p250 Fa23 Exam 1 vA

First Name: UMID: Last Name: Physics/BioPhysics 250 Fall 2023 - Exam #1 Form 1 READ THIS BEFORE YOU BEGIN • Print your name and UMID in the boxes provided above (please write neatly so the scanner can pick it up). • Fill in your name , UMID number , and form number (under “key”) on your scantron sheet • This is a one hour and 50 minute, closed book exam. • You may use a calculator; please do not share calculators. • The exam includes 20 multiple choice questions which will be machine graded. Each question is worth 5 points. No partial credit will be given. • You may use one 8.5×11” sheet of paper with hand-written notes written on both sides. • If you are stuck on a problem, move on and come back to it. Constants you might need 𝑘𝑘 = 8.99 × 10 9 N m 2 /C 2 𝜀𝜀 0 = 8.85 × 10 −12 C 2 /N m 2 𝑔𝑔 = 9.8 m/s 2 Speed of sound in air at 20C = 340m/s Density of air at 20 C = 1.20 kg/m 3 Magnitude of the electron charge: 𝑒𝑒 = 1.6 × 10 −19 C Mass of the proton: 𝑚𝑚 𝑝𝑝 = 1.67 × 10 −27 kg Mass of the electron: 𝑚𝑚 𝑒𝑒 = 9.11 × 10 −31 kg 1 electron volt (1eV) = 1.6 × 10 −19 J Avogadro’s Number: 𝑁𝑁 𝐴𝐴 = 6.02 × 10 23 /mol Threshold of hearing 𝐼𝐼 𝑜𝑜 = 10 −12 W/m 2 milli(m) = 10 −3 , micro( 𝜇𝜇 ) = 10 −6 , nano (n) = 10 −9 pico (p) = 10 −12 , femto(f) = 10 −15 kilo (k) = 10 3 , mega (M) = 10 6 , giga (G) = 10 9

Passage for questions 1-3: The Stethoscope A stethoscope is a medical device for listening to sounds inside the human body. It commonly consists of a chest piece that is connected to PVC tubing, which then splits into two shorter metal tubes that direct the sound to the ears of the physician. The chest piece often has two sides: an open “bell” to better hear low-frequency sounds, and a similarly shaped hollow cavity that is covered with a flexible membrane, the diaphragm, for higher-pitched sounds. Use 𝑣𝑣 = 340 𝑚𝑚 𝑠𝑠 ⁄ for the speed of sound and 𝜌𝜌 = 1.2 𝑘𝑘𝑔𝑔 𝑚𝑚 3 ⁄ for the density of air for the problems below. 1. If the bell has a diameter of 25 mm on the open side that is pointing towards the patient, and the two eartips have circular openings with a diameter of 2.5 mm, by how much has the total sound intensity that reaches the physician’s ears increased, compared to what entered the bell? Assume that there are no losses in the tubing. A. It did not increase. B. By a factor of 5 C. By a factor of 10 D. By a factor of 50 E. By a factor of 1000

4. Four traveling waves are described by the following equations, where all quantities are measured in SI units and y represents the displacement. I. 𝑦𝑦 = 0.12 cos( − 1.5 𝑥𝑥 + 2 𝑡𝑡 ) II. 𝑦𝑦 = − 0.12 cos(2 𝑥𝑥 + 1.5 𝑡𝑡 ) III. 𝑦𝑦 = − 0.02 cos(3 𝑥𝑥 − 4 𝑡𝑡 ) IV. 𝑦𝑦 = 0.02 cos( − 2 𝑥𝑥 + 2.5 𝑡𝑡 ) Which of these waves have the same speed? A. I and II B. I and III C. I and IV D. II and III E. III and IV 5. A phon is an experimentally derived unit of human-perceived loudness. A person perceives sound at 800 Hz to be 40 phons, which corresponds to an intensity level of 40dB (from the graph). The person hears a different sound at 100 Hz to also be 40 phons, which corresponds to 60 dB. Approximately what is the ratio of the intensity of the 800 Hz sound to the 100 Hz sound, i.e. what is 𝐼𝐼 800𝐻𝐻𝐻𝐻 / 𝐼𝐼 100𝐻𝐻𝐻𝐻 ? A. 100 B. 0.050 C. 0.01 D. 20 E. 0.10 Intensity level (dB)

6. Two speakers are set up and connected to the same 1000 Hz signal source. When you stand at a particular location between the two speakers you find that the two speakers individually produce a wave with amplitude 𝐴𝐴 . When both speakers are powered, you expect: A. An amplitude which goes from 0 to 𝐴𝐴 , as the phase difference of the sound coming from the two speakers changes from 0 to 2 𝜋𝜋 radians. B. An amplitude of 𝐴𝐴 when the phase difference of the sound coming from the two speakers at your position is 0. C. An amplitude of 4 𝐴𝐴 when the phase difference of the sound coming from the two speakers at your position is 0 radians. D. An amplitude which goes from 0 to 2 𝐴𝐴 as the phase difference of the sound coming from the two speakers changes from 𝜋𝜋 to 2 𝜋𝜋 radians. E. The phase difference of the sound coming from the two speakers at your position is always zero. 7. A guitar string plays a “G” note with a frequency of 196Hz when it is tuned to a tension of 70.0 N. A guitarist wishes to “bend” the note to a G# with a frequency of 207.65Hz. This is done by changing the tension in the string without appreciably changing its length. What string tension is required to play a G#? A. 72.1 N B. 68.0 N C. 66.1 N D. 74.2 N E. 78.6 N

