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1 © 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Lecture Outlines Chapter 16 Physics, 3 rd Edition James S. Walker

2 Chapter 16 Temperature and Heat

3 Units of Chapter 16 Temperature and the Zeroth Law of Thermodynamics Temperature Scales Thermal Expansion Heat and Mechanical Work Specific Heats Conduction, Convection, and Radiation

4 16-1 Temperature and the Zeroth Law of Thermodynamics Definition of heat: Heat is the energy transferred between objects because of a temperature difference. Objects are in thermal contact if heat can flow between them. When the transfer of heat between objects in thermal contact ceases, they are in thermal equilibrium.

5 16-1 Temperature and the Zeroth Law of Thermodynamics The zeroth law of thermodynamics: If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, then objects A and C will be in thermal equilibrium if brought into thermal contact. That is, temperature is the only factor that determines whether two objects in thermal contact are in thermal equilibrium or not.

6 16-1 Temperature and the Zeroth Law of Thermodynamics

7 16-2 Temperature Scales The Celsius scale: Water freezes at 0° Celsius. Water boils at 100° Celsius. The Fahrenheit scale: Water freezes at 32° Fahrenheit. Water boils at 212° Fahrenheit.

8 Converting between Celsius and Fahrenheit: 16-2 Temperature Scales Converting between Fahrenheit and Celsius :

9 Example 16-1 Temperature Conversions

10 16-2 Temperature Scales The pressure in a gas is proportional to its temperature. The proportionality constant is different for different gases, but they all reach zero pressure at the same temperature, which we call absolute zero:

11 Figure 16-2 A constant-volume gas thermometer

12 Example 16-2 It’s a Gas Trycket vid 0,00° C är 80,0 kPa. Om gasen kan anses ideal, vad är trycket vid 105° C? p(T) = kT med k = 80,0 kPa/273,15 K = 293 Pa/K p = k(273, ) = 111 kPa

13 16-2 Temperature Scales The Kelvin scale is similar to the Celsius scale, except that the Kelvin scale has its zero at absolute zero. Conversion between a Celsius temperature and a Kelvin temperature:

14 16-2 Temperature Scales The three temperature scales compared:

15 16-3 Thermal Expansion Most substances expand when heated; the change in length or volume is typically proportional to the change in temperature. The proportionality constant is called the coefficient of linear expansion.

16 Conceptual Checkpoint 16–1a Compare Expansions (Rätt överdrivna figurer för tydlighetens skull. Vad är en rimlig temperaturdifferens som svarar mot figuren? (ΔT =1/3α = 20 kK för Cu!)

17 16-3 Thermal Expansion Some typical coefficients of thermal expansion:

18 Exercise 16-2 La Tour Eiffel är 301 m högt, en sommardag då temperaturen är 22°C. Hur mycket minskar dess höjd om temperaturen sjunker till 0°C? ΔT = - 22 K = (- 22°C) α = K -1 ger ΔL = α L 0 ΔT = - 0,079 m

19 16-3 Thermal Expansion A bimetallic strip consists of two metals of different coefficients of thermal expansion, A and B in the figure. It will bend when heated or cooled.

20 Figure 16-5 A bimetallic strip

21 16-3 Thermal Expansion The expansion of an area of a flat substance is derived from the linear expansion in both directions: Holes expand as well:

22 16-3 Thermal Expansion The change in volume of a solid is also derived from the linear expansion: For liquids and gases, only the coefficient of volume expansion is defined:

23 Area and Volume Expansions A´= (L+ΔL) 2 = (L + α L ΔT) 2 A’ – A = ΔA ≈ 2 α A ΔT V´= (L+ΔL) 3 = (L + α L ΔT) 3 V’ – V = ΔV ≈ 3 α V ΔT = β V ΔT

24 16-3 Thermal Expansion Some typical coefficients of volume expansion:

25 Example 16-3 Oil Spill En kopparbehållare (V = 150 cm 3 ) fylls till bredden med olivolja och värms därefter upp 25 grader. Hur mycket olja rinner då ut över kanten? ΔV ≈ β V ΔT = 0, /K 150 cm 3 25 K = 2,55 cm 3 ΔV Cu ≈ β V Cu ΔT = /K 150 cm 3 25 K = 0,19 cm 3 så den utrunna volymen olja blir ungefär 2,4 cm 3

26 16-3 Thermal Expansion Water also expands when it is heated, except when it is close to freezing; it actually expands when cooling from 4° C to 0° C. This is why ice floats and frozen bottles burst.

