Maximum score 16*6.25=100 points Average 50.8% Correct answers:

Transcription

1 Midterm Exam 2 Maximum score 16*6.25=100 points Average 50.8% Correct answers: 40% 47% 40 16% 49% Count score 54% 37% 70% 60% 22% 78% 78% 65% 40% 45% 33% 48%

2 This problem only 16% of students solved correctly: W F x 1N 1m 0.5N 1m 1N 1m 0. 5J x

3 Doppler Effect What does it sound like when a plane buzzes your house? With a moving source of sound coming at you, the wave fronts are compressed. Behind, the wave fronts spread out. This change in the frequency due to relative velocity is the Doppler Effect. 12/1/2010 3

4 Doppler Effect Green car honks horn 4

5 Doppler Effect Green car honks horn 5

6 Doppler Effect Green car honks horn 6

7 Doppler Effect Green car honks horn 7

8 Sonic Boom An interesting thing happens when the object moves at a velocity equal to the velocity of sound. Wave fronts pile up on each other creating tremendous pressure. 8

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10 Doppler Effect Summary When source is coming toward you (v s > 0) Distance between peaks decreases Frequency increases When source is going away from you (v s < 0) Distance between peaks increases Frequency decreases 10

11 Lecture 24 Thermodynamics, Heat and temperature Heat and Temp

12 Temperature: a measure of the average kinetic energy of an object.

13 Three most commonly used temperature scales: 1. Celsius scale 2. Fahrenheit scale 3. Kelvin scale Temperature scales Water freezes at: Water boils at: Celsius: 0 ºC 100 º C Fahrenheit: 32 º F 212 º F Kelvin: K K Most commonly used temperature scale in the world is Celsius scale.

14 Temperature conversion Following conversion Celsius Fahrenheit Fahrenheit Celsius Celsius Kelvin 9 T F T 5 5 T C T 9 C F T T Why ? K C It is believed that 0 K ( ºC) is the absolute coldest that anything can get. 0 K represents absolute zero.

15 Temperature (K) Hydrogen Bomb Interior of the Sun Surface of the Sun Copper melts Water freezes Liquid Helium

16 Kelvin

17 Quiz How would you dress if the temperature was 70 o C? A. Doesn t matter I m dead from heat exposure. B. Shorts, T-shirt, and flip-flops. C. Pants, jackets, and shoes. D. Winter coat, knitted hat, and snow pants. E. Doesn t matter I m dead from frost bite.

18 Quiz How would you dress if the temperature was 40 o C? A. Doesn t matter I m dead from heat exposure. B. Shorts, T-shirt, and flip-flops. C. Pants, jackets, and shoes. D. Winter coat, knitted hat, and snow pants. E. Doesn t matter I m dead from frost bite.

19 Quiz How would you dress if the temperature was 100 K? A. Doesn t matter I m dead from heat exposure. B. Shorts, T-shirt, and flip-flops. C. Pants, jackets, and shoes. D. Winter coat, knitted hat, and snow pants. E. Doesn t matter I m dead from frost bite.

20 Thermal Expansion *Consequence of change in average separation between costituent atoms in an object L T L o

21 Coefficient of Thermal Expansion

22 Example A steel railroad track has a length of m when the temperature is 0.0 o C. What is the length when the temperature is 40.0 o C? L L o T 12x10 6 ( 1 K ) *(30.000m)*(40.0K) L 0.014m L new L o L

23 DEMO 3A-03 Bimetallic Strip

24 Example The ends of the rail are rigidly clamped at 0.0 o Cso expansion is prevented. What is the thermal stress set-up in the rails if the temperature increases to 40.0 o C? F A Y L N x10 10 L o m N 9.33x107 m 2 If the cross-sectional area of the rail is 30.0 cm 2 what is the force? F A * F A 9.33x107 N m 2 m 2 *(30cm2 * 10 4 cm ) 2 2.8x105 N

25 Internal Energy The internal energy of a system is the total energy of all of the molecules in the system except for the macroscopic kinetic energy and external potential energy

26 Internal Energy The average translational energy of a single atom depends only on the gas temperature & is given by: K tr 3 2 kt This translational energy is part of what we call internal energy

27 Zeroth Law of Thermodynamics: *Two systems individually in thermal equilibrium with a third system are in thermal equilibrium with each other. Implications: *Temperature is an indicator of thermal equilibrium. *All parts of a system must be in thermal equilibrium if the system is defined by a single temperature.

28 The wooden block and the metal block have been sitting in this room at temperature T room for quite a while. What are the temperatures of the wooden and the metal block? A. Wood > T room, T metal < T room B. T wood = T room, T metal < T room C. T wood = T room, T metal = T room D. T wood = T room, T metal > T room E. None of the above.

29 A common misconception objects that feel cool like metals and smooth stones are usually not cooler than objects (like wood) that do not feel cool. Q T body 37 C T bar 25 C The sensation of coolness and hotness is actually the rate of heat transfer not the temperature.

30 Energy transferred by heat W out W out Q in Heat, Q is energy transferred between two objects due to a difference in temperature, T.

31 Quiz Which of these are reasonable units for heat, Q? A. J B. N.m C. kg.m 2 /s 2 D. All of the above and none of the below. E. None of the above. Heat, Q is energy transferred between two objects due to a difference in temperature, T.

32 Units of Heat 1 cal = amount of heat needed to raise 1 gram of water by 1 o C 1 kcal = Joules

33 Example A student eats a 2000 Calories dinner and wishes to do the equivalent work in the gym by lifting a 50 kg barbell. How many times must he raise it to expend this much energy? Assume he raises it 2.00 m each time he lifts it & regains no energy when he drops it to the floor. Convert calories in dinner to Joules: 2x10 6 cal 2x10 6 cal 4.186J 8.37x10 6 J 1cal Equate work done against gravity with energy W lifting nmgh n W mgh J 50kg *9.81 m s 2 *2.00m 8534times If lift every 5 seconds: 8543times * 5s time * 1hr 3600s 11.86hr

34 Energy transferred by heat U Q in U is called the internal energy.

35 Energy transferred by heat could go into work or internal energy W out U Q in W out U Q in

36 The First Law of Thermodynamics is just Conservation of Energy Any change in = Energy added or Energy taken away E U Q W in out Involves changes In center of mass Involves internal variables, like T Heat Involves T Work Involves F

37 Since Q and W are the energy transferred into a system from the surroundings, it is important to carefully define the system and the surrounds. It is often times useful to actually draw a box and make sure that Q and W are crossing the boundary. Wout Wout E U Q in W out E U Q in W out