QuizWk13Soln

QuizWk13Soln - b. 650 km/s c. 420 km/s d. 1480 km/s...

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Physics 208, Spring 08, Quiz Week 13 Apr. 14, 2008 1. Short problem : A red and blue beam of light have a power of 3 mW and wavelengths respectively of 750 nm and 430 nm. How many photons/second do they produce? [Hint: hc = 1240 eV x nm and 1 eV = 1.6 x 10 -19 J.] Solution: Power = energy/time = Number of photons x hf/time The blue light produces Number of photons/time = Power/hf = Power x lambda/hc = 3 mW x 750 nm/(1240 eV x nm) = 3 x 10 -3 eV x 750 nm/(1.6 x 10 -19 s x 1240 eV x nm) = 1.13 x 10 16 photons/s and the red light produces 6.50 x 10 15 photons/s. Red light has less energy per photon (because it has lower frequency) so it needs more photons than the blue light to produce the same power. Multiple choice 2. What is the maximum velocity of a photoelectron emitted from a surface whose work function is 5.0 eV when illuminated by a light whose wavelength is 200 nm? a. 460 km/s
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Unformatted text preview: b. 650 km/s c. 420 km/s d. 1480 km/s Solution b K max = hf = hc/ - = 1240/200-5=1.2 eV = 1.92 x 10-19 J=> Remember electron mass = 9.11 x 10-31 kg v max = 2 K max m = 2 " 1.92 " 10 # 19 9.11 " 10 # 31 = 650 km / s 3. The light intensity on a metallic surface with a work function of 3 eV produces photoelectrons with a maximum kinetic energy of 2 eV. The frequency of light is doubled. Determine the maximum kinetic energy of photoelectrons in eV. a. 3 b. 7 c. 2 d. 4 Solution: b K max = hf => hf = 5 eV If f is doubled K max = 2hf = 10eV -3 eV = 7 eV. 4. A neutron has de Broglie wavelength is 1.4 x 10-10 m. What is its kinetic energy in eV? The neutron mass is 939.573 MeV/c 2 = 1.67 x 10-27 kg. a. 4 b. 0.4 c. 0.04 d. 40 Solution c. " = h p # K = p 2 2 m = h 2 2 m 2 = ( hc ) 2 2 mc 2 2 = 1240 2 2 $ 939.573 $ 10 6 $ 0.14 2 = 0.04 eV...
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This note was uploaded on 04/29/2008 for the course PHYS 208 taught by Professor Rzchowski during the Spring '08 term at Wisconsin.

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QuizWk13Soln - b. 650 km/s c. 420 km/s d. 1480 km/s...

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