Quantum Physics

878 Chapter 27 Quantum PhysicsACTIVE FIGURE 27.4A circuit diagram for studying thephotoelectric effect. When lightstrikes plate E (the emitter), photoelectronsare ejected from the plate.Electrons moving from plate E toplate C (the collector) create acurrent in the circuit, registered atthe ammeter, A.LightCPhotoelectronsE© Bettmann/CORBISLog into PhysicsNow at www.cp7e.comand go to Active Figure 27.4, whereyou can observe the motion ofelectrons for various frequencies andvoltages.VAMAX PLANCK, German Physicist,(1858 – 1947)Planck introduced the concept of a “quantumof action” (Planck’s constant h) in anattempt to explain the spectral distributionof blackbody radiation, which laid the foundationsfor quantum theory. In 1918, hewas awarded the Nobel Prize for this discoveryof the quantized nature of energy.Current–V sHigh intensityLow intensityApplied voltageACTIVE FIGURE 27.5Photoelectric current versus appliedpotential difference for two lightintensities. The current increases withintensity, but reaches a saturationlevel for large values of V. At voltagesequal to or less than V s ,where V s is the stopping potential,the current is zero.Log into PhysicsNow at www.cp7e.comand go to Active Figure 27.5, whereyou can sweep through the voltagerange and observe the current curvefor different intensities of radiation.Variable powersupplyActive Figure 27.4 is a schematic diagram of a photoelectric effect apparatus.An evacuated glass tube known as a photocell contains a metal plate E (the emitter)connected to the negative terminal of a variable power supply. Another metalplate, C (the collector), is maintained at a positive potential by the power supply.When the tube is kept in the dark, the ammeter reads zero, indicating that there isno current in the circuit. However, when plate E is illuminated by light having awavelength shorter than some particular wavelength that depends on the materialused to make plate E, a current is detected by the ammeter, indicating a flow ofcharges across the gap between E and C. This current arises from photoelectronsemitted from the negative plate E and collected at the positive plate C.Active Figure 27.5 is a plot of the photoelectric current versus the potential differenceV between E and C for two light intensities. At large values of V, the currentreaches a maximum value. In addition, the current increases as the incident light intensityincreases, as you might expect. Finally, when V is negative—that is, when thepower supply in the circuit is reversed to make E positive and C negative—the currentdrops to a low value because most of the emitted photoelectrons are repelled by thenow negative plate C. In this situation, only those electrons having a kinetic energygreater than the magnitude of eV reach C, where e is the charge on the electron.When V is equal to or more negative than V s , the stopping potential, noelectrons reach C and the current is zero. The stopping potential is independent ofthe radiation intensity. The maximum kinetic energy of the photoelectrons isrelated to the stopping potential through the relationshipKE max eV s[27.4]Several features of the photoelectric effect can’t be explained with classicalphysics or with the wave theory of light:• No electrons are emitted if the incident light frequency falls below some cutofffrequency f c , which is characteristic of the material being illuminated. This isinconsistent with the wave theory, which predicts that the photoelectric effectshould occur at any frequency, provided the light intensity is sufficiently high.• The maximum kinetic energy of the photoelectrons is independent of lightintensity. According to wave theory, light of higher intensity should carry moreenergy into the metal per unit time and therefore eject photoelectrons havinghigher kinetic energies.• The maximum kinetic energy of the photoelectrons increases with increasinglight frequency. The wave theory predicts no relationship between photoelectronenergy and incident light frequency.• Electrons are emitted from the surface almost instantaneously (less than 10 9 safter the surface is illuminated), even at low light intensities. Classically, weexpect the photoelectrons to require some time to absorb the incident radiationbefore they acquire enough kinetic energy to escape from the metal.