Download Physics 1010: The Physics of Everyday Life

Physics 1010:
The Physics of Everyday Life
TODAY
• Microwaves
• Sunlight
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Admin Stuff
• Requested to extend Extra Credit, so it is
due on Tuesday
• Today is last new lecture; from Thursday
comprehensive review
• Final is worth 80pts, as much as all midterms
together (you drop one midterm)
• One more EC, published Thursday
• Check your grades, TALK TO THE GRADERS
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Microwaves
Oscillating fields cause electrons to move
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EM wave speed, c = 3 x 108 m/s
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Moving electrons in resistive material
heat it up
Recall: electrical resistance
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piece of metal
Electron slows down because of
resistance in the metal.
Kinetic energy of electron is
transformed into heat.
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Food does not (should not?) contain metal,
so how does it get heated?
A water inside resonates
B the water inside rotates
C magic!
D the wave excites the molecules
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Food does not (should not?) contain metal,
but it does often contain water
positive charges at one end,
negative charges at the other –
H2O
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H2O is polarized.
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Not to scale! Wavelength is far bigger than
a molecule.
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Rotating and bumping causes heating
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λ
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one wavelength
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• Water molecules are
rotational oscillator at
-2.45 GHz
• Rotate and bump into
-+each other
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• Converts rotation to
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-heat (random motion)
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2.45 billion oscillations per sec
= 2.45 GHz = ν
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Which form of water do you think gets heated
more effectively in the microwave oven?
a) water vapor
b) liquid water
c) solid water (ice)
d) vapor and liquid are equally well heated
e) all three are equally well heated
b) Free to rotate, but close enough to bump into
each other
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Liquid water most rapidly heating as free
to rotate, but close enough to bump
Vapor- molecules too far apart
to rub against each other.
Solid- molecules are packed in too
tightly, cannot rotate effectively.
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Water has good absorption
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The arrows indicate the strength and direction of the force that would be
exerted by the electromagnetic wave on an electron (or a negative charge).
Water molecules are polar, so the hydrogens have a positive charge and the
oxygen has a negative charge.
What will this water molecule do in response to this force?
Upward force on positive
parts of molecule
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Downward force on
negative parts of molecule
RESULT IS ROTATION!
a)
b)
c)
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The arrows indicate the strength and direction of the force that would be
exerted by the electromagnetic wave on an electron (or a negative charge).
Water molecules are polar, so the hydrogens have a positive charge and the
oxygen has a negative charge.
What will this water molecule do in response to this force?
Upward force on positive
parts of molecule
--
+
+
--
+
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Downward force on
negative parts of molecule
RESULT IS ROTATION!
Rotate 90 degrees clockwise because the force on the oxygen is down and
the force on the hydrogens is up.
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The arrows indicate the strength and direction of the force that would be
exerted by the electromagnetic wave on an electron (or a negative charge).
Water molecules are polar, so the hydrogens have a positive charge and the
oxygen has a negative charge.
What will this water molecule do in response to this force?
Rotate 90 degrees clockwise because the force on the oxygen is down and the
force on the hydrogens is up.
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-ATTENTION: Net force on
water molecule is zero,
because upward force on
positive hydrogens exactly
balances downward force on
negative oxygen. (Overall,
molecule is neutral.)
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What is the wavelength of the 2.45 GHz
waves in a microwave oven?
a)
b)
c)
d)
8.17 m
8.17 cm
0.122 m
0.122 cm
c) λ = c/ν = 3.e*108/2.45*109 = 0.122 m
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The carbon dioxide molecule has charges distributed
roughly as shown below. How well would liquid carbon
dioxide heat in a microwave oven?
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a) as well as water
b) better than water
c) not as well as water.
c) Not as well as water because not polar
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Would any of these atoms and molecules
not heat up in microwave?
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a. none would heat.
b.
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d.
CH3 c.
Carbon
e.-- c. and d. would not heat
hydrogen tends to suck electrons off of carbon, C,
just like with oxygen, so extra negatives on hydrogen
atom, extra positives on carbon.
ans. e. only b is polar, and so will be rotated by microwave
field. All others, will be no twist on them. Must have positive
and negative charges arranged nonsymmetrically. True in
water, many fats. (oil heats nicely).
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Humans are protected by the metal
surrounding the microwave oven
Incoming microwave
Metal wall
Electrons respond,
reflect waves back
Across perfect conductor, no
voltage. So if perfect, electron
motion causes electric field to
be zero at conductor
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But what about those little holes that we
can see through?
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Light gets through
Light is EM waves
Microwaves are EM waves
Why don’t microwaves get through?
a) Wavelength of MW is longer than light
b) The wavelength of light is LONGER than the MW
c) The holes only let some wavelengths through
d)
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We cannot “see” objects (or features) that are smaller than the wavelength
of the wave we use (sound, EM).
