Friday, February 29, 2008 (Happy Leap Day!)
Announcements:
- There is a physics demonstration show that is free and open
to the public tomorrow at 2:00 pm in Phillips 215, "Energy All Around
Us."
Assignments:
- Answer the recommended conceptual exercises and problems for
Chapter 9, and check your understanding by
looking at the answers in the back of the textbook.
- Submit your topic ideas for the physics Web Project.
Chapter 8 - Light and Electromagnetism
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.
Electric power is the rate at
which energy must be supplied: P =
IV = I*I*R = V*V/R
Resistors in series (end to
end): Rtotal = R1 + R2 + ...
Resistors in parallel (same
voltage): 1/Rtotal = 1/R1 + 1/R2 + ...
If a wire of
resistance R is cut into three equal lengths and connected in parallel,
what is Rtotal?
Demo: Series and
parallel circuits with bulbs
Ponderable: Which has
more resistance: a standard light bulb rated at 60 W or one rated
at 100 W? If these two bulbs were connected to a DC power supply,
which one would be brighter? Lesson: It is important to
understand what is implied by advertised statements.
Kirchhoff's Rules:
Junction rule (conservation
of charge): Total current into a junction must equal total current out.
Loop rule
(conservation of energy): Sum of potential differences around any
loop must be zero.
Induced Electromotive Force
A changing magnetic field can produce a
current in a circuit due to an induced
electromotive force.
The magnitude of the induced emf (and
resulting current) depends on the rate
of change of the magnetic flux.
Demo: Induced emf (coil,
magnet, large galvanometer) - Observe that the amount of deflection of
the
galvonometer needle (which also operates on magnetic induction) depends
on how quickly the magnet moves relative to the coil. The
direction of the induced current changes when any one of the following
conditions are reversed: orientation of the magnet, orientation
of the coil, relative motion of magnet and coil. This
demonstration shows how generators work.
An electric generator uses
mechanical work to
produce electrical energy. An electric
motor is basically an electric generator operated
in reverse.
Demos -
simple motor, Genecon, hand-charged flashlight
Applications of magnetic
induction: generators, motors, transformers, audio
speakers, telephone receivers and speakers, read/write heads for
magnetic media (tape recorders, VCR, disk drives), tuning circuits for
wireless EM devices (radios, cell phones, TVs), electric guitar
pickups, MRI, regenerative breaking systems, metal detectors, magnetic
anti-theft
devices, traffic signal loops in pavement, rechargeable toothbrushes.
Chapter 9 - Electromagnetic radiation
Electromagnetic waves consist
of oscillating
electric and magnetic
fields that are at right angles to each other and in phase.
EM waves travel at the speed of light,
which in a vacuum is exactly (by
definition): c = 299 792 458 m/s
This is fast by human time and
length scales, but slow on astronomical scales (see chart).
The electromagnetic spectrum
is divided into "bands"
of frequency with
special names that relate to the corresponding energy.
Diagram
of EM spectrum with corresponding length scales and atmospheric
opacity. Another spectrum diagram.
Diagram
with ionization potential as it relates to cell-phone health
concerns.
While the speed of light is the same for all EM
waves in a vacuum, the
energy of these waves depends on the frequency: E = hf
As was the case for other waves, the wave speed is
related to the
frequency and wavelength by: c = f*lambda
Radar = Radio Detection and Ranging, uses radio (EM)
waves
IR detection demo using a remote control
The importance of Maxwell's Equations is stated on a clever T-shirt
that says:
And God said:
∇ • E = ρ / εo
∇ • B = 0
∇ x E = - ∂B/∂t
∇ x B = μoJ + μoεo∂E/∂t
Translation: "Let there be light"