8B - 18 Particles-waves_11.ppt

8B - 18 Particles-waves_11.ppt - Modern Physics Major...

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Modern Physics Major discoveries, starting about 1900, changed our understanding of the physics of atomic events.
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Photons - Quanta of light An apparatus used to study the photoelectric effect. The incident light shines on target T, ejecting electrons, which are collected by collector cup C. The electrons move in the circuit in a direction opposite the conventional current arrows. The batteries and the variable resistor are used to produce and adjust the electric potential difference between T and C. Einstein, 1905, Theory of Photoelectric Effect Applet on PE effect.
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First photoelectric experiment Measurements show that for light of a given frequency, K max does not depend on the intensity of the light source. Classically, the force on electron depends on E, so higher E 2 (intensity) should give higher energy to e - it does not!
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Second photoelectric experiment Now we vary the frequency f of the incident light and measure the associated stopping potential, V stop . The Figure is a plot of V stop versus f . Note that the photoelectric effect does not occur if the frequency is below a certain cutoff frequency f 0 or, equivalently, if the wavelength is greater than the corresponding cutoff wavelength λ 0 = c / f 0 . This is so no matter how intense the incident light is.
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Observations on PE effect 1) Instantaneous onset of current. (Classical takes time to heat.) 2) Maximum* e energy is independent of intensity. (Classical E or ˽* ntensity gives acceleration and max speed.) 3) Current only above a threshold frequency of the light. (Classical does not envision f dependence.) This is a complete failure of classical theory. *From the spectrum of e energies can learn about the energy levels of the material; “photoelectron spectroscopy” is an important technique.
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Einstein’s explanation - Photons Einstein said that light has an energy E = hf. An electron in a material must be bound by a minimum potential energy, Φ , which depends on the material. If the electron absorbs one photon and is ejected from the material with a maximum kinetic energy = K M , by conservation of energy: Energy of light in = Δ (electron energy) the photoelectric eqn. kinetic energy hf Φ K M pt’l energy (or “work function”) = K M /e There are many energy levels, Φ I , from which e come spectroscopy of solids.
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Demo - Photoelectric effect Applet: knight photoel effect
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Why do warm things glow? The classical mechanism is that as temperature increases, motion increases and the atomic charges accelerate more frequently and with greater amplitude. In detail, the classical theory fails because it predicts that all the EM waves that can exist in a cavity are excited. In fact, the high frequency waves are missing. Max Planck (1900) found a formula that fit.
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Review of Kinetic Theory of Heat The ideal gas pV = nRT. n = number of moles, R = molar gas constant = 8.3 J/mole K (K is degrees Kelvin, N A = Avogadro’s number, M = mass of 1 mole of gas = m N A , m = mass of one molecule) Pressure, p, of a gas arises from collisions of molecules against the walls; can prove that pV = 2 (total kinetic energy)/3 = nRT.
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8B - 18 Particles-waves_11.ppt - Modern Physics Major...

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