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Unformatted text preview: 304. [(94.14 Kw“; Your Name.
Student ID#. 1Ba Physics for Scientists and Engineers: Oscillations, Waves, Electric
ity and Magnetism Midterm #2. Monday 12 May. 2008 Instructor: Steve Cowley Please do not write in this region —
—
— Open book, open notes — no talking. Please write your answers in the space provided below. Partial credit is
given for answers that are on the right track so show your working. You may use rough paper but please put the‘
working on these sheets. Answers need not be more accurate than two signiﬁcant ﬁgures. If you don’t have much
time you will get almost all of the credit for writing down the right expression (with the right numbers inserted) and leaving the amour}.
No calculators or cell phones are to be used in the exam. Question 1. Listening to Moving Music. 20 points
Sound waves have a. velocity v = 340m/s and you may use the doppler formula derived in class (or the book). r _ Vile is l L V'ilfi" (a) You are standing (stationary) beside the road and a car comes towards you and passes you at velocity
34m/s. In the car the radio is broadcasting an electric guitarist playing a note with frequency 990 3‘1. Calculate the frequencies (of the guitar note) you hear before and after the car passes. 4 points
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I L9 We :IJHM :7 (b) Suppose instead you were driving your motorbike in the opposite direction to the car also at speed 34m/s.
What would be the frequencies you hear in this case (before and after the car passes)? 4 points Towoda 1 3
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{E ‘ 104 (c) You take out your electric guitar (while riding the motorbike!) and play the note (frequency) you hear and ‘5 _ send it out of your motorbike’s speakers! What is the frequency of the sound from these speakers heard by the H " " * people in the car traveling towards you? 4 points in," ’ L x ‘ V‘i :0 ii .‘i i V, (L: #3!" : q [1:o  {1.10 + ‘LCH‘IQJ fluff) Mn  ‘ ‘ V “I '0 1“ I‘ (d) Harder Suppose you are actually moving towards the car at an unknown speed and the people in the
car want to determine your speed from a measurement of: the frequency they hear from your speakers (fL), the
frequency coming from the radio (f0) and their speed (can). Calculate a formula for the speed of your motor bike in terms of these variables. 6 P015733 V fig“ {"0 in {‘0 l l “' ) ’__ : v .. vfh
v.06] “‘3 (fawn! urxf‘ K r’ ﬂ. (e) In principle using sound (sonar) to determine the speed of a car is more accurate than radar (radio waves)
— why? Why then do we use radar? We are looking for thoughtful answers. 2 points .L W W “Mil (bahMIL I'de“\5‘n 4 :lré‘cr‘! #2:.” 31¢qu :23“? 1/
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Question 2. Charged Slab. 20 points VSdMJ 1.; 5:? Consider a plane slab of charge that occupies the volume —a < z < a (for all :r and y). The charge density p0
in this region is a positive constant. I
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(:1) Calculate the total charge per unit area (in the x—y plane) between z and z whetizl< a and when a (/z/ '
3 points
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(b) From symmetry arguments write down the form of the electric ﬁeld. is. it’s direction and how it depends
on the coordinates. 3 points I 4 \
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(c) Find E for z > a. — nee Gauss‘s law and carefully explain the surface you use. 4 points
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P [a ((1) Find E for z < 0... Plot E as a, functiogﬁof z. 4 points
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(e), Harder A stationary particle w1th negaitive c arge, —Q, and mass m is placed at z = (1/2. Calculate the subsequent motion. “That would the motion be if we started the negative charge at z = 2a? 6 points 20: 0/1 thgf :‘g Z' #6. 41!; .135 GM a
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