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Chapter 10 Temperature and Heat

Chapter 10 Temperature and Heat. Temperature Thermodynamics = study of heat and its effect on matter One of the sub-fields of Physics Part of Classical Physics (like Mechanics) developed by 1900 Measuring Temperature What is hot? What is cold? We must compare it to some other object

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Chapter 10 Temperature and Heat

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  1. Chapter 10 Temperature and Heat • Temperature • Thermodynamics = study of heat and its effect on matter • One of the sub-fields of Physics • Part of Classical Physics (like Mechanics) developed by 1900 • Measuring Temperature • What is hot? What is cold? • We must compare it to some other object • Use a thermometer to assign a temperature • Sealed tube with a liquid inside (mercury or ethanol with a red dye) • Volume of the liquid changes faster than the length of glass • Volume increases until thermometer is same temperature as mouth • 101.6 oF is “hotter” than 98.6 oF • Must have a scale of the thermometer to compare temperatures • Could use other properties of matter than volume Length of a rod, electrical resistance, color change, etc…

  2. Thermal Equilibrium = when touching objects no longer have changing properties • Zeroth Law of Thermodynamics = Objects in thermodynamic equilibrium have the same temperature • Temperature Scales • Must have a standard scale to compare temperatures • 1 meter defined by a metal rod of a certain length • Temperature standards use freezing point and boiling points of water • Fahrenheit (1686-1736) • Freezing point of water = 32 oF • Boiling point of water = 212 oF • Used primarily in USA • Celcius (1701-1744) • Freezing point of water = 0 oC • Boiling point of water = 100 oC • Used everywhere else and in science • 1 oC > 1 oF 180 Fo 100 Co

  3. Conversion of Fahrenheit and Celcius Temperatures • 0 oC = 32 oF so there is a +/- 32 added into the conversion factors • Convert 100 oC = ??oF • Convert 98.6 oF = ??oC • Prove to yourself that –40 oC = -40 oF • Absolute Zero • 0 oC is based on freezing point of water, could use ethanol (-115oC) • Arbitrary point defined as zero, so we get negative numbers • What is the real lowest temperature = 0 ?

  4. Pressure of a gas depends on its Temperature (constant volume) • At the lowest possible temperature, P = 0 • Lowest possible temperature = -273.2 oC = Absolute Zero 5) Kelvin Temperature Scale: 0 K = absolute zero • TK = TC + 273.2 • Room Temperature = 22 oC TK = 22 + 273.2 = 295.2 K • Not degrees Kelvin, but simply Kelvins • Can’t ever reach absolute zero

  5. Hot object cools down Cool object heats up • Heat • Zeroth Law = Objects in contact will come to the same temperature • Early explanation was the flow of “caloric” from hot to cold • Amount of caloric in an object = temperature • Also would work for mixture of liquids • Heat = Q being transferred, not caloric, is responsible for equilibration 1) Heat is a form of Energy (unit = J) 2) Another unit is the calorie: 1 calorie = 4.19 J 3) 1 calorie = amount of heat needed to raise the temperature of 1 g of water by 1 oC 4) Different materials require different amounts of heat to raise their temperature by the same amount 5) Specific Heat Capacity = c = amount of heat needed to raise the temperature of 1 g of a material by 1 oC (Units = cal/g oC) Hot Cold

  6. Water has a very large heat capacity, so it cools things very well • Small amount of water takes away a large amount of heat • Temperature of the water is not raised very fast • Coasts are cooler during the day, and warmer at night • Q = mcDT is used to relate mass, heat, heat capacity, and Temperature How much heat could 100 g of water and 100 g of steel take away if there temperatures were raised 50 oC? • Heat and Temperature • Like caloric, Heat flows from Hot to Cold • Added heat increases temperature • Removed heat decreases temperature • Temperature tells us direction of heat flow • Temp depends on heat, mass, c

  7. Heat = energy flowing between objects of different temperatures • Temperature = quantity that indicates direction of heat flow. If no heat is flowing, the objects are the same temperature. • Heat and Change of Phase • Phases of matter depend on how much heat they have (temperature) • Solid < Liquid < Gas • Ice < Water < Steam • Block of ice at –20 oC • Add heat: block warms up to 0 oC • Add more heat: ice melts to water at 0 oC • No temperature change • Change in phase requires heat • Latent Heat of Fusion = Lf = amount of heat required to change the phase of 1 g of a solid to a liquid = 80 cal/g for water • Latent Heat of Vaporization = Lv = amount of heat required to change the phase of 1 g of a liquid to gas = 540 cal/g for water How much heat is required to turn 15 g of liquid water at 100 oC to steam at 100 oC?

  8. Ice added to water • Ice is < 0 oC and the water is > 0 oC • Heat flows from water to ice • Heats the ice to 0 oC • Then it starts melting the ice • If there is enough ice, eventually all of the water will be cooled to 0 oC • Both the ice and water will be at 0 oC = Thermal Equilibrium • Heat from the surroundings eventually melts the ice • After all ice melted, then the temperature will rise > 0 oC • Boiling Water = 100 oC • Add more heat, change some of it into steam • No change in temperature, all the heat goes to Lv • High altitude lowers pressure needed to boil, less heat, b.p. = 96 oC at an altitude of 1 mile (Denver) • Q needed for 200 g ice (c = 0.5 cal/goC) at –10 oC to melt? 17,000 cal

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