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Physics 1010: The Physics of Everyday Life TODAY • Microwaves • Sunlight 1 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 2 Microwaves Oscillating fields cause electrons to move - - EM wave speed, c = 3 x 108 m/s 3 Moving electrons in resistive material heat it up Recall: electrical resistance - piece of metal Electron slows down because of resistance in the metal. Kinetic energy of electron is transformed into heat. 4 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 5 Food does not (should not?) contain metal, but it does often contain water positive charges at one end, negative charges at the other – H2O -- H2O is polarized. + + -+ Not to scale! Wavelength is far bigger than a molecule. + 6 Rotating and bumping causes heating -- + + + + - -- + + λ + + one wavelength + • Water molecules are rotational oscillator at -2.45 GHz • Rotate and bump into -+each other + • Converts rotation to + + -heat (random motion) + 2.45 billion oscillations per sec = 2.45 GHz = ν 7 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 8 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. - - + - - + + + - - + + + - + + - - + + + + + + - + - + - - - + + - + + - + - - + + - - - - - - + - + + + + + + + + + + - - - - + + + + + + + + - + + - - + + - - - + + + - + - + + + + + Water has good absorption 9 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 -- + + -- + + Downward force on negative parts of molecule RESULT IS ROTATION! a) b) c) 10 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 -- + + -- + + 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. 11 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. + + -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.) 12 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 13 The carbon dioxide molecule has charges distributed roughly as shown below. How well would liquid carbon dioxide heat in a microwave oven? - + - a) as well as water b) better than water c) not as well as water. c) Not as well as water because not polar 14 Would any of these atoms and molecules not heat up in microwave? + a. none would heat. b. - 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). 15 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 16 But what about those little holes that we can see through? • • • • 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) 17 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 18 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 19 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. 20 With reflections, places where no electric field occur wall standing wave oscillating back and forth between these two 21 What's the distance between maxima of the intensity? a) 2λ b) λ c) λ/2 d) λ/3 ? wall c) Half wavelength 22 Microwave ovens have hot and cold spots because of reflections • Field is waves in each direction 12:00 23 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. 24 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 25 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 26 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. 27 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 28 Eyes in our skin? • • • • Detect near-visible radiation UV (beyond blue): sun burn Infrared: regular burn Typical digital camera detects all of these 29 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 30 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 31 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. 32 SCATTERING Light scatters from small particles: Rayleigh scattering • • • • • 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 33 Rayleigh scattering make the sky blue • Blue light scatters down to earth • Enough to drown out the light of the stars 34 Rayleigh scattering make the sunrise and sunset red • Blue scatters out • Red left over 35 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) 36