Midterm 2009 Solutions

Name:___________________________Student no: ___________________Group___________

CC2404 Midterm test (Duration: 60 minutes) Instructions: Write down ALL the answers on this test paper. Part I - Multiple Choices Questions (30%) 1. Which of the following is NOT a base unit of Système International (SI) units? a. ampere b. mole c. joule d. kelvin e. kilogram

c

2. When a rock falls freely, which of the following quantities is constant? (Assume no friction in air). a. falling distance b. speed c. potential energy d. mechanical energy d e. kinetic energy 3. The freezing point and boiling point of water are 0 °C and 100 °C respectively. What are the corresponding readings in Fahrenheit? a. b. c. d. e.

23 °F and 221 °F 23 °F and 212 °F 32 °F and 221 °F 32 °F and 212 °F none of the above

d

4. Which of the following thermometers is generally NOT used for core temperature measurement? a. Liquid crystal thermometer b. Mercury thermometer c. Thermistor d. Infrared thermometer a e. None of the above 5. Which of the following statements about rod cells and cone cells on the retina is INCORRECT? a. b. c. d. e.

Rods are not used for differentiating between different colors. Cones have a higher sensitivity in the high light intensity conditions. There are no rods on the blind spot. There are three types of rods. None of the above. 1

d

Name:___________________________Student no: ___________________Group___________ 6. Which of the following statements about the accommodation of the eye is correct? a. b. c. d. e.

The eye is said to be accommodated when focusing on an object at the far point. When an eye is unaccommodated, it has to reduce the power of the eye lens. When an eye is accommodated, there is a change in the shape of the cornea. The iris is responsible for the change in shape of the lens during accommodation. None of the above.

b

7 What is the first overtone frequency of an air tube closed at one end with a fundamental resonant frequency of 128 Hz? a. b. c. d. e.

128 Hz 256 Hz 384 Hz 512 Hz 640 Hz

c

8. The frequency response of the ear is described in the followings. Which is NOT true? a. b. c. d. e.

The frequency response of the ear is based on resonance. When the intensity of the sound increases, the brain perceives this as an increase in pitch. Normal human hearing is about 20 Hz to 20 kHz. Sound of frequencies less than 20 Hz is infrasound. None of the above.

b

9. In ultrasound imaging, which of the following quantities has a significant dependence on ultrasound wavelength in soft tissue? a. b. c. d. e.

Acoustic impedance Resolution Density Speed of sound Thickness of the tissue

b

10. Which of the following physical phenomena is the basis for image transmission in optical fibers? a. b. c. d. e.

Reflection Refraction Total internal reflection Total internal refraction None of the above.

c

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Name:___________________________Student no: ___________________Group___________ Part II - Short Questions (50%) 1. A marathon runner is extremely efficiency. (a)

During the day of race he has 7500 kcal food input and 8.00 MJ useful energy output.

Calculate his efficiency. (1 kcal = 4.186 kJ)

(4

marks) Input energy = 7500*4.186=31395 kJ=31.4 MJ Efficiency = (useful energy)/(input energy)=8MJ/31.4 MJ = 25% (b)

There is an unbalance between input and output energy. Assuming there is no fat stored

in the body. Suggest in what form the missing energy might have changed to.

(2

mark) Heat or thermal energy (c)

The runner should drink a lot of water. Give a reason for this suggestion.

(2

marks) Water becomes sweat and evaporates; this extracts heat and helps to cool body temperature. 2. (a) Name three methods by which the thermal energy can be transferred. Which of these methods can carry thermal energy through a vacuum.

(2 marks)

conduction, convection and radiation. only radiation will carry thermal energy through a vacuum. (b) Give TWO examples of physical properties that varies with temperature and describe how they are used to measure temperature.

(4 marks)

volume – the volume of certain liquids or gases expand when the temperature increases and contract when the temperature decreases. resistance – the resistance of most of the metals increases when the temperature increases and vice verse. (c) State one advantage and one disadvantage for each of them. low cost and ease small size, ease of use, of use and fast response time Infection and

cross-infection; 3

(4 marks)

Name:___________________________Student no: ___________________Group___________ crossmucosal damage contamination

3. An eye has a near point of 15 cm and a far point of 50 cm. The distance of the retina from the front surface of the cornea is 1.9 cm. For all calculations assume that the eye acts as a single thin lens situated at the front surface of the cornea. (a) Calculate the power of the eye when focusing on an object at its far point. P=

1 1 1 1 1 = + = + = 54 .63 D f u v 0.5 0.019

(b) Calculate the power this eye should have when focusing on an object at infinity. P=

(2 marks)

(2 marks)

1 1 1 1 1 = + = + = 52 .63 D f u v ∞ 0.019

(c) Calculate the power of the corrective lens needed by this eye to focus clearly on an object at infinity. Write down the type of eye defect the man has. (2 marks) 52.63D – 54.63D = -2.0 D Short sight (myopia)

(d) Calculate the new near point of the eye when the corrective lens is used. 1 1 1 1 1 P= = + = + = 59 .30 D f u v 0.15 0.019

59 .30 D − 2.0 D =

1 1 1 1 + = + ; u ' = 0.21m u ' v u ' 0.019

4

(4 marks)

Name:___________________________Student no: ___________________Group___________

4. (a) The graph on the right shows the equal loudness contours for a person with normal hearing. Estimate the minimum sound level can be detected by a person, at a frequency of 3 kHz. (2 marks) From 0 phon curve, the sound level at a frequency of 3 kHz for a healthy hearing person is -8 dB.

