POLARIZATION - STICK MODEL

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Thursday 11/06

- Started radiation, by showing this excellent applet:
http://webphysics.davidson.edu/applets/retard/Retard_FEL.html
The applet illustrates the concept that when a charge accelerates, distortions in the electric field lines propagate outward at the speed of light. These distortions amount to the electric part of electromagnetic waves. Try setting the demo button in the upper right on "inertial", with v=0. Then suddenly slide the slider to nonzero velocity, and watch the shift in electric field lines propagate outward. Notice that the kink becomes more and more perpendicular to the direction of propagation as it goes out. The kink represents a "sideways" electric field vector, showing the polarization of the wave. Next, set the button to "SHO", simple harmonic oscillator, and put the velocity slider on, say v = 0.7. (This is the speed in units of the speed of light, i.e. v = 0.7 c.)

- To get deeper into the physics, I asked how the electric field in the wave is created at a given spot far from the charge. The answer:

We learned before that changing magnetic fields produce induced voltages, which means is that they can accelerate a charge that starts out at rest, which means that they must have generated an electric field! So a changing B field makes an E field. But then what makes the changing B field, far from the accelerating charge? The answer is a new phenomenon, that we didn't know about before, and which James Clerk Maxwell discovered in 1861: just as a changing B field makes an E field, so too a changing E field makes a B field! Maxwell inferred that this must be the case if the theory is to be consistent. So what happens is that the accelerating charge makes a changing E field which makes a changing B field which makes a changing E field ... etc, and the pattern of changing fields spreads out into empty space at the speed of light.

- The speed of light is fast: about one foot per nanosecond! In fact, it;s closer to 11.8 inches per nonosecond, within 2% of one foot. Not bad. In meters per second it's 2.998 E8 m/s, very close to 3 E8 m/s, which is easier to remember. Another useful way to put it: 300,000 km/s.

- The rest of what was covered is more or less in the book and/or in the demos shown.

Demos shown:

K7-61 TESLA COIL

K8-01 ELECTROMAGNETIC WAVE - MODEL

K8-03 LIGHT NANOSECOND

K8-05 ELECTROMAGNETIC PLANE WAVE MODEL

K8-11 MICROWAVES - STANDING WAVES

K8-42 RADIOWAVES - ENERGY AND DIPOLE PATTERN

K8-51 MICROWAVE OVEN

K8-52 MICROWAVE MAGNETRON

M7-01 MICROWAVES - POLARIZATION

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Tuesday 11/04

- More about induction
- Generators and motors

Demos shown:

K1-12 CATHODE-RAY TUBE - DEFLECTION BY MAGNET

K1-03 FORCE ON CURRENT IN MAGNETIC FIELD

K2-02 INDUCTION IN A SINGLE WIRE

K2-03 FARADAY’S EXPERIMENT ON INDUCTION

K2-04 FARADAY’S EXPERIMENT - EME SET - 20, 40, 80 TURN COILS

K2-43 LENZ’S LAW - PERMANENT MAGNET AND COILS

K2-44 EDDY CURRENT PENDULUM

K4-01 AC/DC GENERATOR

K4-21 ST. LOUIS MOTOR

K4-41 MOTOR-GENERATOR PAIR

K4-25 DC MOTOR - HOMEMADE

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Thursday 10/30

-  Covered most of what is in section 11.2, plus more. In particular, I tried to explain more about the nature of forces on currents, and the nature of induction. Here is a kind of outline of the flow of ideas, as I presented them. All these things were illustrated using the demos.

1. Parallel currents attract, antiparallel repel.

2. This can be understood more fundamentally this way: the moving charge in one wire is deflected by the magnetic field set up by the current in the other wire.

3. The direction of the magnetic force on a moving charge is perpendicular to the plane formed by the velocity vector of the charge and the magnetic field vector. In the example of two parallel wires, the magnetic field set up by one wire is concentric with the wire (see Fig. 11.1.9), and the velocity of the charges in the other wire is along the wire,  so the force is towards or away from the other wire. (Think about it.)

