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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).
 Spring '07
 GIAMBATTISTA,A
 Energy, Power

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