Chapter 11Chapter 11
Energy in Thermal Energy in Thermal ProcessesProcesses
Graph of Ice to SteamGraph of Ice to Steam
FeedbackFeedback I am having a slight time understanding I am having a slight time understanding
the connection between Kthe connection between KBB constant and R. constant and R. For instance in problem two, when For instance in problem two, when
calculating rms speed of a nitrogen calculating rms speed of a nitrogen molecule i used the mass of Nmolecule i used the mass of N22 because because nitrogen is diatomic however i am not nitrogen is diatomic however i am not completely sure this was correct.completely sure this was correct.
Please explain why vapor bubbles in a pot Please explain why vapor bubbles in a pot of boiling water get larger as they of boiling water get larger as they approach the surface. – Problem 10.35approach the surface. – Problem 10.35
Units of HeatUnits of Heat
CalorieCalorie An historical unit, before the An historical unit, before the
connection between thermodynamics connection between thermodynamics and mechanics was recognizedand mechanics was recognized
A A caloriecalorie is the amount of energy is the amount of energy necessary to raise the temperature of necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C .1 g of water from 14.5° C to 15.5° C .
A Calorie (food calorie) is 1000 calA Calorie (food calorie) is 1000 cal 1 cal = 4.186 J1 cal = 4.186 J
This is called the This is called the Mechanical Mechanical Equivalent of HeatEquivalent of Heat
Specific HeatSpecific Heat
Every substance requires a unique Every substance requires a unique amount of energy per unit mass to amount of energy per unit mass to change the temperature of that change the temperature of that substance by 1° Csubstance by 1° C
The The specific heat, c,specific heat, c, of a substance of a substance is a measure of this amountis a measure of this amount
Tm
Qc
Units of Specific HeatUnits of Specific Heat
SI unitsSI units J / kg °CJ / kg °C
Historical unitsHistorical units cal / g °Ccal / g °C
Heat and Specific HeatHeat and Specific Heat
Q = m c ΔTQ = m c ΔT ΔT is always the final temperature ΔT is always the final temperature
minus the initial temperatureminus the initial temperature When the temperature increases, ΔT When the temperature increases, ΔT
and ΔQ are considered to be positive and ΔQ are considered to be positive and energy flows into the systemand energy flows into the system
When the temperature decreases, ΔT When the temperature decreases, ΔT and ΔQ are considered to be negative and ΔQ are considered to be negative and energy flows out of the systemand energy flows out of the system
Warming IceWarming Ice
Start with one Start with one gram of ice at –gram of ice at –30.0º C30.0º C
During A, the During A, the temperature of temperature of the ice changes the ice changes from –30.0º C to from –30.0º C to 0º C0º C
Use Q = m c ΔTUse Q = m c ΔT
Consequences of Different Consequences of Different Specific HeatsSpecific Heats
Water has a high Water has a high specific heat specific heat compared to landcompared to land
On a hot day, the On a hot day, the air above the land air above the land warms fasterwarms faster
The warmer air The warmer air flows upward and flows upward and cooler air moves cooler air moves toward the beachtoward the beach
Melting IceMelting Ice
Once at 0º C, the Once at 0º C, the phase change phase change (melting) starts(melting) starts
The temperature The temperature stays the same stays the same although energy although energy is still being is still being addedadded
Use Q = m LUse Q = m Lff
Phase ChangesPhase Changes
A A phase changephase change occurs when the occurs when the physical characteristics of the physical characteristics of the substance change from one form substance change from one form to anotherto another
Common phases changes areCommon phases changes are Solid to liquid – meltingSolid to liquid – melting Liquid to gas – boilingLiquid to gas – boiling
Phases changes involve a change Phases changes involve a change in the internal energy, but in the internal energy, but no no change in temperaturechange in temperature
Latent HeatLatent Heat During a phase change, the During a phase change, the
amount of heat is given asamount of heat is given as Q = m LQ = m L
L is the L is the latent heatlatent heat of the of the substancesubstance Latent means hidden or concealedLatent means hidden or concealed
Choose a positive sign if you are Choose a positive sign if you are adding energy to the system and a adding energy to the system and a negative sign if energy is being negative sign if energy is being removed from the systemremoved from the system
Warming WaterWarming Water
Between 0º C and Between 0º C and 100º C, the 100º C, the material is liquid material is liquid and no phase and no phase changes take placechanges take place
Energy added Energy added increases the increases the temperaturetemperature
Use Q = m c ΔTUse Q = m c ΔT
Boiling WaterBoiling Water
At 100º C, a At 100º C, a phase change phase change occurs (boiling)occurs (boiling)
Temperature does Temperature does not changenot change
Use Q = m LUse Q = m Lvv
Heating SteamHeating Steam
