chapter_12
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chapter_12

Course Number: PS 104, Fall 2008

College/University: Embry-Riddle FL/AZ

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PS104 Chapter 12 Temperature and Heat 1. Common Temperature Scales Converting from a Fahrenheit to a Celsius temperature Tc = (TF - 32)(5/9) Converting from a Celsius to a Fahrenheit temperature TF = TC (9/5) + 32 2. The Kelvin temperature scale The kelvin is the SI base unit for temperature. T (kelvin) = TC + 273.15 A change of 1 Co is equal to a change of 1 K The constant-volume gas thermometer Problem...

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12 Temperature PS104 Chapter and Heat 1. Common Temperature Scales Converting from a Fahrenheit to a Celsius temperature Tc = (TF - 32)(5/9) Converting from a Celsius to a Fahrenheit temperature TF = TC (9/5) + 32 2. The Kelvin temperature scale The kelvin is the SI base unit for temperature. T (kelvin) = TC + 273.15 A change of 1 Co is equal to a change of 1 K The constant-volume gas thermometer Problem 2 On the moon the surface temperature ranges from 375 K during the day to 1.00 102 K at night. What are these temperatures on the (a) Celsius and (b) Fahrenheit scales? 3. Thermometers Thermometric property a physical property that changes with temperature length of a merucry column pressure of a gas in a constant-volume gas thermometer potential difference in a thermocouple electrical resistance thermometers made of platinum wire. 1 4. Linear thermal expansion The length Lo of an object changes by an amount L when its temperature changes by an amount T : L = Lo T where is the coefficient of linear expansion and L = L - Lo . for steel is (Steel) = 12 10-6 (C o )-1 Thermal Stress Stress = Y (L/Lo ) = YT where Y = Young's modulus and Stress = pressure = F/A. The bimetallic strip The expansion of holes The maximum density of water (4o C) Problem 11 Find the approximate length of the Golden Gate bridge if it is known that the steel in the roadbed expands by 0.53 m when the temperature changes from +2 to +32o C 5. Volume Thermal Expansion The volume of a normal material increases as the temperature increases. The volume Vo of an object changes by an amount V when its temperature changes by an amount T : V = Vo T Problem 26 A swimming pool contains 110 m3 of water. The sun heats the water from 17 to 27 o C. What is the change in the volume of the water? Answer: 0.228 m3 2 6. Heat and internal energy Heat is energy that flows from a high-temperature object to a lowertemperature object because of the difference in temperatures. The SI unit of heat is the joule (J) Object are hotter because of their internal energy (i.e., their molecular kinetic energy) 7. Heat and temperature change For a limited temperature range, experiments show that the amount of heat Q (joules) is directly proportional to the in change temperature T and to the mass m. The heat Q that must be supplied or removed to change the temperature of a substance of mass m by an amount T is: Q = cmT where c is the specific heat capacity SI unit for specific heat capacity: J/(kgCo ) Table of specific heats: cwater = 4186 J/(kgCo ) at 15 o C Gases: the specific heat capacity depends upon whether or not the heat exchanged is done at constant pressure or constant volume. This will be discussed later in chapter 15. 1 calorie = 4.186 joules 1 kcal = 4186 joules Calorimetry: In a closed system, the total heat exchanged between systems in thermal contact must add up to zero. Q1 + Q2 + Q3 = 0 Q is positive if heat enters the system m1 c1 T1 + m2 c2 T2 + m3 c3 T3 = 0 Problem 47 In a passive solar house, the sun heats water stored in barrels to a temperature of 38 o C. The stored energy is then used to heat the house on cloudy days. Suppose that 2.4 108 J of heat are needed to maintain the inside of the house at 21 o C. How many barrels (1 barrel = 0.16 m3 ) of water are needed? Answer: 21.1 barrels 3 8. Heat and phase change: Latent heat Three phases of mattergas, liquid, and solid Phase transition: solid liquid solid gas (sublimation) or liquid gas or During phase transitions, heat is exchanged but the temperature does not change. Q = mL where is the latent heat of the substance The SI unit of latent heat: J/kg Latent heat of fusion, Lf Water at 0 o C Lf = 33.5 104 J/kg Water at 100 o C Lv = 22.6 105 J/kg Latent heat of vaporization, Lv Problem 51 How much heat must be added to 0.45 kg of aluminum to change it from a solid at 130o C to a liquid at 660o C (its melting point)? The latent heat of fusion for aluminum is 4.0 105 J/kg. Answer: 3.9 105 J Problem 56 The latent heat of vaporization of H2 O at body temperature (37.0 o C) is 2.42 106 J/kg. To cool the body of a 75-kg jogger [average specific heat capacity = 3500 J/(kgCo )] by 1.5 o C, how many kilograms of water in the form of sweat have to be evaporated? Answer: 0.163 kg Extra Problem Problem 78 A 10.0-kg block of ice has a temperature of -10.0o C. The pressure is one atmosphere. The block absorbs 4.11 106 J of heat. What is the final temperature of the liquid water? Answer: 13.4o C 4
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