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Unformatted text preview: Platt, David – Homework 28 – Due: Nov 14 2005, 4:00 am – Inst: Ken Shih 1 This printout should have 8 questions. Multiplechoice questions may continue on the next column or page – find all choices before answering. The due time is Central time. 001 (part 1 of 1) 10 points In 1965 Penzias and Wilson discovered the cosmic microwave radiation left over from the Big Bang expansion of the universe. The energy density of this radiation is 9 . 44 × 10 14 J / m 3 . The speed of light 2 . 99792 × 10 8 m / s and the permeability of free space is 4 π × 10 7 N / A 2 . Determine the corresponding electric field amplitude. Correct answer: 0 . 146025 V / m. Explanation: Let : μ o = 4 π × 10 7 N / A 2 , c = 2 . 99792 × 10 8 m / s , and u = 9 . 44 × 10 14 J / m 3 . The average poynting vector is given by h S i = E 2 2 μ o c . (1) From this expression we can solve for E in terms of the energy density u knowing that u = h S i c and from this we get the electric field as E = p 2 μ o c 2 u (2) = q 2(4 π × 10 7 N / A 2 ) × q (2 . 99792 × 10 8 m / s) 2 × q (9 . 44 × 10 14 J / m 3 ) = . 146025 V / m ....
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This note was uploaded on 07/26/2009 for the course PHY 303L taught by Professor Turner during the Fall '08 term at University of Texas.
 Fall '08
 Turner
 Physics, Work

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