Examples:
A electron microscope
B
C
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1) We use electron microscopes to see viruses, because viruses are smaller
than the wavelength of visible light
2) We use blue lasers (RlueRay) for HDTV DVD because in order to pack a
higher density disk (~40GB) the dots on the disk have to be smaller than
the wavelength of red light
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This Hot PocketTM comes with a cardboard tube
coated on the inside with a thin layer of a weakly
conducting material. This is supposed to brown the
Hot PocketTM. How does this work?
a) Microwaves enter the thin layer and accelerate electrons,
which give up energy due to resistance heating. Hot metal
then warms the bread.
b) The thin layer helps focus microwaves into the surface of the
Hot PocketTM.
c) The thin material contains excess water, which heats up well in
the microwave.
a) Dry bread (just before browning) has little
water. So heat something near.
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With reflections, places where no electric
field occur
wall
standing wave
oscillating back
and forth between
these two
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What's the distance between maxima
of the intensity?
a) 2λ
b) λ
c) λ/2
d) λ/3
?
wall
c) Half wavelength
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Microwave ovens have hot and cold spots
because of reflections
• Field is waves
in each
direction
12:00
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Radio waves also have hot and cold spots, because of reflection
from buildings and so on. Suppose you're listening to a radio
station with frequency 890 kHz (wavelength = 337 m), but the
reception is poor. About how far would you have to move to find
the nearest hot spot?
a) 337 m
b) 674 m
c) 168 m
d) 84 m
Answer: d) hot spots are spaced by 1/2 wavelength, so from
cold to hot is 1/4 wavelength.
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SUNLIGHT
Visible light is a small part of
the electromagnetic spectrum
• Spectral simulation for Sun
(5800 ºC)
http://www.colorado.edu/physics
/phet/simulations/blackbody/bl
ackbody.swf
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Sun emits light over the spectrum: the
range of frequencies
• The sun emits 41% of its radiation in the visible
spectrum,
• 9% in the ultraviolet spectrum (short wave), and
• 50% in the infrared spectrum (long wave).
Image from:
http://www.uwsp.edu/geo/faculty/ritter/geog101/uwsp_lectures/lecture_radiation_energy_concepts.html#Radiation
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Humans have three types of cones with broad sensitivity.
Your eye has red, green, and blue color receptors (cone cells);
each receptor is sensitive to nearby wavelengths as well.
When yellow light hits your eye, it stimulates both your red and
green receptors; stimulating your red and green receptors
simultaneously with red and green light looks like yellow to you.
There is no magenta light; you see magenta when your red and
blue receptors are stimulated. We call a mixture of all
wavelengths “white” light.
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Green light has a wavelength of
510 nm = 510x10-9m = 5.1x10-7m = 0.5 µm
What is its frequency?
a)
b)
c)
d)
1.96x106 Hz
5.9x1014 Hz
106.7 MHz
1.96x1015 Hz
b) f = c/λ = 3x108/5.1x10-7 = 5.9x1014
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Eyes in our skin?
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Detect near-visible radiation
UV (beyond blue): sun burn
Infrared: regular burn
Typical digital camera detects all of these
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Light slows down in materials
• Atomic polarization
• Light must polarize
material as it passes
through, slows velocity
to v, which is less than c
• Index of refraction:
n=c/v
Material
Vacuum
Air
Water
Glass
n
1
1.003
1.3
1.5
E
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What is the speed of light in glass (index
of refraction = 1.5) ?
a)
b)
c)
d)
1x108 m/s
2x108 m/s b) Speed of light in material = c/n
8 (m/s)/1.5 = 2x108 m/s.
=
3x10
3x108 m/s
4.5x108 m/s
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Green light has frequency =
5.9x1014 Hz. What is its wavelength in glass?
(1 nm = 10-9 meters)
a) 510 nm
b) 765 nm
c) 340 nm
c) Speed of light in glass = 2x108 m/s.
Wavelength = speed/frequency
= 2x108 (m/s)/5.9x1014 Hz = 3.4x10-7 m or
340 nanometers.
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SCATTERING
Light scatters from small particles:
Rayleigh scattering
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Good antenna: 1/4 wavelength
Atoms are 0.5 nm across
Light is 500 nm in wavelength
Bad antenna
Worse for longer wavelengths,
as farther from 1/4
wavelength
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Rayleigh scattering make the sky blue
• Blue light
scatters down
to earth
• Enough to drown
out the light of
the stars
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Rayleigh scattering make the sunrise and
sunset red
• Blue scatters
out
• Red left over
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Reviews for final
• Cover all material
• Final similar to mid-terms, but worth twice as
many points (as much as all mid-terms
together)
• Second part of surveys: extra credit (10pts)
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