# Mid Exam Source

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Inha University Department of PhysicsChapter 2 Problem Solutions1. If Planck's constant were smaller than it is, would quantum phenomena be more or less conspicuous than they are now?3. Is it correct to say that the maximum photoelectron energy KEmaxis proportional to the frequency of the incident light? If not, what would a correct statement of the relationship between KEmaxand be? Sol Planck s constant gives a measure of the energy at which quantum effects are observed. If Planck s constant had a smaller value, while all other physical quantities, such as the speed of light, remained the same, quantum effects would be seen for phenomena that occur at higher frequencies or shorter wavelengths. That is, quantum phenomena would be less conspicuous than they are now. Sol No: the relation is given in Equation (2.8) and Equation (2.9),), (max oh h KE So that while KEmaxis a linear function of the frequency of the incident light, KEmaxis not proportional to the frequency.Inha University Department of Physics5. Find the energy of a 700-nm photon.7. A 1.00-kW radio transmitter operates at a frequency of 880 kHz. How many photons per second does it emit? Sol The number of photons per unit time is the total energy per unit time(the power) divided by the energy per photon, or Sol From Equation (2.11),eV. 77 1m 10 700m eV 10 24 19 -6.. EOr, in terms of joules,J 10 84 2 m 10 700 m/s) 10 s)(3.0 J 10 63 61998 34 .. (E. photons/s 10 72 1Hz) 10 s)(880 J 10 63 6J/s 10 00 1303 343 .. (. hPEPInha University Department of Physics9. Light from the sun arrives at the earth, an average of 1.5 x 1011m away, at the rate of 1.4 x 103W/m2of area perpendicular to the direction of the light. Assume that sunlight is monochromatic with a frequency of 5.0 x 1014Hz. (a) How many photons fall per second on each square meter of the earth's surface directly facing the sun? (b) What is the power output of the sun, and how many photons per second does it emit? (c) How many photons per cubic meter are there near the earth? Sol (a) The number of photons per unit time per unit are will be the energy per unit time per unit area (the power per unit area, P/A), divided by the energy per photon, or). .. /2 2114 34 -2 3m s photons/( 10 2 4Hz) 10 s)(5.0 J 10 (6.63W/m 10 4 1 hA P(b) With the reasonable assumption that the sun radiates uniformly in all directions, all points at the same distance from the sun should have the same flux of energy, even if there is no surface to absorb the energy. The total power is then,, . ) . ( ) . ( ) / ( W 10 0 4 m 10 5 1 4 W/m 10 4 1 426 2 11 2 3 2 S ER A Pwhere RE-Sis the mean Earth-Sun distance, commonly abbreviated as 1 AU, for astronomical unit. The number of photons emitted per second is this power divided by the energy per photon, or . photons/s 10 2 1Hz 10 0 5 s J 10 (6.63J/s 10 0 44514 34 -26 .) . )(.Inha University Department of Physics11. The maximum wavelength for photoelectric emission in tungsten is 230 nm. What wavelength of light must be used in order for electrons with a maximum energy of 1.5 eV to be ejected? Sol Expressing Equation (2.9) in terms of = c/ and 0= c/0, and performing the needed algebraic manipulations,(c) The photons are all moving at the same speed c, and in the same direction (spreading is not significant on the scale of the earth), and so the number of photons per unit time per unit area is the product of the number per unit volume and the speed. Using the result from part (a),. ..) .3 1382 21 photons/m 10 4 1 m/s 10 0 3m s photons/( 10 2 4 nm. 180m eV 10 24 1m 10 230 eV 5 11 nm) 2301169101]1

