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TEST1_ENSC3313_SP2005_KEY

# TEST1_ENSC3313_SP2005_KEY - ENSC 3313 Spring 2005 TEST 1...

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1 ENSC 3313, Spring 2005 TEST 1 - KEY Name_________________________ Answer all 10 questions, show all work, 2points each 1. Each color pixel in a video image of a standard television is renewed 30 times/s. The pixel light intensity decays exponentially with time after being renewed. Calculate the decay constant for the phosphor pixel if 80% or less of the intensity is to remain at the next renewal. Answer: Exponential decay problem τ = t 0 e I I τ = t I I ln 0 solve for τ : = τ 0 I I ln t We need the intensity to decay to 80%, so I/I 0 =0.80 and this must occur by t =1/30 s. Plug in numbers: s 15 . 0 ) 80 . 0 ln( 30 / 1 = = τ 2. A silicon single crystal is grown such that, when sliced into wafers, the (001) plane of atoms is exposed. Calculate the number of silicon atoms per cm 2 on the (001) plane. Note: Si has diamond cubic crystal structure (illustrated) of side length a=0.543 nm, atomic radius = 0.118 nm, and N A = 6.023 × 10 23 mol -1 . Answer: The (001) plane is the top face of the unit cell. There are two atoms per unit area on this plane, hence the planar density of atoms is: 2 14 2 7 cm atoms Si 10 8 . 6 nm cm 10 nm 543 . 0 2 . D . P × = × = x y z

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