HNRS 227 Lecture 5 and 6 Chapter 4 and Chapter 5. Heat and Temperature Wave Motion and Sound presented by Prof. Geller

Transcription

1 HNRS 227 Lecture 5 and 6 Chapter 4 and Chapter 5 Heat and Temperature Wave Motion and Sound presented by Prof. Geller

2 Recall from Chapters 1, 2, 3 Units of length, mass and time, and metric Prefixes Density and its units The Scientific Method Speed, velocity, acceleration Force Falling objects Newton s Laws of Motion Newton s Law of Universal Gravity Work Potential Energy and Kinetic Energy Conversion of Energy Types/Sources of Energy

3 Main Concepts for Chapters 4 and 5 Kinetic Molecular Theory Temperature Heat Phases of matter Thermodynamics Forces, Vibrations and Wave Motion Sound Waves and their characteristics Reflection, Refraction, Resonance

4 1st Law of Thermodynamics In an isolated system, the total amount of energy, including heat energy, is conserved. ENERGY IS CONSERVED

5 Temperature A relative term reflecting how vigorously atoms in a substance are moving and colliding Alternative definition the average kinetic energy of the molecules in a region

6 Temperature Units Fahrenheit Celsius Kelvin Where is absolute zero? At what temperature does water freeze? Boil?

7 Heat Heat is a form of energy the energy flows from a warmer object to a cooler object Units of heat calorie amount of temperature needed to raise temperature of 1 gram of water 1 degree Celsius kilocalorie (kcal or Calorie) amount of temperature needed to raise temperature of 1 kg of water 1 degree Celsius

8 Specific Heat Ability of a material to absorb heat energy proportional to mass proportional to change in temperature Q = m*c*δt c is the SPECIFIC HEAT of the substance amount of energy needed to increase temperature of 1 gram of substance 1 degree Celsius

9 Transfer of Heat Conduction movement of heat by collisions between vibrating atoms or molecules Convection transfer of heat by the physical motion of masses cooler liquids or gasses descend while warmer liquids or gasses rise

10 Phases and Phase Diagram (NOT IN TEXT but IMPORTANT)

11 2nd Law of Thermodynamics Two key components heat flows from a warmer body to a cooler body entropy increases remains constant or increases in time

12 Question for Thought Define temperature and heat. Temperature is a measure of the average kinetic energy of the molecules of a substance. Heat is the total internal energy of the molecules involved in an energy transfer.

13 Question for Thought Why do most materials become less dense as their temperature is increased? As the temperature of a solid increases, the vibrations of the individual molecules become larger. When these vibrations become larger, the average distance between the molecules increases to accommodate these larger oscillations, and the solid expands. In a liquid or a gas, the individual molecules move faster as the temperature increases, and the collisions between individual molecules become more violent. Since the molecules are moving faster, they move farther apart as they travel a larger distance in the time between collisions.

14 Question for Thought Would the tight packing of more insulation, such as glass wool, in an enclosed space increase or decrease the insulation value? Explain. Tight packing would tend to decrease the insulation value of glass wool because it would squeeze the wool together and give the heat more paths to travel. It is the presence of many small pockets of air, with unattached molecules, that gives glass wool and other similar insulation materials their insulating properties.

15 Question for Thought A true vacuum bottle has a double-walled, silvered bottle with the air removed from the space between the walls. Describe how this design keeps food hot or cold by dealing with conduction, convection and radiation. The vacuum between the walls prevents heat transfer by means of convection or conduction, while the silvered walls reflect radiated energy back into (or away from) the food, preventing energy transfer by radiation.

16 Question for Thought Why is cooler air found in low valleys on calm nights? Cooler air is denser than warmer air. This denser air weighs more per volume than the warmer air and pushes the warmer air out of the way as it sinks down to its lowest level. The warmer, less dense air sits on top of the cooler air because it weighs less per volume.

17 Question for Thought Why is air a good insulator? Air is not very dense and conduction is not very efficient at transferring energy because the molecules are much farther apart than they are in solids or liquids.

