Thermal Physics by liaoqinmei

VIEWS: 3 PAGES: 43

									               Thermal Physics
          Thermometric property - a
characteristic of an object that varies
                     with temperature
         Heat versus Temperature
 Temperature: average kinetic energy of
  each particle in a substance
 degree of hotness or coldness
 indicates direction of heat flow
          Heat versus Temperature
 Heat - total thermal energy (internal energy)
  absorbed or transferred
 Internal energy - KE and PE
 PE due to bond energy (chemical energy stored
  in bonds) and electromagnetic forces between
  particles
 Example: bucket vs thimble of water at 100°C
   Measurement of Temperature
 Alcohol vs. mercury thermometers
 Alcohol - used at lower temperatures
 Alcohol freezes at -114°C and boils at
  78.5°C
 Mercury - used at higher temperatures
 Mercury freezes at -39°C and boils at
  357°C(675F)
                       Thermometers



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     Constructing Thermometers
 Bulb - thin glass for quick heat transfer
 Clinical thermometer - restriction
  prevents backflow
 Vacuum above liquid
 Calibrated using two fixed points (0C and
  100C), mark scale evenly
     Thermometers-constructed
  using thermometric properties
 Expansion of liquid in capillary tube
 Resistance in wire (thermistor)
 Different rates of expansion of metals
  (bimetallic strips)
 Volume of gas at constant pressure (gas
  expansion rate is linear)
 Color change of solid when
  heated(pyrometers
                           Conversions
 K = °C + 273
 °F = 9/5 °C + 32
 Absolute zero - no motion of particles in
  substance at 0 K
 Water freezing point 32°F, 0°C, 273 K
 Water boiling point 212°F, 100°C, 373 K
 Room temperature 20°C
           Relating temperature to
               velocity of particles
 Kinetic energy = 1/2 mv2
 Kinetic energy = 3/2 kT
 k = Boltzmann constant 1.38 x 10-23 J/K
 T = Temperature in Kelvin
 m = mass in kilograms
 v = root mean square velocity (rms)
                            rms velocity
 Number of particles at different speeds is
  not a normal distribution - some particles
  move VERY fast
 Peak velocity is most probable velocity
 Vrms = (vav2)1/2
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      Heat Transfer - Conduction
 Occurs in solids, liquids, gases
 Temperature difference causes transfer of
  thermal energy from hot to cold by
  particle collision without net movement
  of substance
      Heat Transfer - Convection
 Occurs in fluids (liquids and gases)
 Temperature difference causes mass
  movement of fluid particles - density
  differences
 Convection cells (convection currents)
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        Heat Transfer - Radiation
 No medium required
 Heat travels as electromagnetic waves
 Most reflected at atmosphere, some is
  absorbed
                            Heat Capacity
 Objects with a high heat capacity take in heat at
  a slower rate - heat slowly, and also cool slowly
 Heat capacity Q/T units are JK-1
 Specific heat capacity - heat capacity per unit
  mass - heat required to raise the temperature of
  1 kg by one Kelvin
 Variable for specific heat….c
                        Heat Equation
 Heat required to produce a temperature
  change
 Q = mcT
 Can heat a substance using electrical
  energy
 Electrical energy = VIt = mcT
                                Mixtures
 Heat lost by one substance equals heat
  gained by the other substance - total heat
  gained or lost by the system is 0
 Calorimeter - allows minimal energy loss
  to surroundings
Calorimeter



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                     Example #1
Mix 100 grams of water at 20°C
 and 200 grams of water at 40°C.
 Find the final temperature of the
 water.
 Solution:
 Heat lost by one sample plus heat gained by
  other sample equals 0
mcT  mcT  0
                    1    1
0.1kg(4180 kJkg K )(Tf  20) 
                  1 1
   0.2kg(4180 kJkg K )(Tf  40)  0
 418Tf  8360 836Tf  33440  0

              Tf  33C
                    Example #2
A 100 gram block of Ag at
 100°C is placed in 100 grams
 of water at room temperature.
 Find the final temperature.
 Solution:
 Heat lost by silver plus heat gained by water
  equals 0
mcT  mcT  0
                   1   1
0.1kg(235 kJkg K )(Tf 100 ) 
                  1 1
   0.2kg(4180 kJkg K )(Tf  20)  0
 23.5Tf  2350 418Tf  8360  0

