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## Thermal Properties

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**Thermal Expansion**• When a solid is heated, its atoms spread out in all dimensions. • Consequences of thermal expansion may be desirable or undesirable!**Thermal Expansion**• To calculate a change in length due to thermal expansion: Δl = αliΔt**Thermal Expansion**• α is the coefficient of thermal expansion • different for each material • units: °C-1 or K-1 Δl = αliΔt**Thermal Expansion**• Δl and li must be in the same units of length • Δt and α must use the same temperature units Δl = αliΔt**Thermal Expansion**• bimetallic strips take advantage of different rates of expansion of different materials • thermostats**Thermal Expansion**• Expansion of volume can be similarly calculated: • β is the coefficient of volume expansion. • β = 3α ΔV = β ViΔt**Thermal Expansion**• Liquids respond to temperature changes much more than solids do. • The same equation can be used for liquid expansion. ΔV = β ViΔt**Thermal Expansion**• Example 14-3: • How much does the gasoline expand? • How much does the tank expand? • How much gas spills?**Thermal Expansion**• Gases also expand in volume with changes in temperature; this is discussed in more detail later in the chapter.**Thermometers**• Galileo—thermoscope • drawback: also responded to changes in air pressure**Thermometers**• Gas thermometers are more accurate but less convenient. • Now liquid thermometers use liquids sealed in glass tubes under vacuum.**Liquid Thermometers**• must be calibrated • must be sensitive enough that a small change in temperature produces a noticeable change in liquid level**Liquid Thermometers**• should respond quickly to changes in temperature • mass must be small in relation to the bulk of the object whose temperature is being measured**Liquid Thermometers**• medium used in thermometer must be appropriate for the range of temperatures being measured**Thermometric**• A thermometric property of a substance is any measurable property that changes in proportion to a corresponding change in temperature.**Thermometric**• other temperature-measuring instruments: • resistive temperature detectors (RTD) • thermocouple junctions**Thermometric**• other temperature-measuring instruments: • mechanical thermometers • digital thermometers**Temperature Scales**• Fahrenheit • After trying other things, the reference points became 32° (freezing point of water) and 212° (boiling point of water).**Temperature Scales**• Celsius • This scale sets the freezing point of water at 0° and the boiling point of water at 100°.**Temperature Scales**• Absolute or Kelvin • emphasized an “absolute zero” • absolute zero is 0K or about -273.15°C**Temperature Scales**• Absolute or Kelvin • also uses triple point of water as a reference • the degree symbol (°) is not used**Conversions**• Celsius (tC) and Kelvin (T): • T = tC + 273.15 • tC = T – 273.15**Conversions**• Fahrenheit (tF) to Celsius (tC): • tC = 5/9(tF + 40°) – 40° • tC = 5/9(tF – 32°)**Conversions**• Celsius (tC) to Fahrenheit (tF): • tF = 9/5(tC + 40°) – 40° • tF = 9/5(tC) + 32°**Conversions**• To convert between Kelvin and Fahrenheit, first convert to Celsius and then to the desired unit.**Ideal Gas**• Ideal gas particles have no volume. • Ideal gas particles do not interact except when they have perfectly elastic collisions.**Charles’s Law**• deals with the effect of temperature on the volume of a gas at a constant pressure • related to the computation of absolute zero temperature**V1**V2 = T1 T2 Charles’s Law • temperature must be expressed in Kelvin for this and all other gas laws**Charles’s Law**• When doing problems involving these laws, consider in advance: Will the final quantity be larger or smaller than the original?**Boyle’s Law**• deals with the effect of pressure on the volume of a gas at a constant temperature • invented what we call a “Boyle’s law tube” to investigate this**Boyle’s Law**• volume is inversely proportional to pressure, if the temperature is constant V1P1 = V2P2**P1**P2 = T1 T2 Gay-Lussac’s Law • deals with the effects of pressure and temperature of a gas at a constant volume**P1V1**P2V2 = T1 T2 Combined Gas Law • does not require that any single quantity remain constant**Ideal Gas Law**• PV = KT • K is a temperature-dependent constant • K = kN • N = number of particles • k = 1.381 × 10-23 J/K**Ideal Gas Law**• k = 1.381 × 10-23 J/K is called Boltzmann’s constant • k · NA provides the universal gas constant (R): R 8.315 J/(K·mol)**Ideal Gas Law**• The ideal gas law is usually written as: PV = nRT • The units used for R must be consistent with those given in the data!**Ideal Gas Law**• standard temperature and pressure (STP): 273.15 K 1.013 × 105 Pa**Real Gases**• The ideal gas model is not as accurate at higher pressures and lower temperatures.**Real Gases**• Other more accurate and more complicated equations have been developed. • Johannes van der Waals