27 16-4 Heat and Mechanical Work Experimental work has shown that heat is another form of energy. James Joule ( ) used a device similar to this one to measure the mechanical equivalent of heat:

28 16-4 Heat and Mechanical Work One kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of 1 kg of water from 14.5° C to 15.5° C. Through experiments such as Joule’s, it was possible to find the mechanical equivalent:

29 16-4 Heat and Mechanical Work Another unit of heat is the British thermal unit (Btu). This is the energy required to heat 1 lb of water from 63° F to 64° F.

30 Example 16-4 Stair Master

31 Example 16-4 Stair master En person med massan 74,0 kg dricker en milkshake (305 kcal). Hur många trappsteg à 20 cm måste han ta för att förbränna det energiintaget? Q = 3, cal = (1 cal = 4,186 J) = J För att ta ett steg åtgår energin E = mgh = 74,0 kg9,81 m/s 2 0,20 m = 145,188 J Så antalet steg i trappan blir Q/E (= 8794) ≈ 8800

32 16-5 Specific Heats The heat capacity of an object is the amount of heat added to it divided by its rise in temperature: Q is positive if Δ T is positive; that is, if heat is added to a system. Q is negative if Δ T is negative; that is, if heat is removed from a system.

33 Exercise 16-3 Värmekapaciteten C för 1,00 kg vatten är 4186 J/K. Vad blir temperaturändringen om värmet a) 505 J tillförs b) 1010 J bortförs? Q = CΔT [= cmΔT] ΔT = J/4186 J/K = 0,121 K ΔT = J/4186 J/K = - 0,241 K

34 16-5 Specific Heats The heat capacity of an object depends on its mass. A quantity which is a property only of the material is the specific heat:

35 16-5 Specific Heats Here are some specific heats of various materials:

36 16-5 Specific Heats A calorimeter is a lightweight, insulated flask containing water. When an object is put in, it and the water come to thermal equilibrium. If the mass of the flask can be ignored, and the insulation keeps any heat from escaping: 1.The final temperatures of the object and the water will be equal. 2. The total energy of the system is conserved. This allows us to calculate the specific heat of the object.

37 Active Example 16-2 Find the final Temperature Vatten med massan 550 g och med en begynnelsetemperatur på 32°C släpps ned i en aluminiumbehållare, som väger 210 g med ursprungstemperaturen 15°C. Vad blir sluttemperaturen då man antar att inget värmeutbyte sker med omgivningen. Q behållare = cmΔT = 9000,210(T- 15°C ) Q vatten = cmΔT = 41860,550(T - 32°C) om Q behållare + Q vatten = 0 fås T = 31°C (30,7°C)

38 Example 16-5 Cooling Off

39 Ett metallblock som väger 0,50 kg och med en begynnelsetemperatur på 54,5°C släpps ned i en behållare, som innehåller 1,1 kg vatten med ursprungstemperaturen 20,0°C. Sluttemperaturen blir 21,4°C då man bortser från behållarens inverkan och antar att inget värmeutbyte sker med omgivningen. Beräkna metallens specifica värmekapacitet. Q block = cmΔT = c0,5(21,4 - 54,5)= - c16,55kgK Q vatten = cmΔT = 41861,1(21,4 -20,0)= 6446,44 J c ≈ 390 J/(kgK) (dvs Cu?) om Q block + Q vatten = 0

40 16-6 Conduction, Convection, and Radiation Conduction (värmeledning), convection (konvektion), and radiation (strålning) are three ways that heat can be exchanged. Conduction is the flow of heat directly through a physical material.

41 16-6 Conduction, Convection, and Radiation Experimentally, it is found that the amount of heat Q that flows through a rod: increases proportionally to the cross- sectional area A increases proportionally to the temperature difference from one end to the other increases steadily with time decreases with the length of the rod

42 16-6 Conduction, Convection, and Radiation Combining, we find: The constant k is called the thermal conductivity of the rod.

43 Conceptual checkpoint 16-3 The Feel of Tile När vi vaknar på morgonen och kommer från sovrummets matta till badrummets kakel, uppfattar vi då kaklet som varmare, kallare eller ha samma temperatur som mattan?

44 16-6 Conduction, Convection, and Radiation Some typical thermal conductivities: Substances with high thermal conductivities are good conductors of heat; those with low thermal conductivities are good insulators.

45 Example 16-6 What a Pane Ett fönster (pane) har mått enligt figur. Hur mycket värme förloras genom rutan på en dag, om innertemperaturen är 21°C och utetemperaturen är 0,0°C?

46 Example 16-6 What a Pane Värmeflödet blir Q pane = kAΔTt/L = = 0,84 W/(mK)(1 m) 2 21 K s/0, m = = 300 MWs = 300 MJ

47 Conceptual Checkpoint 16-4 Compare the Heat Flow Sker värmeflödet i den vänstra uppställningen a) bättre b) sämre c) med samma värmeflöde som i den högra?