18 Question for Thought Explain the meaning of the mechanical equivalent of heat. Mechanical energy can be converted to heat as it changes from external mechanical energy to the internal kinetic energy of the molecules. A given quantity of mechanical energy always yields a known amount of heat.

19 Question for Thought What do people really mean when they say that a certain food has a lot of Calories? When people refer to the Calorie content of food, they are referring to the amount of chemical energy available from the food. One way to measure the chemical energy of foodstuff is to find out how much heat is released by complete oxidation. A Calorie (kcal) is a measure of the heat release and thus is a measure of the chemical energy released.

20 Question for Thought A piece of metal feels cooler than a piece of wood at the same temperature. Explain why. The metal is more efficient at conducting heat away from your hand than wood, so it feels cooler because your hand senses heat leaving your body.

21 The latent heats of fusion and vaporization go into or are released from internal energy during a phase transition. There is no temperature change associated with these heat transfers, so they are hidden. Question for Thought Explain how latent heat of fusion and latent heat of vaporization are hidden.

22 Question for Thought What is condensation? Explain on a molecular level, how the condensation of water vapor on a bathroom mirror warms the bathroom. Condensation occurs when more vapor molecules are returning to the liquid state than are leaving the liquid state. When a water vapor molecule joins a group of liquid water molecules, it has to give up its latent heat of vaporization. This heat is transferred to the surrounding air molecules such as the air in the bathroom.

23 Question for Thought Which provides more cooling for a styrofoam cooler, one with 10 pounds of ice at 0 degrees C or one with 10 pounds of ice water at 0 degrees C? The 10 pounds of ice provide more cooling because as the ice undergoes the phase change into water, it absorbs heat. Ten pounds of ice water simply absorbs heat according to the value of its specific heat until it reaches room temperature and therefore absorbs less heat.

24 Question for Thought Explain why a glass filled with a cold beverage seems to sweat. Would you expect more sweating inside a house during the summer or during the winter? Explain. Water condenses out of the air onto the cooler surface of a glass because the air near the glass is cooled, lowering its temperature to the dew point. Since the warmer air can hold more water vapor in the summer, it would have more water vapor to condense. Therefore, you would expect more condensation in the summer.

25 Question for Thought Why is a burn from steam at 100 degrees C more severe than a burn from 100 degrees C water? One hundred degree Celsius steam contains more energy (540 cal/g) than 100 C water, so the steam burn would be more severe.

26 Question for Thought Relative humidity typically increases after sunset. Explain how this is possible when no additional water vapor is added or removed from the air. Cooling of air reduces the capacity of air to hold water vapor. Relative humidity is a ratio of water in the air to how much water it can hold. Thus a decrease of capacity increases the relative humidity, even when the amount of water vapor in the air is constant.

27 Question A 1.0 kg metal head of a geology hammer strikes a solid rock with a velocity of 5.0 meters per second. Assuming that all of the energy is retained by the hammer head, how much will the temperature increase? (specific heat of hammer head is 0.11 kcal/kgc)

28 Answer 4. Q = ΔKE and ΔKE = 1 2 mv2 Q = 1 2 mv 2 Q = mcδt mcδt = 1 2 mv 2 ΔT = T = kg ( ) 5.0 m s ( 1.0 kg) 0.11 kcal = 0.03 C 2 ( 1.00 cal) kg C J ( ) 1.00 kcal 1, cal 1 2 mv cal J mc

29 Question Lead is a soft, dense metal with a specific heat of kcal/kgc, a melting point of o C and a heat of fusion of 5.5 kcal/kg. How much heat must be provided to melt a kg sample of lead with a temperature of 20.0 o C?