              Tf  24C
                         Kinetic Theory
 All matter is composed of extremely
  small particles
 All particles are in constant motion
 If particles collide with other particles,
  KE is conserved
 A mutually attractive force exists
  between particles
                                    Matter
 Matter - has mass and occupies space
 Four phases - solid, liquid, gas, plasma
 Plasma - made by heating gas atoms until
  they ionize - separate into positively and
  negatively charged particles - sun, other
  stars composed of plasma
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Characteristic   Solid         Liquid       Gas
Shape            definite      variable     variable
Volume           definite      definite     variable
Compressibility almost          slightly     highly
                 incompressible compressible compressible

Diffusion        small         slow         fast
Density          highest       high         low
Bonds            strong        relatively   weak
                               strong
Particle         closely       larger       very large
spacing          packed        spaces       spaces
PE               high          higher       highest
Thermal          least         middle       most
energy
                                Latent Heat
 Latent heat of transformation: heat required to
  change 1kg of a substance from one phase to
  another
 Equation: Q = mL
 No temperature change during a phase change -
  heat is used to change PE - heat needed to break
  bonds, heat released when bonds are formed
                             Latent Heat
 Latent heat of fusion: solid to liquid (heat
  is absorbed) or liquid to solid (heat is
  released)
 Latent heat of vaporization: liquid to gas
  (heat is absorbed) or gas to liquid (heat is
  released)
 Heating curves:
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                     Example #1
Find the heat required to melt
 10 kg of gold. (latent heat of
 fusion = 6.3 x 104 JKg-1)
                   Solution
Q  mH f
                   1
 10kg(6.3x10 Jkg )
             4


       6.3x10 J
             5
                     Example #2
Find the heat required to
 vaporize 100 grams of lead
 (latent heat of vaporization =
 2.04 x 104 JKg-1)
                  Solution

      Q  mHv
                           1
        0.1kg(2.04x10 Jkg )
                       4



          2.04x10 J
                  3


 
                            Example #3
 Find the heat required to change 2 kg of
  ice at -10°C to steam at 120°C
 Specific heat of ice 2060 Jkg-1K-1
 Latent heat of fusion 3.34 x 105 Jkg-1
 Specific heat of water 4180 Jkg-1K-1
 Latent heat of vaporization 2.26 x 106
  Jkg-1
 Specific heat of steam 2020 Jkg-1K-1
                   Solution:
Heat ice from -10°C to 0°C
Melt ice
Heat water from 0°C to 100°C
Evaporation of water
Heat vapor from 100°C to 120°C
Q  mcT  mH f  mcT  mHv  mcT
            1   1
2 kg(2060Jkg K )(10C) 
                   1
    2kg(3.34x10 Jkg ) 
               5

                       1   1
          2 kg(4180Jkg K )(100C) 
                               1
                2kg(2.26x10 Jkg ) 
                           6

                                 1       1
                 2 kg(2020Jkg K )(20C) 
                                    6
     
                      5.5x10 J
                               Example #4
 Heat is added to a mass of 5 kg at room
  temperature (20°C) at a rate of 500 watts for 1
  minute until the substance begins to melt at
  300°C. The substance takes 3 minutes to melt.
 A. Sketch a graph of temperature vs. time
 B. Find the specific heat of the solid
 C. Find the latent heat of fusion
                    Solution-Part A.


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300°C




 20°C
               1     4
        Times and temperatures on graph?
         B. Q  mcT
           500watts x 60s  5kg(c)(280C)
                       1   1
           c  21.4Jkg K
         C. Q  mH f
            500watts(180sec) 5kg(H f )
                                 1
               Hf  1.8x10 Jkg
                             4



    
                      Evaporation
        Evaporation takes place at all
temperatures and results in the cooling
                             of a liquid
                           Evaporation
 Change from liquid state to gaseous state,
  occurs at a temperature below boiling
  point
 Particles near surface have enough KE to
  overcome attractive forces of nearby
  particles, lowers KE of substance

								
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