48 Example 16-7 Parallel Rods

49 Två stavar, 0,525 m långa, den ena av bly, den andra av koppar, är förbundna mellan två metallplattor som håll vid temperaturerna 2,00 och 106 °C respektive. Stavarna är kvadratiska med kantlängden 1,50 cm. Hur stort värmeflöde sker genom stavarna på 1,00 s? Inget värmeutbyte sker med omgivningen. Q Pb = kAΔTt/L= 34,3 W/(mK)(0,015 m) K 1s/0,525 m = 1,53 J Q Cu = kAΔTt/L = 395 W/(mK)(0,015 m) K 1s/0,525 m = 17,6 J Q = 1,53 J + 17,6 J = 19,1 J

50 16-6 Conduction, Convection, and Radiation Convection is the flow of fluid due to a difference in temperatures, such as warm air rising. The fluid “carries” the heat with it as it moves. konvektion = värmetransport i ett medium (gas eller vätska)

51 Figure Countercurrent heat exchange in the human arm

52 16-6 Conduction, Convection, and Radiation All objects give off energy in the form of radiation, as electromagnetic waves – infrared, visible light, ultraviolet – which, unlike conduction and convection, can transport heat through a vacuum (dvs inget medium behövs). Objects that are hot enough will glow – first red, then yellow, white, and blue. Objects at body temperature radiate in the infrared, and can be seen with night vision binoculars.

53 16-6 Conduction, Convection, and Radiation The amount of energy radiated by an object due to its temperature is proportional to its surface area and also to the fourth (!) power of its temperature. It also depends on the emissivity, which is a number between 0 and 1 that indicates how effective a radiator the object is; a perfect radiator would have an emissivity of 1.

54 16-6 Conduction, Convection, and Radiation This behavior is contained in the Stefan- Boltzmann law: Here, e is the emissivity, and σ is the Stefan- Boltzmann constant:

55 Exercise 16-4 Beräkna den utstrålade effekten från en sfär med radien 5,00 cm och temperaturen 355 K. Anta att emissiviten = 1. P = εσAT 4 = = 5, W/(m 2K 4 )4π(0,05 m) 2 (355 K) 4 = 28,3 W

56 Example 16-8 Human Polar Bears

57 Beräkna den utstrålade effekten från en ”polar bear” som har arean 1,15 m 2 och yttemperaturen 303 K när han befinner sig i omklädningsrummet (där temperaturen är 293 K) och när han står ute (där temperaturen är 273 K). Anta att emissiviten = 0,9 (observera att detta värde också anger måttet för absorptionen!) P = εσA(T 4 - T in 4 ) = 0,95, W/(m 2K 4 )1,15 m 2 (303 K) 4 - 0,95, W/(m 2 K 4 )1,15 m 2 (293 K) 4 = = 494,6 W – 432,5 W = 62,1 W P = εσA(T 4 – T ut 4 ) = 494,6 W - 0,95, W/(m 2 K 4 )1,15 m 2 (273 K) 4 = 494,6 W – 326,0 W ≈ 169 W

58 Figure The Thermos bottle (p.538) (med en utmärkt reflektor, ε ≈ 0 och därför en dålig “strålare”) invented by Sir James Dewar ( ) skotsk fysiker och kemist.

59 Summary of Chapter 16 Heat is the energy transferred between objects due to a temperature difference. Objects are in thermal contact if heat can flow between them. Objects that are in thermal contact without any flow of heat are in thermal equilibrium. Thermodynamics is the study of physical processes that involve heat. If objects A and B are both in thermal equilibrium with C, they are in thermal equilibrium with each other.

60 Summary of Chapter 16 Temperature determines whether two objects will be in thermal equilibrium. Celsius scale: water freezes at 0° C, boils at 100° C The lowest attainable temperature is absolute zero. Kelvin: absolute zero is 0 K; water freezes at K and boils at K

61 Summary of Chapter 16 Temperature scale conversions: Most substances expand when heated. Linear expansion: Volume expansion: Water contracts when heated from 0° C to 4° C.

62 Summary of Chapter 16 Heat is a form of energy: Heat capacity of an object: Specific heat is heat capacity per unit mass: Energy is conserved in heat flow.

63 Summary of Chapter 16 Conduction: heat exchange from one part of a material to a cooler part, with no bulk motion of the material. Heat exchanged in time t : Convection is heat exchange due to the bulk motion of an unevenly heated fluid. Radiation is heat exchange due to electromagnetic radiation.

64 Summary of Chapter 16 Radiated power as a function of temperature: Stefan-Boltzmann constant:


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