PHY 1033 Fall 2010 Exam #3 In class (1 hr. 40 min. ending 4:40 pm) Ihas Nov. 9 You are allowed to consult one 8.5”x11” sheet of paper with anything written on it in your own hand, and to use a calculator, ruler, and pencils or pens. Number your answer to each question clearly, answering them in numerical order in a “Blue/Green Book” test form. If your exam book had enough pages left in it from the last exam, you may use it again. Clearly put your name on the front of the blue/green book with “Exam #3” and today’s date. Do not violate the UF honor code. Write on the front of the blue/green book “I have neither given nor received help from anyone during this exam”, and sign your name. Answers without the required justification or explanations will be counted wrong, even if they are correct. You may quietly ask me any question after raising your hand, but my answers will only clarify the exam questions. Good luck! c(light in air) = 3x108 m/s c(sound in classroom air) = 341 m/s n(water) = 1.33 f(microwave oven) = 2450 MHz h = 6.6x10‐34 J/Hz Q(electron) = 1.6x10‐19 Coulombs n(diamond) = 2.5 1. A demo in class showed two pendula swung into a magnetic field. The one with slits in the copper sheet swung thru the magnet gap, while the solid copper sheet was stopped between the poles of the magnet. This demonstrates which law? Explain. Lenz’s or Faraday’s Law: The opposing induced current is much bigger in the solid sheet. He slots in the other sheet break up thecurrents, making the opposing field it produces much smaller. 2. A 341 Hz tuning fork excites what wavelength of sound in a column of class room air 25 cm long? Λ=c/f=341/341=1m. I gave the 25 cm which is what you would measure in the lab experiment we did=1/4 λ. You did not need it here. 3. Two tuning forks as in problem #2 are made to oscillate in phase. One is 1.5 m directly in front of you and the other 2.0 m in front of you. What do you hear? Explain, perhaps drawing a picture. The tuning forks send sound from positions ½ λ apart, so they DESTRUCTIVELY interfere, cancelling each other, so that you hear no sound. 4. How far apart are the hot spots when cooking in a microwave oven (no turn table)? Show your calculation. λ/2=c/f=3x108/2450x106=12 cm (λ/2 because that is the distance between maxima in the standing wave). 5. You rub a plastic straw, charging it, and wave it up and down 3 times per second. What is the frequency of E‐M radiation produced? Show calculation. T=1/3 sec so f=1/T=3 Hz 6. In what part of the E‐M spectrum is the radiation in problem #5? Very long wavelength radio waves. 7. The work function ф is the amount of energy it takes to pry an electron from a metal. For copper ф = 4.5 eV. What is the lowest frequency light which can free an electron from copper? E=фq=4.5x1.6x10‐19 Joules and f=E/h so f=4.5x1.6x10‐19 /6.2x10‐34 = 1.2x1015 Hz 8. What is the wavelength of the photon in problem #7? Show you equations. λ=c/f=3x108/1.2x1015=2.5x10‐7 m=250nm 9. What part of the E‐M spectrum is the photon in problem #7? Show your logic in case you got #7 wrong. UV 10. What is the momentum p (in Kg m/s) of the photon in problem #7? Show your logic in case you got #7 wrong. Hint: If you use one system of units (m, s, Coulombs, Hz) it will come out correctly.p=E/c=hf/c=h/λ=6.2x10‐34/2.5x10‐7=2.5x10‐27kg‐m/s2 11. How does the product of the answer for #8 and #10 compare to what you expect for ΔxΔp from the uncertainty principle? λp=2.5x10‐7 x 2.5x10‐27= 6.2x10‐34 which is h (Planck’s constant) as it should be! (within a factor of 2 or so). 12. What physical property does not change when a light wave enters a block of clear glass from air? f 13. (2 pts) In the Compton Effect a photon collides with an atom, with another photon coming away from the collision. How are the wavelengths of the incoming and outgoing photons related (incoming greater than outgoing, the same as, or less than)? Are they the same photon? Wavelength increases (energy lost from photon), so it must be a different photon than that coming in. 14. (2 pts) Which bends more on diffracting, red or blue light? Which bends less on refracting, red or blue light? Red; red 15. A standing wave is composed of how many waves doing what? Be specific. 2 waves of same f moving in opposite directions. 16. What is Brewster’s angle (in degrees) for light reflecting off a facet of a diamond? tanӨ=n so arctan2.5=68°=Brewster’s angle 17. Why does putting two polarizers oriented at right angles to each other in front of each slit in Young’s interference experiment prevent the interference pattern from being seen? Be concise. Each polarizer will pass photons of different polarization, which means the photons coming through each slit are from different atomic transitions—so these photons can not be in phase—only coherent beams of light can interfere—this shows that the interference pattern is really a collection of single photons interfering with themselves. 18. (5 pts) You take the following data on the photo‐electric effect: V = 2.0 volts λ = 400 nm V = 1.5 volts λ = 530 nm V = 1.2 volts λ = 670 nm V = 0.9 volts λ = 900 nm Make a neat graph of E(photon) vs. f(photon). Use your ruler to get the scales correct, in Joules (y‐axis) and Hz (x‐axis). Extra credit (2 pts): What is the slope of the best straight–line fit to the plotted points in units of Joule‐sec? 3.40E-019
Photon Energy in Joules
3.20E-019
3.00E-019
SLOPE=4.2E-34 J-s
note: h=6.2E-34 J-s
2.80E-019
2.60E-019
2.40E-019
2.20E-019
2.00E-019
1.80E-019
1.60E-019
1.40E-019
1.20E-019
3.00E+014
4.00E+014
5.00E+014
6.00E+014
7.00E+014
8.00E+014
Photon Frequeny in Hz