Theories of Heat. all substances contain tiny, constantly moving particles Kinetic Theory.
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Transcript of Theories of Heat. all substances contain tiny, constantly moving particles Kinetic Theory.
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Theories of Heat
Theories of Heat
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• all substances contain tiny, constantly moving particles
Kinetic TheoryKinetic Theory
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• the sum of the kinetic energy of the random motion of the particles
• average kinetic energy of particles is proportional to the temperature
Thermal EnergyThermal Energy
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• matter can be subdivided• diffusion: the spreading of
a substance through particle motion alone
• much faster in gases than liquids
DiffusionDiffusion
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• gas molecules move faster than liquid molecules
• easily demonstrated with substances like ammonia and bromine vapor
DiffusionDiffusion
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• affects microscopic particles
• caused by random, asymmetrical collisions of liquid or gas molecules against the particles
Brownian MotionBrownian Motion
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• claimed heat is a material fluid (caloric) that flows from hot bodies to cold bodies
• evidence for the kinetic theory eventually destroyed this idea
Caloric TheoryCaloric Theory
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• studied conversion of mechanical energy to thermal energy
• mechanical equivalent of heat
Joule’s ResearchJoule’s Research
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• various experiments gave slightly different results
• currently accepted value of the mechanical equivalent of thermal energy:
Joule’s ResearchJoule’s Research
4.186 N·m = 1 cal (at 15°C)
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Joule’s ResearchJoule’s Research
In his honor, the N·m was renamed the “joule,” the SI
derived unit of energy, work, and heat.
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Thermal Energy and Matter
Thermal Energy and Matter
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Heat CapacityHeat Capacity• It is not always the hottest
object that has the greatest amount of thermal energy!
• Heat Capacity (C): amount of thermal energy required to raise the temperature of entire object 1°C.
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Heat CapacityHeat Capacity• Heat (Q): amount of
thermal energy added to or taken from a system
• SI unit: J/°C
C =Δt
Qobject
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Heat CapacityHeat Capacity• Δt = change in temperature• technically incorrect to say
that a system has a certain amount of heat
C =Δt
Qobject
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Specific HeatSpecific Heat• analogous to the specific
density of a material• specific heat capacity =
heat capacity per gram
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Specific HeatSpecific Heat• specific heat (csp) of a
substance is the amount of thermal energy required to raise the temperature of 1 g of the substance by 1°C
• SI units: J/g·°C
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Specific HeatSpecific Heat• specific heat of water:
• 1 cal/g·°C (at 15°C)• 4.18 J/g·°C (near room
temperature)
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Specific HeatSpecific Heat• to calculate specific heat:
csp = mΔt
Q
• and by definition:
C = m(csp)
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ConservationConservation• when an object gains heat,
its surroundings lose that same amount of heat
• heat-balance equations:
Qsystem = -Qsurroundings
Qsystem + Qsurroundings = 0 J
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ConservationConservation• adiabatic vessel: one that
allows no heat to enter or leave its contents
• calorimeter: container designed to minimize the exchange of thermal energy with its surroundings
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ConservationConservation• In computational work, it
must be remembered to include the calorimeter’s gain or loss of heat.
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Heat and Phase Transitions
Heat and Phase Transitions
• an amount of heat is required to melt a solid or to vaporize a liquid• adding this heat will not
change the temperature while melting or vaporizing occurs
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Heat and Phase Transitions
Heat and Phase Transitions
• latent heat of fusion (Lf): amount of thermal energy required to melt 1 kg of the substance at its melting point
Lf = mQmelt
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Heat and Phase Transitions
Heat and Phase Transitions
• latent heat of vaporization (Lv): amount of thermal energy required to vaporize 1 kg of the substance at its boiling point
Lv = mQboil
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Heat and Phase Transitions
Heat and Phase Transitions
• Example 15-6: Why are there five parts to this?
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Mechanisms for Heat Transfer
Mechanisms for Heat Transfer
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ConductionConduction• the flow of thermal energy
from one object to another through contact
• conductors: materials that conduct thermal energy easily
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ConductionConduction• conductors are more likely
to have free electrons• insulators: materials that
do not conduct thermal energy easily
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ConvectionConvection• the transfer of thermal
energy from one place to another by the physical translation of particles between locations
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ConvectionConvection• most liquids and gases rise
when heated• water has unusual
properties
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RadiationRadiation• travels without the use of
an intervening medium• converted to thermal
energy when absorbed by matter
• all objects radiate thermal energy
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RadiationRadiation• Stefan’s law gives the
correspondence between absolute temperature (T) and the power of its radiant energy (S):
S = σT4
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RadiationRadiation• Stefan-Boltzmann constant:
σ = 5.67 × 10-8 W/(m2·K4)• S is proportional to
temperature to the fourth power
S = σT4
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RadiationRadiation• black objects absorb and
radiate radiant energy better than other objects
• blackbody: a perfect (ideal) radiator and absorber