+ 1]1

+ +.) )( . (() / (maxmax hcKK hchc ooInha University Department of Physics13. What is the maximum wavelength of light that will cause photoelectrons to be emitted from sodium? What will the maximum kinetic energy of the photoelectrons be if 200-nm light falls on a sodium surface?15. 1.5 mWof 400-nm light is directed at a photoelectric cell. If 0.10 percent of the incident photons produce photoelectrons, find the current in the cell. Sol The maximum wavelength would correspond to the least energy that would allow an electron to be emitted, so the incident energy would be equal to the work function, and Sol Because only 0.10% of the light creates photoelectrons, the available power is (1.0x10-3)(1.5x10-3W) = 1.5x10-6W. the current will be the product of the number of photoelectrons per unit time and the electron charge, or nm 539eV 3 2m eV 10 24 16 ..max hcwhere the value of for sodium is taken from Table 2.1.From Equation (2.8),eV. 3.9 eV 3 2 m 10 200m eV 10 24 196 ..max hch KA 48 0m eV 10 1.24m 10 400 J/s 10 5 116 -9 6 .) )( . () (/ ehcPehcPeEPe IInha University Department of Physics17. A metal surface illuminated by 8.5 x 1014Hz light emits electrons whose maximum energy is 0.52 eV The same surface illuminated by 12.0 x 1014Hz light emits electrons whose maximum energy is 1.97 eV From these data find Planck's constant and the work function of the surface . Sol Denoting the two energies and frequencies with subscripts 1 and 2,. , max, max, 2 2 1 1 h K h KSubtracting to eliminate the work function and dividing by 1- 2,s eV 10 1 4Hz 10 5 8 Hz 10 0 12eV 52 0 eV 7 191514 141 21 2 .. .. .max, max, K Khto the allowed two significant figures. Keeping an extra figure givess J 10 6.64 s eV 10 14 434 - 15 . hThe work function may be obtained by substituting the above result into either of the above expressions relating the frequencies and the energies, yielding = 3.0 eV to the same two significant figures, or the equations may be solved by rewriting them as , , max, max, 1 1 2 1 2 2 2 1 2 1 h K h Ksubtracting to eliminate the product h12and dividing by 1- 2to obtaineV 0 3Hz) 10 5 8 Hz 10 (12.0Hz) 10 eV)(12.0 52 0 Hz) 10 eV)(8.5 7 1914 1414 141 22 1 1 2... ( . (max, max, K K(This last calculation, while possibly more cumbersome than direct substitution, reflects the result of solving the system of equations using a symbolic-manipulation program; using such a program for this problem is, of course, a case of "swatting a fly with a sledgehammer".) Inha University Department of Physics19. Show that it is impossible for a photon to give up all its energy and momentum to a free electron. This is the reason why the photoelectric effect can take place only when photons strike bound electrons. Sol Consider the proposed interaction in the frame of the electron initially at rest. The photon's initial momentum is po= Eo/c, and if the electron were to attain all of the photon's momentum and energy, the final momentum of the electron must be pe= po= p, the final electron kinetic energy must be KE = Eo= pc, and so the final electron energy is Ee= pc + mec2. However, for any electron we must have Ee2= (pc)2+ (mec2)2. Equating the two expressions for Ee2( ) ( ) ( ) ( ) , ) ( ) ( ) (22 2 22222 2 22 c m c m pc pc c m pc c m pc E e e e e e + + + + ( ). ) (22 0 c m pc eorThis is only possible if p = 0, in which case the photon had no initial momentum and no initial energy, and hence could not have existed.To see the same result without using as much algebra, the electron's final kinetic energy ispc c m c m c p e e +2 4 2 2 2for nonzero p. An easier alternative is to consider the interaction in the frame where the electron is at rest after absorbing the photon. In this frame, the final energy is the rest energy of the electron, mec2, but before the interaction, the electron would have been moving (to conserve momentum), and hence would have had more energy than after the interaction, and the photon would have had positive energy, so energy could not be conserved.Inha University Department of Physics21. Electrons are accelerated in television tubes through potential differences of about 10 kV. Find the highest frequency of the electromagnetic waves emitted when these electrons strike the screen of the tube. What kind of waves are these? Sol For the highest frequency, the electrons will acquire all of their kinetic energy from the accelerating voltage, and this energy will appear as the electromagnetic radiation emitted when these electrons strike the screen. The frequency of this radiation will beHz 10 4 2s eV 10 14 4V) 10 10 118153 ..)( ( eheVhEwhich corresponds to x-rays.23. The distance between adjacent atomic planes in calcite (CaCO3) is 0.300 nm. Find the smallest angle of Bragg scattering for 0.030-nm x-rays. Sol Solving Equation (2.13) for with n = 1,od9 2 nm 0.300 2 nm 030 02 ..ar cs in ar cs in

,_

,_

Inha University Department of Physics25. What is the frequency of an x-ray photon whose momentum is 1.1 x 10-23kg m/s? Sol From Equation (2.15),Hz 10 0 5s J 10 63 6m/s) kg 10 m/s)(1.1 10 0 3183423 - 8 ... (hcp27. In See. 2.7 the x-rays scattered by a crystal were assumed to undergo no change in wavelength. Show that this assumption is reasonable by calculating the Compton wavelength of a Na atom and comparing it with the typical x-ray wavelength of 0.1 nm. Sol Following the steps that led to Equation (2.22), but with a sodium atom instead of an electron, m, 10 8 5 kg) 10 m/s)(3.82 10 (3.0s J 10 63 61726 - 834 ..,NaNa CcMhor 5.8 x 10-8nm, which is much less than o.1 nm. (Here, the rest mass MNa=3.82 x 10-26kg was taken from Problem 2-24.)Inha University Department of Physics29. A beam of x-rays is scattered by a target. At 45ofrom the beam direction the scattered x-rays have a wavelength of 2.2 pm. What is the wavelength of the x-rays in the direct beam? Sol Solving Equation (2.23) for , the wavelength of the x-rays in the direct beam, pm 5 1 45 pm)(1 426 2 pm 2 2 1 . ) cos . ( . ) cos