Assignment_7_09 - TA's Name_Section Your Name Physics 2214 Assignment 7 Concepts Energy and Power in Waves Maxwell's equations in Differential Form

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TA's Name:_____________________________________Section: ____ Your Name: ____________________________________ Physics 2214 Assignment 7 Concepts: Energy and Power in Waves Electromagnetic Waves Maxwell's equations in Differential Form Plane Waves Relations between E, B and v Reading: Y&F, Vol. 1, Chapters 15 and 16; Vol. 2, Chapter 32 Assignment: Due in lecture on Thursday, March 12. Please turn in this sheet stapled to the top of your work. Physics Problems: 1. Energy and Power in Sound Waves. Let's focus on harmonic waves. (a) For fixed average intensity, how do the ampitude of (i) the molecular displacements s m and (ii) pressure p m vary with the frequency of a sound wave? (b) Two 1 kHz sound waves, one propagating in water (B=2.2 x 10 9 Pa, v=1480 m/s) and one in air (B=1.4 x 10 5 Pa, v=343 m/s) have equal average intensities. What is the ratio of the (i) the molecular displacements s m and (ii) the pressure modulations p m for these two waves? Does your answer make sense? (Explain briefly). (c) The perceived loudness of a sound wave is roughly proportional to the sound level, given by β (in dB)=10 log 10 (I/I 0 ), where I 0 =10 -12 W/m 2 corresponds to the threshold of hearing. (Note that β is dimensionless, but we express it in fake "units" of decibels (dB); this just means that we've calculated 10 log 10 (I/I 0 ). ) Suppose that you have a 1 W point source of sound waves, and that there are no floors, walls or other objects nearby to reflect the sound. (i) If you double your distance r from the source, what happens to the sound level, and thus to the loudness you hear? Does your initial distance from the source affect the change in loudness?
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This note was uploaded on 04/01/2009 for the course PHYS 2214 taught by Professor Giambattista,a during the Spring '07 term at Cornell University (Engineering School).

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Assignment_7_09 - TA's Name_Section Your Name Physics 2214 Assignment 7 Concepts Energy and Power in Waves Maxwell's equations in Differential Form

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