Thermal Physics (PowerPoint download)

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					Thermal Physics



      Chapter 11
States of Matter
 The state of matter of an object depends on
     Molecular separation
          Large in gases
          Close to the same for liquids and solids
     Intermolecular forces
          Increase from gas to liquid to solid
     Speed of Molecules
          Generally decrease from gas to liquid to solid
          Exceptions can occur if pressure changes
Temperature (Pass 1)
 General use definition:
      Temperature is a number assigned to an
       object to indicate its degree of warmth
           more useful than comparative warmth
           Measured with thermometer (more later)
 Initial definition:
      Objects are in thermal equilibrium if they are
       at the same temperature
Thermometers
 Thermometer: an physical system whose
 properties change predictably with
 temperature and is calibrated and marked off
 to assign numbers with those changes
     Changing properties can include:
          Volume of gas or liquid
          Length of a solid
          Electrical resistance of a conductor
          Light transmission of a crystal
Common Temperature Scales
 Fahrenheit:
     BP H2O = 212°F
     FP H2O = 32°F
 Celsius:
     BP H2O = 100°C
     FP H2O = 0°C
 Kelvin:
     BP H2O = 373.15 K
     FP H2O = 273.15 K
Temperature Scale Conversions
 Fahrenheit to Celsius:

        TC  9 TF  32 
             5

 Celsius to Kelvin:


         TK  TC  273 .15
Thermal Expansion of Solids
 Most solids expand when heated and contract when
  cooled
      Predictable in linear, area, and volume
 Mathematical analysis:
   Linear expansion

       L  L0T
       where α is the linear thermal expansion coefficient
         determined for each material experimentally

         units of °C
                     -1
Thermal Expansion of Solids (cont.)
     Volumetric expansion
       V  V0 T
      where β is volume thermal expansion coefficient and
             3
          same unit as α
          note equations have same form
Mechanical Equivalent of Heat
 Heat: energy transfer when a temp.
  difference exists between two objects
     Heat is measured in metric by the calorie
          Amount of heat required to raise the temp. of 1 g
           of water by 1°C.
     We’ll want to use SI joule to be consistant
          1 calorie = 4.187 joules
 Since heat is a form of energy, we can
  convert between heat and the other forms of
  energy we’ve used
Calorimetry and Change of Phase
 Heat capacity: amount of heat needed to change
  an object’s temp. by 1°C
 Heat required to change an object’s temp. by ΔT°C
  Q  mcT
  where
      Q is heat
      c is specific heat
Calorimetry and Change of Phase
(cont.)
 Heat of transformation: heat required or released
  as an object transitions between one phase and
  another
      Heat of fusion: heat for solid-liquid phase change
      Heat of vaporization: heat for liquid-gas phase
       change
 Heat absorbed or released during phase change
   Q  mL
  where L is heat of transformation
Heat Transfer
 Three methods
    Conduction:
       transfer w/o net motion of the material

       transfer through direct contact

       relatively slow

    Convection:
       transfer through mass motion or flow of fluid

       transfer through direct contact

       more rapid than conduction

    Radiation:
       transfer without contact or mass motion

       more rapid than convection
Rate of Heat Flow
 Conduction:
    Heat flows through a uniform material according to
     the following:
      Q
          KA
              T2  T1 
      t          L
       where ΔQ/Δt is time rate of heat flow, A is the area and L
       is the thickness of material, and K is the thermal
       conductivity (characteristic of material) in units of
       W/m·°C
Rate of Heat Flow (cont.)
      Effectiveness of insulation (resistance to heat flow)
       can be expressed as an R value
          L
       R
          K
           Used to rate home insulation, windows, and coats
 Radiation
    Rate of radiation is expressed as power

         P  eAT     4
        where σ = 5.67 × 10-8 W·m-2·K-4, e is the emissivity
        constant (dimensionless, between 0 and 1), and A
        is surface area of object

				
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posted:4/29/2012
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