9. An oscillator creates periodic waves on a stretched sting. The string is long enough that you don’t have to consider reflections from the end. If you halve the frequency of the oscillator, but otherwise don’t change any string properties how will the wave’s period, wavelength, and wave speed change? A. The period is halved, the wavelength is halved, and the wave speed does not change. B. The period is halved, the wavelength is doubled, and the wave speed does not change. C. The period is halved, the wavelength is halved, and the wave speed increases. D. The period is doubled, the wavelength does not change, and the wave speed doubles. E. The period is doubled, the wavelength doubled, and the wave speed does not change. 10. A duck is travelling with a speed of 0.90 m/s toward its companion, which is sitting still in the water. The moving duck paddles its feet 2.0 times per second. The paddling of the feet creates waves in the water which travel at a speed of 2.0 m/s. As the waves pass the stationary duck, it oscillates up and down. How many times per second does the stationary duck oscillate? A. 3.6 Hz B. 5.0 Hz C. 8.0 Hz D. 2.0 Hz E. 1.6 Hz

11. You produce a note by blowing over the hole of an open bottle. When you a blow a little harder you hear the same note but louder. Which of the following is the most likely explanation? A. As you blow harder the frequency increases, producing greater intensity for the same displacement amplitude. B. The speed with which you blow over the hole is proportional to the speed of sound. Increasing the sound speed increases the loudness. C. As you blow harder, the loudness increases through the Doppler Effect. D. As you blow harder, the amplitude of the standing waves in the bottle increase, producing greater amplitude sound waves. E. As you blow harder, sound produced by the bottle arrives in-phase at the location of the listener, producing constructive interference. 12. Sound passing though sea water is attenuated according to Beer’s Law: 𝐼𝐼 = 𝐼𝐼 0 𝑒𝑒 −𝛽𝛽𝛽𝛽 with 𝛽𝛽 the attenuation coefficient 𝐼𝐼 the intensity, 𝐼𝐼 0 the intensity at the surface, and 𝑥𝑥 the depth. The graph shows the relative intensity ( 𝐼𝐼 / 𝐼𝐼 0 ) of the sound as a function of depth in sea water. Approximately what is the attenuation coefficient for this frequency sound in sea water? A. 0.0050 m -1 B. 0.10 m -1 C. 50 m -1 D. 170 m -1 E. 0.02 m -1 1.0 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 Relative intensity 0 10 20 30 40 50 Depth (m)

15. Two “source” charges, Q1 and Q2 , of opposite sign, sit apart on an axis. Charge Q2 has greater magnitude than charge Q1 . Now a “test” charge q can be put in any location within any of the three regions, a, b, and c, on the axis, with no initial velocity before releasing. In which of the regions will there be a point at which the charge can experience a net force of zero? A. Only region a. B. Only region b. C. Only region c. D. All three regions. E. Either region b or c. 16. The figure shows three “source” point charges labeled Q 1 , Q 2 , and Q 3 . Electric field vectors are also plotted in the region surrounding the charges. What are the signs of the three charges? A. All three charges are positive. B. All three charges are negative. C. Q 1 is positive, Q 2 is negative, Q 3 is positive. D. Q 1 is negative, Q 2 is positive, Q 3 is negative. E. Q 1 is positive, Q 2 is positive, Q 3 is negative.

17. Two identical small conducting spheres are separated by 0.5 m. The spheres carry different amounts of charge, and each sphere experiences an attractive electric force of 12.8 N. The total charge on the two spheres is - 20 μC. The two spheres are now brought to touch before being separated again by 0.5 m. The electric force on each sphere is closest to which of the following? A. Zero. B. Repulsive force of 6.4 N. C. Attractive force of 6.4 N. D. Repulsive force of 3.6 N. E. Attractive force of 3.6 N. 18. A proton is placed in an electric field of intensity 1 kN/C. What are the magnitude and direction of the acceleration of this proton due to this field? Given: m proton = 1.67 × 10 -27 kg, e = 1.60 × 10 -19 C. A. 9.58 × 10 9 m/s 2 opposite to the electric field. B. 9.58 × 10 9 m/s 2 in the direction of the electric field. C. 95.8 × 10 10 m/s 2 in the direction of the electric field. D. 9.58 × 10 10 m/s 2 opposite to the electric field. E. 9.58 × 10 10 m/s 2 in the direction of the electric field.