heat and thermodynamics
TRANSCRIPT
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H t d Th d iHeat and Thermodynamics
Physics Rapid Learning Series
Rapid Learning Centerwww.RapidLearningCenter.com/© Rapid Learning Inc. All rights reserved.
Wayne Huang, Ph.D.Keith Duda, M.Ed.
Peddi Prasad, Ph.D.Gary Zhou, Ph.D.
Michelle Wedemeyer, Ph.D.Sarah Hedges, Ph.D.
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Learning Objectives
Temperature: Understand the definition of temperature.
By completing this tutorial, you will learn:
Energy in Thermal Processes. Understand the relationship between heat and internal energy.Thermal Processes in your world. Global warming and greenhouse gasses.
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Laws of Thermodynamics. Apply the laws of thermodynamics to real life problems such as engines and human metabolism.
Concept MapPhysics
Studies
Previous content
New content
Thermodynamics
The field of
Matter and EnergyMatter and Energy
Heat Energy
One type of energy is
2nd Law1st LawExpansionThermal
Expansion
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Heated objects Heated objects expand
Energy is
destroyed
Energy is neither created nor
destroyed
Entropy
increases
Entropy always
increases
States thatStates that States that
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ThermodynamicsThermodynamics and Thermal Expansion
Thermometers and Temperature Scales. Thermal expansion of Solids and Liquids
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Thermal expansion of Solids and Liquids
Thermodynamics and Temperature
How does temperature affect materials and how is this used to measure temperature?
Linear expansion
Volume expansion
The physics of thermostats
The physics of
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thermometers
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Definition - Thermodynamics
Thermodynamics - The branch of physics that deals with conversionsphysics that deals with conversions between heat energy and other forms of energy.
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Thermal ExpansionAs solids and liquids are heated, they expand because their molecules gain kinetic energy and move around more.
Expansions of solids can be predicted using the linear expansion equation.
Expansions of liquids can be predicted using the volume expansion equation.
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p q
The ticking noises heard after a hot car is shut down are due the shape changes of engine parts as they cool.
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Linear Expansion Equation
The linear expansion equation is of the form:
Where:
L = length of objectT = temperature
∆TαL∆L ××=
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pα = coefficient of linear expansion [units = per degree]∆ = Greek letter “delta”, change in.
Volume Expansion Equation
Similarly, the volume expansion equation is of the form:
∆TβV∆V ××=
Where:
V = volume of objectT = temperature
∆TβV∆V ××=
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ß = coefficient of volume expansion [units = per degree]∆ = Greek letter “delta”, change in.
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Expansion Coefficients
Note the units of the expansion coefficients:
1∆Lα ×= 1∆Vβ ×=
The linear and volume expansion coefficients are
Thus, length and volume cancel. Both expansion coefficients have units of “per degree”
∆TLα ×
∆TVβ ×
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The linear and volume expansion coefficients are related to each other by the following relationship:
Typical values for expansion coefficients may be found in your physics textbook.
α3β ×=
Sample Problem: Thermal ExpansionQuestion: How much will the volume of a 2 cm3 aluminum cube change if the temperature changes from 12°C to 32°C? The linear expansion coefficient, α, of aluminum is 23x10-6/°C.
S l tiSolution:
Step 1: Use the volume expansion equation.∆V = Vß * ∆T
Step 2: Calculate ∆T.∆T = Tfinal –Tinitial = 32°C – 12°C = 20°C
St 3 C l l t ß
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Step 3: Calculate ß.ß = 3α = 3 * 23 x10-6/°C = 69 x 10-6/°C
Step 4: Plug in the values.∆V = Vß * ∆T = (2cm3) * (69x10-6/°C) * (20°C)
Answer: ∆V = 2.76x10-3 cm3
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Note - Units and Temperature Change
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Thermostats - 1Thermostats are designed to measure temperature by applying the principle of linear expansion.
