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Temperature and Heat Chapter 12 Expectations After this chapter, students will: know what temperature is be familiar with common temperature scales calculate the changes in the linear dimensions of objects, and the volumes of substances, with temperature understand heat as a form of energy Expectations After this chapter, students will: Use the concept of specific heat capacity to relate temperature changes to gains and losses of heat energy Calculate the energy transfers accompanying phase changes of materials Analyze situations in which different phases of matter exist in equilibrium Temperature Temperature is a rational numerical characterization of the hotness or coldness of an object or a substance. If a hotter (higher temperature) object touches a colder (lower temperature) one, heat energy tends to flow from the hotter to the colder. Temperature Scales Temperature scales are typically based on the phase changes of a very common material: water. The ice point is the temperature at which liquid water and ice are in equilibrium, at one atmosphere of pressure. The steam point is the equilibrium for liquid water and steam, at one atmosphere. Temperature Scales: Celsius Ice point: 0 °C Steam point: 100°C Anders Celsius 1701 – 1744 Swedish astronomer Temperature Scales: Fahrenheit Ice point: 32 °F Steam point: 212°F Daniel Gabriel Fahrenheit 1686 – 1736 German physicist Temperature Scales: Absolute Ice point: 273.15 K Steam point: 373.15 K William Thomson (Lord Kelvin) 1824 – 1907 Scottish mathematician and physicist Temperature Scales: Rankine Ice point: 491.67 °R Steam point: 671.67 °R William Rankine 1820 – 1872 Scottish engineer Temperature Conversions Fahrenheit / Celsius TC TF 32 5 9 TF TC 32 9 5 Celsius / Absolute TA TC 273.15 TC TA 273 .15 Temperature Style and Grammar Temperature differences or changes are expressed in “Celsius degrees” (C°). The temperature of an object or substance is expressed in “degrees Celsius” (°C). The unit of absolute temperature is the “kelvin” (K). There is no such thing as a “degree Kelvin” (°K). Temperature Expansion – Linear Most materials expand when they get hotter. As they expand, all their linear dimensions (length, width, height, diameter, etc.) expand proportionally. The amount of the expansion depends on: The amount of temperature change The original size of the dimension A material property: coefficient of linear expansion Temperature Expansion – Linear L aL0 T SI unit for coefficient of linear expansion: (C°)-1 Values of a for some materials are tabulated on p. 342 of your textbook. Temperature Expansion – Volume V bV0 T SI unit for coefficient of volume expansion: (C°)-1 Values of b for some materials are tabulated on p. 342 of your textbook. Temperature and Heat Heat is the energy that moves from an object or substance at higher temperature to an object or substance at lower temperature because of their temperature difference. SI unit of heat: the joule (J). Temperature and Heat When heat flows from a hotter object into a colder one, the internal energy of the hotter object decreases, and the internal energy of the colder object increases. The internal energy consists of several forms of molecular kinetic and potential energy. Temperature is not a measure of an object’s total internal energy. Temperature and Heat Does the same amount of heat energy, flowing into or out of a variety of objects, change every object’s temperature by the same amount? No. The change in temperature depends on: the amount of heat lost or gained; the mass of the object; and the material that the object is made of. Temperature and Heat The material property is called the specific heat capacity: temperature Q cmT change heat specific mass required heat capacity SI units of specific heat capacity: J / (kg C°) Heat: Other Units Satan’s units* for heat: calorie (cal): the amount of heat that increases the temperature of 1 gram of water by 1 C° kilocalorie (kcal): increases 1 kg H2O by 1 C° nutritional Calorie: = 1000 regular calories = 1 kcal British Thermal Unit (BTU): increases the temperature of 1 pound of water by 1F° (*Satan loves to spread misery and confusion.) Heat and Phase Change To change the phase of a material (melt ice, freeze water, boil water, condense steam) we must add heat, or remove heat. At the phase-changing temperatures (melting point or boiling point), heat is added or removed without changing the temperature until the phase change is complete. Heat and Phase Change Heat and Phase Change The amount of heat required to accomplish the phase change depends on the mass of material involved, what kind of material it is, and what phase change we are considering. heat mass In general: Q mL latent heat SI units of latent heat: J / kg Heat and Phase Change Phase change names solid liquid gas melting sublimation from solid to: ----- (fusion) (vaporization) boiling or freezing from liquid to: ----- evaporation (fusion) (vaporization) condensation condensation from gas to: ----- (vaporization) (vaporization) Equilibrium Between Phases most things Fusion curves: contract when they temperatures and freeze pressures at which the solid and liquid phases are in equilibrium. water expands when it freezes Equilibrium Between Phases Vaporization curves: temperatures and pressures for liquid – gas equilibrium. water Relative Humidity Partial pressure: when a number of different molecular species are present in a mixture of gases, the total pressure of the mixture is the sum of the partial pressures due to each constituent. If the partial pressure of water vapor in the atmosphere reaches the equilibrium pressure (from a vaporization curve), water leaves the atmosphere at the same rate it enters. (Fog or rain.) Relative Humidity Relative humidity tells how much water vapor is in the air, compared to how much can be in the air. % relative humidity or partial pressureof water vap in the air 100% or equilibriu m pressureof water vap at the current te mperature Relative Humidity Given an existing partial pressure of water vapor in the air, if the air is cooled, it will reach a temperature for which the equilibrium vapor pressure of water decreases to be equal to the existing partial pressure of water vapor. At that temperature, the relative humidity is 100%, and water starts coming out of the atmosphere. This temperature is called the dew point.
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