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2053Lecture13 - Temperature Heat& Transfer Mechanisms...

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Unformatted text preview: Temperature, Heat & Transfer Mechanisms Transfer Topics of Discussion Topics Temperature Scales & Conversions Heat & Units of Heat Types of Heat Transfer Mechanisms Thermal Expansion Latent & Specific Heat Energy Conservation Three Temperature Scales Three The Fahrenheit temperature scale is The Fahrenheit based on the freezing and boiling points of water. Water freezes at 32˚F and boils at 32 212˚F. 212˚F The Celsius temperature scale is based on The Celsius the freezing and boiling points of water. Water freezes at 0˚C and boils at 100˚C. 100˚C The Kelvin temperature scale is used in The Kelvin low temperature physics. low Temperature Conversion Temperature Let F, C and K be the temperature in Let Fahrenheit, Celsius and Kelvin. Then we can convert from any temperature scale to any other by using the following equations. any 9 F = C + 32 5 C = K − 273.15 5 C = ( F − 32 ) 9 Definition of Heat Definition The energy transferred, called the The heat Q, iis the result of a temperature s difference between a thermodynamic system and its surroundings. system Surroundings Q Q Thermodynamic System Units of Heat Units The unit of heat is the Joule ( J ). The The calorie is the energy necessary to raise the he calorie temperature of 1g of water from 14.5˚C to 14.5 15.5˚C. 15.5 The energy required to raise the temperature of The 1lb of water from 63˚F to 64˚F is called the 63˚ 64˚ BTU. BTU The mechanical equivalent of heat is given by The 1cal = 4.186 J. Heat Transfer Mechanisms Heat Thermal conduction is a process that directly transfers heat through a substance. substance. Heat that is transferred by the movement Heat of a substance is called convection. convection Radiation is a process that transfers heat by means of electromagnetic waves. by Thermal Conduction of a Slab Thermal Let ∆x be the thickness of a slab and A its Let ∆x cross-sectional area. Suppose that the temperature on each side of the slab is TH and TC such that TH >TC. Empirically, the rate of Empirically, energy transfer P is proportional to the crosssectional area and the temperature difference, sectional and inversely to the thickness, that is P ∝ AΔT/Δx. For a slab of thickness L, the the energy transfer by means of conduction is given by P = kA(TH – TC)/L, where k is the thermal where kA conductivity in SI units J/sm˚C. conductivity J/sm˚C Radiation Radiation Stefan’s Law states that the rate at which an Stefan’s object emits energy is proportional to the fourth power of its temperature, and is given by –5 P=σAeT4, where σ = 5.669 × 10 –5 W/m2K2, e is P= called the emissivity and A is the surface area of emissivity the object. the If the initial and final temperatures are T0 and T, If then the energy gain or loss is given by P=σAe(T4–T04). P= An ideal absorber is a material with e=1. An ideal An ideal reflector is a material with e=0. An ideal Thermal Expansion Thermal Thermal expansion of an object occurs when a change in volume is the result of a temperature change. For a rod of original length L0, the change in length ΔL is given the by ΔL = L – L0 = αL0(T – T0), where α is by the coefficient of expansion whose SI unit coefficient is ˚C–1. ˚C Latent Heat Latent The amount of heat Q that must be added The or removed from a substance of mass m in order to change its state is given by order Q = mL, where L is called the latent heat latent mL of the substance. The SI unit of the latent heat of a substance is J/kg. J/kg Types of Latent Heat Types The latent heat of fusion Lf refers to the The latent change between the solid and liquid phases of a substance. phases The latent heat of vaporization Lv refers to The latent the change between the liquid and gas phases of a substance. phases The latent heat of sublimation Ls refers to The latent the change between the solid and gas phases of a substance. phases Specific Heat Specific The amount of heat Q added to a The substance of mass m in order to change its temperature by ∆T is called the specific ∆T heat c, and is given by c = Q / mΔT, and whose SI unit is J/kg˚C. J/kg˚C A General Form of the Conservation of Energy Conservation The total energy before equals the total energy The after, that is after, KEafter + PEafter + Qoutput = = KEbefore + PEbefore + Qinput where Qinput and Qoutput are the energies added where or removed from the thermodynamic system. or ...
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