A thermostat uses two fused strips of metal, each with a different linear expansion coefficient. As the strip heats up one strip lengthens faster so the strips
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bend away from the longer strip.Typically the bent strips will make contact with a circuit.
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Thermostats - 2Imagine a thermostat made up of two metal strips as shown below. The yellow strip has a larger linear expansion coefficient.
Heat
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Heat
The strip bends towards the side with the lower linear expansion coefficient.
ThermometersThermometers are designed to measure temperature by applying the principle of thermal expansion.Most home thermometers use mercury or alcohol inside a narrow glass tube. As the temperature rises, both the glass and liquid expand.The liquid has a higher volume
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The liquid has a higher volume expansion coefficient than the glass tube so it rises within the tube to tell you the temperature.
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Thermal Expansion: Stop-And-Think
Question: Why would engineers need to take thermal expansion into account in the design of bridges?
On hot days, the metal supports of a bridge will expand and buckle if the bridge is not designed to expand. Similarly, the bridge will crack as the supports shrink on a cold day.
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Energy in Thermal Processes
Heat and Internal EnergySpecific Heat
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Specific HeatCalorimetryLatent Heat and Phase ChangeEnergy TransferGlobal Warming and Greenhouse Gasses
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System vs. Surroundings
Calculations of heat and internal energy are based on the definition of a system and its surroundings.
System - The system refers to the object being studied.
Surroundings -
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gEverything not included in the system.
Example - System v. Surroundings
Example: a beaker
The system refers to the contents of the beaker.
Th di f
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The surroundings refer to everything else.
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Heat and Internal Energy Equation
Internal energy of a system is defined as: 1) The ability to do work on an object.2) The ability to transfer heat to an object.
U = q + w
Heat
Heat energy of the system
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q
Internal Energy Work
System internal energy
Work done on the system or energy available for the
system to do work.
Definition - Energy Units
Calorie (cal) - Heat is measured in calories. A calorie is the amount of heat required to heat 1 gram of water 1°C.
Joule (J) - Energy is measured in Joules. One calorie (cal) is equal to 4 18J
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4.18J.
4.18 J = 1 cal
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Definition - State Function
State Function - n. A state function is a quantity of the object that isa quantity of the object that is dependent on the current state. A state function is independent of the path taken to reach the current state.
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Temperature, kinetic energy, and potential energy are all state functions.
Temperature is a State Function
Temperature is an example of a state function. Your coffee can be 40°C whether you heated it in the microwave or on the stove.
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Note: the Food Calorie
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Definition - Specific Heat
Specific Heat (CP) - n. Specific heat p ( P) pis a material property that defines the quantity of heat, q, required to raise 1g of a specific material 1°C.
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units of cp = [cal/gram*K]
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Specific Heat Equation
T T
The specific heat may be used as follows:
Q = CPm * ∆T
Specific Heat Change in Temperature
Tfinal - Tinitial(K) or (°C)
Proportionality ConstantJ/g*K or cal/g*K
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P
HeatTotal Heat Energy
(J)
MassMass of Object
(g)
Interpretation of Specific HeatWhat are the practical implications of specific heat?
High Specific Heat Low Specific Heat
Large amount of energy to change temperature
Small amount of energy can change temperature
Heats up slowly
Cools down slowly
Small temperature changes with condition changes
e g Water Cast iron
Heats up quickly
Cools down quickly
Quickly readjusts to new conditions
e g Air aluminum foil
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e.g. Water, Cast-iron e.g. Air, aluminum foil
A pool takes a long time to warm up and remains fairly warm over night.The air warms quickly on a sunny day, but cools quickly at night
A cast-iron pan stays hot for a long time after removing from oven.Aluminum foil can be grabbed by your hand from a hot oven because it cools so quickly
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Specific Heat and Heat Capacity
Thus, if the specific heat and the mass of any object are known, then the amount of heat need to raise the temperature by any given amount may be calculated.
