photoelectric lab - Abstract: Plancks constant is the...

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Abstract: Planck’s constant is the relation between a frequency of light, and its energy. This experiment seeks to measure Planck’s constant from the principles of the photoelectric effect. This is achieved by determining the energy of electrons, which have been imparted with energy from photons of different frequencies. We will also explore the photoelectric effect by varying the intensity of the incident light. We predicted a value of 2.841 ± .254 eVs for Planck’s constant, and inconclusively proved the lack of relation between incident intensity and stopping potential. We were unable to discern a relationship between incident intensity and photocurrent, even though theory predicts it. Motivation and Theory: Energy in light propagates in discrete packets called photons. Their energy can be related to the frequency by this expression: Incident photons on a conductor are absorbed and their energy is transferred to electrons. If the energy imparted is above a specific threshold, called the work function, the electron will be emitted from the metal, containing all the energy of the photon, minus the work function energy. With a constant stream of incident photons on metal, a photocurrent of photoelectrons can be created. The electrons will have a range of kinetic energies because some will have collided with atoms on their way out of the metal. The most energetic of these photoelectrons will contain all of the energy, minus the work function, imparted by the photon. This is only true for the most energetic electrons because they did not lose energy to collisions. The relation, with K as kinetic energy, h as Planck’s constant, ν as the incident light frequency, and Φ as the work function, is: The formula only gives the energy of the most energetic electrons. We can determine K max by applying a retarding voltage across the anode and cathode. When the retarding voltage is high enough, it will stop the most energetic electrons, and the current will go to zero. Therefore, the retarding voltage, at the point where the current cancels out, gives us a way to find the exact energy needed to stop the electrons, and therefore, the amount of energy they possess. Once we have this relation for multiple frequencies, we can illustrate the Planck constant with the slope of K max versus frequency, according to the above relation. Another factor to consider is the effect of intensity. As we vary intensity with polarizers, the photocurrent should increase; however, the maximum kinetic energy should not depend on the current. The reason for this is that the energy imparted to the electrons depends only on the incident frequency of the individual photons and not on
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This document was uploaded on 10/31/2011 for the course PHYSICS 286 at UMass (Amherst).

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photoelectric lab - Abstract: Plancks constant is the...

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