Monday, February 25, 2008
Announcements:
- I have finally posted the group assignments for the Web
Project. Communicate with your group members as soon as possible
and decide on a topic for your Web Project.
Assignments:
- Answer the recommended conceptual exercises and problems for
Chapter 8, and check your understanding by
looking at the answers in the back of the textbook.
- Read Chapter 9 and come to class prepared to discuss.
- Submit your topic ideas for the physics Web Project.
Chapter 8 - Light and Electromagnetism
What are some sources of light (Sun, fire, light bulbs, lighting,
lightning bugs, etc.). These are very different; is the
underlying cause the same in all of them? (Lightning, for instance, we
know is an electrical phenomenon, but it's certainly not clear that
fire has anything to do with moving charges.)
Discuss: What is light?
A wave is a disturbance traveling through a medium (although a medium is not required for light waves).
Waves are characterized by their amplitude
and frequency.
Wave speed is related to frequency and wavelength: v = f*lambda
Ponderable: Do high frequency sound waves travel faster than low
frequency waves? Do they travel farther or shorter? What
about for light waves?
The adding of waves (superposition)
can result in an increased amplitude (constructive
interference) or reduced amplitude (destructive interference).
What are some similarities and differences between sound and light waves?
Light waves interfere much like sound waves, but
since light waves have much shorter wavelengths, the effects appear
quite different.
Monochromatic light
has only one frequency, and hence a single color.
Coherent light
waves (like those from a laser) have a constant phase relationship,
which is a necessary
condition for the creation of interference patterns.
Interference effects from light waves are similar to those from water
waves (see figures in book with X's and O's).
Diffraction
- Light
changes direction and spreads out when it encounters an edge.
Demo: When light from a
laser illuminates a straight vertical edge (like that from a card), the
light on a distant screen spreads out horizontally instead of a
half-moon image as we would expect if light behaves only as particles.
Young's Two-Slit Experiment -
When light of wavelength lambda passes through two narrow slits
separated
by a distance d, an interference pattern will be produced with bright
fringes at angles theta: d*sin(theta) = m*lambda, m = 0, +/-1,
+/-2,... and dark fringes at d*sin(theta) = (m-0.5)*lambda, m =
0, +/-1, +/-2,...
Demo: A single slit
produces a diffraction pattern with
dark fringes located at: Wsin(theta) = m*lambda, m = +/-1,
+/-2,...
Which color of light produces the wider diffraction
pattern for the same slit width: red or green light?
Frequency and wavelength of light are related to wave speed: c =
f*lambda
Sources of light revisited: electromagnetic radiation
(including visible light) results from accelerated charges. When
an electron is in an excited state and drops to a lower energy level, a
photon of light is radiated.
- Fluorescent lights operate this way. Mercury is vaporized,
charge flows through, mercury's electrons excite then fall, when they
fall, they lose energy, emitted as light. But it's UV, which we can't
see. However, it hits the surface of the tube, which is coated
with a phosphorescent material. Its electrons are excited, and when
they fall, they emit visible light.
- Black lights are similar, but when its UV rays hit your teeth,
they don't just reflect; they glow.
- A glowing fire poker is similar- electrons jumping down emit
light. This is how an incandescent light bulb radiates light
(~10%) and heat (~90%).
Rewind back to the two slit experiment. Weird thing is, electrons
behave this way too...Video: Down the
Rabbit Hole - Double Slit Experiment
Quantum mechanics is weird, no doubt, but sometimes people say silly
things. The end of this video clip is misleading. It talks about how
observing destroys the interference pattern, and makes you think
observing is just staring at the system (note the big eye staring at
the system). That would be weird, but in fact, to observe,
you have to physically disturb the system.
Electric charges are either positive
or negative
(arbitrarily assigned by
Benjamin Franklin).
Charges are quantized in
"bits" of e, the magnitude of
the fundamental charge on an electron or proton. e = 1.6 x 10-19
C
- Why is the charge on an electron such a small unit?
Charges are transferred, but not created or destroyed (conservation of charge)
Demo - electrostatic charging
(rods, fur, balloon), triboelectric
series
Demo: Van de Graaff
generator to show charging by rubbing
Polarization of charge - A
conductor or insulator can become charged by induction (without
touching) due to a re-alignment of charges within the object.
Demo - charged balloon and stream of water.
Electric force is similar to gravitational force: F = k*q1*q2/d^2
An insulator does not allow
charges to move, while a conductor
allows excess charges to move freely. Excess charges reside on
the outer surface of the object due to mutual repulsion. Semiconductors can act like
conductors or insulators depending on their composition (purity,
doping). Photoconductive
materials are useful in many modern devices (laser printers,
photocopiers, light sensors).
Electric current is the flow of
electric charge due to an electrical
potential difference (voltage). I = dQ/dt.
1 A = 1 ampere = 1 amp = 1 C/s
Typical currents in common
electrical devices.
When a switch is closed so that current can flow in a circuit, the
reponse is very fast (approximately the speed of light), but the
average speed of a typical electron is much slower. Why?
Approximately how slow?
Demo: Rubber ball model
of current and resistance
What could be done to increase the current in this
demonstration?
What are the corresponding parameters to resistance?
Electrical
resistance in a wire depends on the resistivity of the
conductor, the length of the wire, and its cross-sectional area:
R = rL/A
Ohm's law is a useful relation
that is valid for many (but not all) resistive loads: V = IR, or
more properly, I = V/R (Why is this form better?)
The resistivity of most metals
increases with temperature
(ex. tungsten), but there are exceptions (ex. carbon and other
semiconductors).
Application: Thermal resistors (thermistors) are used in digital
thermometers.
Superconductivity
- below a certain critical
temperature, Tc, certain materials have zero resistance.
Exercise: Sketch and
label a graph of current as a function of voltage (I-V plot) for a
light bulb that has a cold resistance of 10 ohms and a hot resistance
of 100 ohms at its operating voltage of 12 V.
Electric power is the rate at
which energy must be supplied: P =
IV = I*I*R = V*V/R