4. A current can be INDUCED in a wire by moving the wire in a magnetic field. This is really the same thing as before: The charges in the moving wire have velocity since the wire is moving, so they feel a force as described above. If the force they feel is along the wire, a current will flow, provided the wire is a conductor.

5. If instead of moving a wire towards a magnet, you move the magnet towards the wire, current will still flow in the wire, since only the relative motion matters.

6. If instead of moving the magnet, you somehow change the strength of the magnetic field, it would be the same as if the magnet were moved, so a changing magnetic field will also generate a current.

7. The direction of the current you generate can be determined by Lenz's law: as the book puts it (p. 360), the effects of magnetic induction oppose the changes that produce them. More specifically, the induced current produces a magnetic field that opposes the change of magnetic field that induced the current. Were it not so, there could be a runaway process, in which induction generates more induction.

8. An application is concentric circular loops: an increasing current in one loop generates an increasing magnetic field, which induces an OPPOSITE current in the other loop, and the two loops repel each other. This was demonstrated in class, with the ring that flies off the iron core inside the other coil. This example is with AC current, so the direction of the induced current keeps switching, 60 times per second, but each time, the induced current is opposite, so the ring flies off. It was also demonstrated with the can exploder, where we released the charge built up on a massive capacitor through three turns of thick copper wire surrounding an aluminum soda can. The induced, opposite current was pushed inwards and outwards, away from the fixed copper wire, and the can was torn apart.

9. Transformers are explained in the textbook. All I'll say here is that I'd put it this way: V_s/V_p = N_s/N_p, where V_s,p is the voltage across the pimary and secondary coils, and N_p,s is the number of turns in the two coils. This is true since each turn of wire contributes the same amount to the induced voltage.

Demos shown:

J7-13 CURIE POINT OF NICKEL

K1-03 FORCE ON CURRENT IN MAGNETIC FIELD

K1-05 FORCE BETWEEN CURRENT-CARRYING COILS

K2-02 INDUCTION IN A SINGLE WIRE

K2-61 THOMSON'S COIL

K2-62 CAN SMASHER - ELECTROMAGNETIC

K3-03 DEMOUNTABLE TRANSFORMER - V VS N - PROJ METER

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Tuesday 10/28

- Magnetism introduced, material from section 11.1 covered.
- Explained that isolated magnetic poles don't exist, and I don't advocate Bloomfield's choice to present the material this way. The basic physics is that parallel currents attract and opposite currents repel. This means also that loops of current in the same sense attract, and opposite repel. You can invent the idea of "poles" and account for this by assigning to each loop a dipole - a positive-negative or North-South pole pair. But there is no unique pair. Only the product of the magnetic pole strength and the separation between the poles has any physical consequence. That is, there is no unique answer to the question "What are the pole strengths for a given loop of current?"
- Discussed magnetic field of the earth, its changing in time, and the geomagnetic pole reversal phenomenon.

Demos shown:

K1-01 FORCE BETWEEN CURRENT-CARRYING WIRES

K1-05 FORCE BETWEEN CURRENT-CARRYING COILS

J5-05 MAGNET MODEL - FIELD LINES

J5-14 MAGNETIC FIELD AROUND SINGLE AND PARALLEL WIRES

J5-34 DECLINATION AND INCLINATION NEEDLE

J5-35 MAGNETS INTERACTING ON PIVOTS

J6-04 LOW POWER - HIGH FORCE ELECTROMAGNET

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Thursday 10/23

- Explained van der Waals attraction between neutral objects, with applications to molecular attraction and gecko feet.
- Covered concepts from section 10.3

Demos shown:

K5-01 PIEZOELECTRICITY

K5-04 PIEZOELECTRIC GUN

K5-12 BATTERY AND CURRENT - WORKING MODEL

K5-32 RESISTANCE VS DIAMETER AND LENGTH

K5-36 RESISTORS AT LN TEMPERATURE - LIGHT BULB INDICATOR

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Thursday 10/16

Introduction to electricity.

