Theories of Heat

Theories of Heat

• all substances contain tiny, constantly moving particles

Kinetic TheoryKinetic Theory

• 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

• matter can be subdivided• diffusion: the spreading of

a substance through particle motion alone

• much faster in gases than liquids

DiffusionDiffusion

• gas molecules move faster than liquid molecules

• easily demonstrated with substances like ammonia and bromine vapor

DiffusionDiffusion

• affects microscopic particles

• caused by random, asymmetrical collisions of liquid or gas molecules against the particles

Brownian MotionBrownian Motion

• 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

• studied conversion of mechanical energy to thermal energy

• mechanical equivalent of heat

Joule’s ResearchJoule’s Research

• 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)

Joule’s ResearchJoule’s Research

In his honor, the N·m was renamed the “joule,” the SI

derived unit of energy, work, and heat.

Thermal Energy and Matter

Thermal Energy and Matter

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.

Heat CapacityHeat Capacity• Heat (Q): amount of

thermal energy added to or taken from a system

• SI unit: J/°C

C =Δt

Qobject

Heat CapacityHeat Capacity• Δt = change in temperature• technically incorrect to say

that a system has a certain amount of heat

C =Δt

Qobject

Specific HeatSpecific Heat• analogous to the specific

density of a material• specific heat capacity =

heat capacity per gram

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

Specific HeatSpecific Heat• specific heat of water:

• 1 cal/g·°C (at 15°C)• 4.18 J/g·°C (near room

temperature)

Specific HeatSpecific Heat• to calculate specific heat:

csp = mΔt

Q

• and by definition:

C = m(csp)

ConservationConservation• when an object gains heat,

its surroundings lose that same amount of heat

• heat-balance equations:

Qsystem = -Qsurroundings

Qsystem + Qsurroundings = 0 J

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

ConservationConservation• In computational work, it

must be remembered to include the calorimeter’s gain or loss of heat.

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

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

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

Heat and Phase Transitions

Heat and Phase Transitions

• Example 15-6: Why are there five parts to this?

Mechanisms for Heat Transfer

Mechanisms for Heat Transfer

ConductionConduction• the flow of thermal energy

from one object to another through contact

• conductors: materials that conduct thermal energy easily

ConductionConduction• conductors are more likely

to have free electrons• insulators: materials that

do not conduct thermal energy easily

ConvectionConvection• the transfer of thermal

energy from one place to another by the physical translation of particles between locations

ConvectionConvection• most liquids and gases rise

when heated• water has unusual

properties

RadiationRadiation• travels without the use of

an intervening medium• converted to thermal

energy when absorbed by matter

• all objects radiate thermal energy

RadiationRadiation• Stefan’s law gives the

correspondence between absolute temperature (T) and the power of its radiant energy (S):

S = σT4

RadiationRadiation• Stefan-Boltzmann constant:

σ = 5.67 × 10-8 W/(m2·K4)• S is proportional to

temperature to the fourth power

S = σT4

RadiationRadiation• black objects absorb and

radiate radiant energy better than other objects

• blackbody: a perfect (ideal) radiator and absorber