Chapter_5_Thermal_Properties_of_Material - Chapter 5...

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Unformatted text preview: Chapter 5 Thermal Properties of Materials Materials photo source: Units of Energy Measurement Units (things you should learn in physics and won’t) All forms of energy can be measured by the All amount of heat they produce: amount One calorie is the amount of heat required to raise the One temperature of one gram of water by 1 degree C. temperature One Btu (British Thermal Unit) is the amount of heat One required to raise the temperature of one pound of water by 1 degree F. water One Btu = 232 Calories A joule is a unit of measure used for all forms of joule energy. It is equal to the amount of work done by a force of one Newton moving through a distance of one meter. meter. One calorie = 4.185 Joules Photo source: One Power is the rate of energy production, consumption or conversion consumption Power is measured over time 1 Watt = 1 Joule per second (1 J/s) A 100 Watt light bulb consumes 100 Joules 100 of energy per minute of Energy is the total amount of power Energy consumed over time. consumed Generally measured in kilowatts per hour photo source: E=MC2 Unit of Energy 1 cal 1 kcal 1 Btu 1 kWh Value in Joules 4.185 j 4,185 j 1,050 j 306(10)6 j Relationship Between Energy Units Relationship Modes of Heat Transfer Modes Conduction Heat transfer between molecules in a solid Convection Heat transfer by molecular buoyancy motion of a Heat gas or liquid gas Radiation Heat transfer between two objects in the form of Heat electromagnetic waves electromagnetic photo source: Heat flow between 2 objects (nature always seeks balance) Transfer of solar heat through a brick cavity wall brick Thermal Resistance Thermal Thermal resistivity ( ρ ) is a constant Thermal is property of a material representing the property ability of the material to resist the flow of heat through it. heat R-value is a measure of the ability of a R-value measure material of a specified thickness to resist the flow of heat through it. the A material with a high R-value resists the material flow of heat, and is called a thermal insulator. insulator. A material with a low R-value allows heat to material flow through it easily, and is called a thermal conductor. photo source: photo thermal Calculating R-Value Calculating R = ρL (thermal resistance X thickness) If the ρ value of concrete is 0.15 (see Table If 5.3), what is the R-value of a 6 inch concrete wall? concrete R = (0.15)(6) = .9 (0.15)(6) If the ρ value of fiberglass insulation is 3.5 (see Table 5.3), what is the R-value of 6 inch thick blanket of fiberglass insulation? thick R = (3.5)(6) = 21 photo source: Estimating R-Value Element 4-in.-thick brick wall 2-in.-thick polystyrene board 8-in.-thick brick wall R-value 3.625(0.2) = .0725 2.0(5.0) = 10.0 7.625(0.2) = 1.525 Rt = 0.725 + 10 + 1.525 = 12.25, round down to R-12 0.725 Heat flow through wall cavity Heat photo source: Emissivity Emissivity The property of the The surface of an object that refers to its potential to emit radiation. radiation. photo source Adding an emissive layer to one side of a material in a vertical air cavity increases R-value increases : photo source: Infrared Thermal Imaging photo source: Infrared Thermal Imaging photo source: ...
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