Demos shown:

J1-01 TRIBOELECTRICITY - CHARGING BY FRICTION

J1-05 CHARGED BALLOONS

J1-12 INDUCTION - ELECTROSCOPE

J1-13 ELECTROSTATIC INDUCTION

J1-21 ELECTROSTATIC ATTRAC AND REPULS - CHARGED CYLINDERS

J1-24 ELECTROSTATIC HAIR RAISING

J2-03 VAN DE GRAAFF GENERATOR WITH GROUND SPHERE

J2-14 LIGHTNING ROD SIMULATOR

J3-06 ELLIPSOIDAL CONDUCTOR

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Tuesday 10/14

Covered section 9.2. Explained nature of overtones and harmonic intervals in music. Discussed harmonic vs. well-tempered tuning, twelve tone scale, and illustrated the sound with an online applet. Demonstrated and explained "beats" using a guitar string.

Discussion of harmonic and tempered tuning
http://www.physicsandmusic.com/reader.php?section=10-2

Tones and scales applet
http://pages.globetrotter.net/roule/js/acc.htm

Demos shown:

G3-28 SUSPENDED SLINKY

G1-15 PENDULA WITH 4 TO 1 LENGTH RATIO

H4-31 VIOLIN

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Thursday 10/09

Covered section 9.1, plus a few things not in the textbook:
 + Used dimensional analysis to show that the period of a pendulum is proportional to the square root of L/g. Why? L is in m, g is in m/s^2, and the only way to combine these to    get s (seconds) is Sqrt[m/(m/s^2)], i.e. Sqrt[L/g]. It's pretty striking and powerful that you get some real physics info from such a simple argument! This kind of derivation can be done all over in physics.
 + Showed a graph illustrating how the period of a pendulum depends on the amplitude.
 + Pointed out that for a simple harmonic oscillator, the motion is described by a sine function, i.e. the displacement is proportional to sin(2pi t/T), where T is the period of oscillation. It's interesting to see a trig function appear in describing motion.

Demos shown:
G1-15 PENDULA WITH 4 TO 1 LENGTH RATIO
G1-52 STRINGLESS PENDULUM
G2-01 MASS ON SPRING - HAND HELD
H4-31 VIOLIN
H1-23 SPEED OF SOUND IN ALUMINUM

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Summary of concepts, first unit

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Tuesday 9/30:

Demos shown:
C8-14 JUMPING CLAMP
C8-34 POWER - INSTRUCTOR DRAGGING CONCRETE
K4-07 BICYCLE GENERATOR
I5-15 ADIABATIC EXPANSION OF CARBON DIOXIDE
DIGITAL THERMOMETER, showing cooling by evaporation of water

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Supplement for Chapter 8:
Entropy change as a quantitative concept

As discussed in the textbook, entropy is a measure of disorder. When heat Q goes into a system in equilibrium at a temperature T, the disorder increases. If the system is already very hot, it is already very disordered, so the disorder increases less than it would if the same heat went into a colder system. Entropy gives a way of quantifying the disorder: when heat Q goes into (or out of) a system at temperature T, the entropy increases (or decreases) by Q/T.

Entropy change =  Q/T = (heat into system)/(temperature of system)

This entropy change for a given heat transfer is inversely proportional to the temperature. For example, at twice the temperature, the entropy change is half. With Q in Joules, and T in degrees Kelvin, the units of entropy are J/K. The concept of entropy was originally formulated purely using the concepts of heat and temperature in this way. The connection to disorder, which can be quantified using probability theory, came later.

Using the above relation we can understand the otherwise mysterious formula (8.1.2) relating the heat, work, and temperature for an ideal heat pump as follows. Suppose heat Q_c is pumped from a cold system at temperature T_c, using an amount of work W by the pump. Then since the total energy is conserved (the first law of thermodynamics) the total heat deposited in the hot system at temperature T_h is

Q_h = Q_c + W. 

