Chapt39_VGC - Chapter 39. Quantization Chapter 39....

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Chapter 39. Quantization
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Topics: The Photoelectric Effect Einstein’s Explanation Photons Matter Waves and Energy Quantization Bohr’s Model of Atomic Quantization The Bohr Hydrogen Atom The Hydrogen Spectrum Chapter 39. Quantization
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The Photoelectric Effect • In 1886 Hertz noticed, in the course of his investigations, that a negatively charged electroscope could be discharged by shining ultraviolet light on it. • In 1899, Thomson showed that the emitted charges were electrons. • The emission of electrons from a substance due to light striking its surface came to be called the photoelectric effect. • The emitted electrons are often called photoelectrons to indicate their origin, but they are identical in every respect to all other electrons.
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Lenard’s Observations 1. Current I is proportional to light intensity 2. Current appears without delay 3. Photo electrons are emitted only if light frequency exceeds threshold f0. 4. The value of the threshold frequency depends on the type of metal 5. If potential difference is positive current is independent of potential. If potential is sufficiently negative (V stop ) no current is observed independent of light intensity. 6. Stopping potential is independent of intensity
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Lenard’s Experiment
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V stop depends on frequency of light
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Characteristics of the Photoelectric Effect 1. The current I is directly proportional to the light intensity. 2. Photoelectrons are emitted only if the light frequency f exceeds a threshold frequency f 0 . 3. The value of the threshold frequency f 0 depends on the type of metal from which the cathode is made. 4. If the potential difference ! V is positive, the current does not change as ! V is increased. If ! V is made negative, the current decreases until, at ! V = ! V stop the current reaches zero. The value of V stop is called the stopping potential. 5. The value of V stop is the same for both weak light and intense light. A more intense light causes a larger current, but in both cases the current ceases when ! V = ! V stop .
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Compare with ionization energy for H, 13.6 eV What would make an electron leave a solid?
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Thermionic Emission Heating electrons to high enough energy will make them leave a solid. cathode anode T ! 10 3 " K heater electrons Heated cathode produces electrons of sufficient energy to overcome work function, E 0 . Process used in vacuum tubes and cathode ray tubes. T ! 10 3 " K ! .1 eV Fraction of electrons with high enough energy for tungsten f ! exp[ ! E 0 kT ] = exp[ ! E 0 kT ] = exp[ ! 4.55 .1 ] = 1.7 " 10 ! 20 # of free electrons/cm 3 - 10 23 + other factors give about 10 A/cm 2 Raise T but don’t melt the cathode.
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Electrons leave cathode with kinetic energy K i K i = E elec ! E 0 Energy in metal Work function Electrons have kinetic energy K f when they reach the anode. K
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This note was uploaded on 12/28/2011 for the course PHYSICS 270 taught by Professor Drake during the Fall '08 term at Maryland.

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Chapt39_VGC - Chapter 39. Quantization Chapter 39....

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