(b) The threshold of hearing at a frequency of 3 kHz for a person with defective hearing is 3.2× 10-10 Wm-2. Calculate the difference of sound level between this person and a normal one. (5 marks) \ The sound level (intensity level) at a frequency of 3 kHz for a person with defective hearing threshold of 3.2 x 10-10 W/m2, is 10 log

I 3.2 ×10 −10 = 10 log = 25 dB Io 1.0 ×10 −12

Loss = 25 dB – (-8 dB) = 33 dB

(c) What is the sound level for a sound of 8 kHz with loudness of 80 Phons?

(2 marks)

80 phon = 90 dB at 8 kHz.

(d). What is the sound intensity in W/m2 at 100 Hz for a sound with loudness indicated by curve P? (3 marks) Curve P is for 40 phon. Which correspond to about 62 dB at 100 Hz This gives sound intensity of 1.6 x 10-6 W/m2 (anything between 1 to 3 W/m2 acceptable) 5

Name:___________________________Student no: ___________________Group___________

5. (a) Write down ONE medical applications of ultrasound for therapy and TWO applications for diagnostics . (6 marks) Therapy: pulverization of tumor; diathermy (deep heat treatment) Diagnostics: Doppler effect for blood flow rate measurement; ultrasound imaging (A scan or B scan counted as ONE application); thickness measurement.

(b). An air bubble is inside the plate. An ultrasound pulse is sent from the top surface of a plate. Two reflected pulses are detected, due to reflections from the two sides of the bubble. A time difference of 15× 10-6 seconds between the two pulses is recorded. Calculate the thickness of the bubble. Speed of sound is 5000 ms-1 in the plate and 340 ms-1 in air. (4 marks)

Ultrasound tranducer Plate

If d m is the thickness of plate, 2d = v; t

d=

vt 340 ×15 ×10 −6 = = 2.55 mm 2 2

6

Air

Name:___________________________Student no: ___________________Group___________ Part III Long question (20%) 1. Assume that a person’s eardrum has a diameter of 1 cm and that the maximum power the ear can accept without damage is 10-5W. (a)

What is the minimum safe distance between the ear and a 25 W speaker? Assume the

speaker emit sound equally in all directions.

(8

marks) Maximum intensity allowed I max = Since I max =

(b)

Pspeaker A

=

Pspeaker 4πR

2 min

p max 10 −5 10 −5 = = = 0.127 W/m2 2 area πr π ⋅ (0.5 ⋅10 −2 ) 2 Pspeaker

, Rmin =

4πI max

=

25 =3.95 m 4 ⋅ π ⋅ 0.127

The distance between the ear and the speaker is now twice the minimum safety distance

in (a). Calculate the sound level in dB experienced by the person. (5 marks) Imax=0.127 W/m2, I=

Pspeaker A

=

Pspeaker 4πR

2

=

Pspeaker 4 * 4πR

2 min

=

1 I max = 0.03175W / m 2 4

Corresponding sound level β =10log

(c)

0.03175 = 105 dB 10 -12

By keeping the same ear –speaker distance as in (b), what amount of energy in joules

falls on the same eardrum for continuous listening of 8 hours?

(4

marks) E=Pt=IAt= 0.03175 ⋅ π ⋅ (0.005 ) 2 ⋅ 8 ⋅ 3600 =0.0718 J (d)

State THREE possible effects of noise.

(3

marks) tinnitus. Hearing loss, feelings of annoyance inability to think clearly dizziness or sickness (> 125 dB) pain in the ears (> 130 dB) permanent deafness (~ 190 dB, short-term exposure) 7

(ANY THREE)

Name:___________________________Student no: ___________________Group___________ - End of exam paper -

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Name:___________________________Student no: ___________________Group___________ Physical constants and equations Physical constants g c

acceleration due to gravity speed of light in vacuum

10 m/s2 3.00 × 108 m/s

Equations

∆f =

v = fλ

2 fv cos θ vw

Intensity =

V=331√(T/273)m/s P=

1 1 1 = + f u v

Power Area

β = 10 log 10

nvw , n = 1,3,5 (open - closed tube) 4L nv f n = w , n = 1,2,3 (open - open tube) 2L fn =

- End of exam paper -

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