After all the water is After all the water is converted to steam, converted to steam, the steam will heat upthe steam will heat up
No phase change No phase change occursoccurs
The added energy The added energy goes to increasing the goes to increasing the temperaturetemperature
Use Q = m c ΔTUse Q = m c ΔT
CalorimetryCalorimetry
Analysis performed using a calorimeterAnalysis performed using a calorimeter Conservation of energy applies to the Conservation of energy applies to the
isolated systemisolated system The energy that leaves the warmer The energy that leaves the warmer
substance equals the energy that enters substance equals the energy that enters the waterthe water QQcoldcold = -Q = -Qhothot Negative sign keeps consistency in the sign Negative sign keeps consistency in the sign
convention of ΔTconvention of ΔT
ConductionConduction
The transfer can be viewed on an The transfer can be viewed on an atomic scaleatomic scale It is an exchange of energy between It is an exchange of energy between
microscopic particles by collisionsmicroscopic particles by collisions Less energetic particles gain energy Less energetic particles gain energy
during collisions with more energetic during collisions with more energetic particlesparticles
Rate of conduction depends upon the Rate of conduction depends upon the characteristics of the substancecharacteristics of the substance
Conduction exampleConduction example The molecules vibrate The molecules vibrate
about their equilibrium about their equilibrium positionspositions
Particles near the Particles near the flame vibrate with flame vibrate with larger amplitudeslarger amplitudes
These collide with These collide with adjacent molecules adjacent molecules and transfer some and transfer some energyenergy
Eventually, the energy Eventually, the energy travels entirely through travels entirely through the rodthe rod
Conduction, equationConduction, equation
The slab allows The slab allows energy to transfer energy to transfer from the region of from the region of higher higher temperature to temperature to the region of the region of lower lower temperaturetemperature
x
TTkA
t
QP ch
ConvectionConvection
Energy transferred by the Energy transferred by the movement of a substancemovement of a substance When the movement results from When the movement results from
differences in density, it is called differences in density, it is called natural conductionnatural conduction
When the movement is forced by a When the movement is forced by a fan or a pump, it is called fan or a pump, it is called forced forced convectionconvection
RadiationRadiation
Radiation does not require physical Radiation does not require physical contactcontact
All objects radiate energy All objects radiate energy continuously in the form of continuously in the form of electromagnetic waves due to electromagnetic waves due to thermal vibrations of the thermal vibrations of the moleculesmolecules
Rate of radiation is given by Rate of radiation is given by Stefan’s LawStefan’s Law
Radiation exampleRadiation example
The electromagnetic waves carry the The electromagnetic waves carry the energy from the fire to the handsenergy from the fire to the hands
No physical contact is necessaryNo physical contact is necessary
Radiation equationRadiation equation
P = σAeTP = σAeT44
P is the rate of energy transfer, in P is the rate of energy transfer, in WattsWatts
σ = 5.6696 x 10σ = 5.6696 x 10-8-8 W/m W/m22 K K44
A is the surface area of the objectA is the surface area of the object e is a constant called the e is a constant called the emissivityemissivity
e varies from 0 to 1e varies from 0 to 1 T is the temperature in KelvinsT is the temperature in Kelvins
Energy Absorption and Energy Absorption and Emission by RadiationEmission by Radiation
With its surroundings, the rate at With its surroundings, the rate at which the object at temperature T which the object at temperature T with surroundings at Twith surroundings at Too radiates is radiates is PPnetnet = σAe(T = σAe(T44 – T – T44
oo)) When an object is in equilibrium with When an object is in equilibrium with
its surroundings, it radiates and its surroundings, it radiates and absorbs at the same rateabsorbs at the same rate
Its temperature will not changeIts temperature will not change
Resisting Energy TransferResisting Energy Transfer
Dewar flask/thermos bottleDewar flask/thermos bottle Designed to minimize Designed to minimize
energy transfer to energy transfer to surroundingssurroundings
Space between walls is Space between walls is evacuated to minimize evacuated to minimize conduction and convectionconduction and convection
Silvered surface minimizes Silvered surface minimizes radiationradiation
Neck size is reducedNeck size is reduced
Global WarmingGlobal Warming
Greenhouse exampleGreenhouse example Visible light is absorbed and re-Visible light is absorbed and re-
emitted as infrared radiationemitted as infrared radiation Convection currents are inhibited by Convection currents are inhibited by
the glassthe glass Earth’s atmosphere is also a good Earth’s atmosphere is also a good
transmitter of visible light and a transmitter of visible light and a good absorber of infrared radiationgood absorber of infrared radiation
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