30 Answer 13. Q 1 = mcδt ( ) kcal = 250.0kg = 2,156 kcal kg C C ( ) Q 2 = ml f ( ) 5.5 kcal = kg = 1,375 kcal kg Q Total = Q 1 + Q 2 = ( 2,156 kcal)+ ( 1,375 kcal) = 3,531 kcal = 3,500 kcal

31 Forces and Vibrations Vibration back and forth motion Amplitude extent of displacement from the equilibrium position Cycle one complete vibration Period time required to complete one cycle Frequency number of cycles per second Relationship between period and frequency T = 1 / f f = 1 / T

32 Waves Longitudinal disturbance that causes particles to move closer together or farther apart IN THE SAME DIRECTION the wave is moving Transverse disturbance that causes motion PERPENDICULAR to the direction that the wave is moving In general, liquids carry longitudinal waves but not transverse waves Same terms as vibrations v = λ * f

33 Sound Waves Sound does not travel in a vacuum Sound moves through solids faster than any gas Velocity of sound is effected by composition and temperature of gas

34 Reflection, Refraction and Interference Reflection waves bouncing back off of a boundary Refraction change in direction of wave crossing a boundary Interference interaction of waves destructive interference constructive interference

35 Resonance Natural frequency frequency of vibration determined by the object s composition and shape Resonance when frequency of external force matches natural frequency

36 Doppler Effect and Sonic Boom Doppler Effect Apparent change in frequency of a wave caused by the relative motion of the source or observer pitch of train approaching, departing Sonic boom shock wave caused by object moving at speed of sound or faster

37 Question for Thought What is a wave? A wave is a disturbance that moves through a medium such as a solid or the air.

38 Question for Thought Is it possible for a transverse wave to move through air? Explain. The book view is No, because there is no force acting on the air to return it to its original position after the wave has dislocated it. However, what about EM.

39 Question for Thought A piano tuner hears three beats per second when a tuning fork and a note are sounded together and six beats per second after the string is tightened. What should the tuner do next, tighten or loosen the string? Explain. Loosen. Since the beat frequency depends upon the difference between the two frequencies, you wish to go in the direction of fewer beats per second.

40 Question for Thought Why do astronauts on the moon have to communicate by radio even when close to one another? There is no medium such as air to transmit sound on the moon.

41 Question for Thought What is resonance? The condition where the frequency of an external force matches the frequency of an object is resonance.

42 Question for Thought Explain why sound travels faster in warm air than in cool air. Gas molecules have a greater kinetic energy and move faster in warm air than in cold air. These molecules are able to transfer an impulse from one molecule to the next faster.

43 Question for Thought Do all frequencies of sound travel with the same velocity? Explain using the wave equation. Longer wavelengths have lower frequencies. Since the velocity of sound is equal to the product of the frequency times the wavelength, the velocity is a constant.

44 Question for Thought What eventually happens to a sound wave traveling through the air? The energy of the sound wave is eventually dissipated into heat.

45 The presence and strength of various overtones determine the characteristic sound of a musical note. Question for Thought What gives a musical note its characteristic quality?

46 Question for Thought Does a supersonic aircraft make a sonic boom only when it cracks the sound barrier? Explain. The sonic boom is from the building up of a pressure wave in front of the moving aircraft. Since this pressure wave is present as long as the plane is moving faster than the speed of sound, the aircraft continually makes a sonic boom.

47 An echo is the return of a sound wave to its source after the wave has been reflected. Question for Thought What is an echo?

48 They all produce standing waves or resonance in whatever is oscillating. Question for Thought Why are fundamental frequencies and overtones also called resonant frequencies?

49 Question The distance between the center of a condensation and the center of an adjacent rarefaction is cm. If the frequency is Hz, how fast are these waves moving?

50 Answer 2. v = fλ f = Hz v = s λ = 2 ( cm) ( cm) = cm / s (or m / s)

51 Question A warning buoy is observed to rise every 5.0 seconds as crests of waves pass by it. What is the period of these waves? What is the frequency?

52 Answer 3. () a T = 5.0s () b f = 1 T 1 = 5.0s = 0.2 Hz

53 Question Sound from the siren of an emergency vehicle has a frequency of 750 Hz and moves with a velocity of 343 meters per second. What is the distance from one condensation to the next?

54 Answer 4. v = fλ λ = v f v = m s f = Hz λ = m s s = = m m s s 1