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Unformatted text preview: ECE344, Midterm Exam 1: Solutions October 19, 2009 1. Problem: The workfunction of a material is defined as the energy required to remove an electron from the material and is usually indicated by the Greek letter (pron: khi). The workfunction of gold is Au = 4 . 9 eV, the workfunction of cesium is Cs = 1 . 9 eV. a. Calculate the maximum wavelength (in nm) of light required to remove an electron from Au via the photoelectric effect. b. Do the same for Cs. Solution: Since = c = 2 hc h = 2 h E , (1) or, expressing the energy in eV: = 2 h eE , (2) using E = Au we have = 253 nm and using E = Cs we get = 653 nm. 2. Problem: Electrons in a swarm at thermal equilibrium have each an average kinetic energy of (3 / 2) k B T . a. Calculate the value of (3 / 2) k B T in eV. b. Calculate the velocity (in cm/s) of an electron having a kinetic energy equal to the average energy (3 / 2) k B T . This is the so-called thermal velocity. c. Calculate its average de Broglie wavevelength in cm. This is called the thermal wavelength....
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