(This is different from (8.1.2) because the  textbook  seems to have taken -Q_c to mean the heat removed from the hot object, rather than Q_c as done here.)
The minimal work required to pump the heat Q_c is achieved when Q_h is as small as possible. The lower limit on Q_h is set by the requirement that the entropy cannot decrease (the second law of thermodynamics). In an ideal heat pump, i.e. one that uses the least possible amount of work, the total entropy remains unchanged. In this case,

Q_c/T_c = Q_h/T_h = (Q_c + W)/T_h. 

Solving yields

W = Q_c(T_h/T_c - 1) = Q_c(T_h - T_c)/T_c,

or inverting,

Q_c = W T_c/(T_h - T_c).  This is the same as (8.1.2) except for the sign.

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Thursday 9/25: class taught by Prof. Einstein

Demos shown:
I5-11 ADIABATIC PROCESS - AIR PISTON WITH THERMISTOR
I5-21 HEATING AIR BY COMPRESSION
I5-22 FIRE SYRINGE

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Tuesday 9/23: class taught by Prof. Einstein

Demos shown:
I4-11 BOILING AT REDUCED PRESSURE
I4-14 CHANGE OF STATE WITH BANG
I4-19 CONDENSATION OF STEAM - SODA CAN COLLAPSE
I4-31 ICE BOMB
I5-12 ADIABATIC EXPANSION OF AIR - FOG IN BOTTLE

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Here's a report on recent research into the microscopic nature of friction and frictionless surfaces: http://focus.aps.org/story/v22/st10

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Thursday 9/18:

Demos shown:
I1-11 THERMAL EXPANSION - BALL AND HOLE
I1-13 THERMAL EXPANSION - BIMETAL STRIP
I1-18 BIMETALLIC STRIP THERMOMETERS
I1-63 HYDROGEN EXPLOSION
I2-11 THERMOS BOTTLE
I2-22 THERMODYNAMICS BY TOUCH
I2-43 CONVECTION - HOT PLATE
I4-33 CRYOPHORUS

I found the original article where the "Cryophorus" was invented. Wollaston used a salt and snow mixture to cool one end, rather than liquid nitrogen.  He also mentions that a certain Mr. Lesie previously devised a method using sulfuric acid to absorb the water vapor. It's interesting to see how the concept of heat as thermal energy was not yet current, and even the fact there there is no such thing as "positive cold", i.e. that cold is only the absence of heat, was apparently not yet  fully established.
On a Method of Freezing at a Distance
William Hyde Wollaston
Philosophical Transactions of the Royal Society of London, Vol. 103 (1813), pp. 71-74
http://www.jstor.org/stable/107389

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Tuesday 9/16:

Demos shown:
F1-01 FLUID PRESSURE VS. DEPTH
F2-12 HOT AIR BALLOON
I3-33 HELIUM BALLOON ON LIQUID NITROGEN
I3-42 BOYLED MARSHMALLOWS
F2-21 REACTION TO BUOYANT FORCE

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Tuesday 9/9:

Demos shown:
B3-01 LEVER AND LOADED WAGON
3-13 PULLEY VS NO PULLEY
C6-11 SLIDING FRICTION - LECTURE TABLE AND FELT

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Thurday 9/4:

Demos shown:
C8-01 GIANT PENDULUM
G1-14 PENDULA WITH DIFFERENT MASSES
C8-21 ROCK AND WASTE BASKET

Video shown:
Jackass vomit comet ride

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Tuesday 9/2:

International System of Units (SI)
This links to a website at NIST, the National Institute of Standards and Technology, located in Gaithersburg, Maryland. It has really nice brief summaries of the historical context of the SI base units, and their present definitions.

Demos shown:
C8-01 GIANT PENDULUM
A2-22 MAGNETIC VECTORS - LECTURE HALL
C3-04 INERTIA - LEAD BRICK AND HAND
C3-05 INERTIA - PEN IN BOTTLE
C3-12 PENCIL AND PLYWOOD

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