The heat capacity C of an object may be determined from its specific heat CP and its mass m as follows:
CmC ×
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C = heat capacityCP = specific heat
m = mass of object
pCmC ×=
Definition - Heat Capacity
Heat Capacity (C) - The amount of heat Q required to raise the temperature of an object by ∆T is given by:
Q = C * ∆T
Heat Capacity
Proportionality Constant
J/K or cal/K
object by ∆T is given by:
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Q = C * ∆T
Heat Change in TemperatureTfinal - Tinitial(K) or (°C)
Total Heat Energy(J)
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Definition - Enthalpy (H)
Enthalpy of Reaction (H) - The enthalpy of a reaction is the heat energy (J) gained or lost during a reaction. Enthalpy is a state function.during a reaction. Enthalpy is a state function.
ReactionBonds of reactants broken. Bonds of products formed.
Bonds of reactants have hi h th
Bonds of products have higher energy than
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higher energy than bonds of products.
higher energy than bonds of reactants.
Exothermic Endothermic
Heat given off by system
Heat absorbed by
system
Endothermic vs. Exothermic
Exothermic - If the system looses net heat during a reaction, the reaction is g ,exothermic.
Endothermic - If the system gains net
∆Hsystem = Hfinal – Hinitial < 0
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y gheat energy during a reaction, the reaction is endothermic.
∆Hsystem = Hfinal – Hinitial > 0
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Definition - Calorimetry
Calorimetry. n. Calorimetry is aCalorimetry is a laboratory technique used to determine the amount of heat Q taken up or given off
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by a reaction.
Procedure - Calorimetry
A small beaker is immersed in an insulated 1
How to determine the heat of a reaction using calorimetry.
beaker filled with a known amount of water.
The initial temperature of the water is noted.A known amount of the reactants are placed in the small reaction beaker.The temperature of the water is recorded at
1
2
3
4
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regular intervals as the reaction proceeds.
The change is temperature is plotted versus time.
4
5
6The heat given off by the reaction is calculated from the total change in temperature of the water.
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Example - Calorimetry
(1) Large, insulated beaker
(2) Water initial(2) Water, initial temperature Tinitial
(3) Reaction beaker
(4) Initiate reaction
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(5) Flow of heat energy to water
(6) Measure final water temperature, Tfinal
(7) Calculate ∆H from ∆Twater and the specific heat of water, CP.
Endothermic vs. Exothermic Reactions
Endothermic Exothermic
Heat moves from Heat moves from system surroundings to system
Internal energy of system increases
KEAVE of system molecules increases
Internal energy of system decreases
KEAVE of the system decreases
to surroundings.
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Temperature of the system increases
Bonds of reaction products have greater energy than bonds of reactants
Temperature of the system decreases.
Chemical bonds of reactants have greater energy than bonds of products.
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Stop and Think- Calorimetry
Question: If the temperature of the water in a l i t i t i i th ticalorimetry experiment rises, is the reaction
endothermic or exothermic?
Answer: The reaction is exothermic because heat energy is moving from the system (the reaction) to
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energy is moving from the system (the reaction) to the surroundings (the water).
Latent Heat and Phase Change
A rise in temperatureInternal energy increases
When a solid is heated it undergoes the following:
Internal energy increases
Molecules oscillate more rapidly
A phase change from solid to liquidMolecules oscillate so rapidly they loose their crystalline form
There is no increase in temperature during the transition
Another rise in temperature
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Another rise in temperature
A phase change from liquid to gasMolecules oscillate so rapidly they escape the intermolecular forces that bind them together and enter the gas phase
There is no increase in temperature during the transition.
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Heats of Fusion and Vaporization
The amount of heat Qfus needed to melt m grams of a substance is given by the following equation:
fusfus Lm∆Q ×=
Qfus = heat energy = [calories]m = mass = [grams]Lfus = Heat of Fusion = [calories/gram]
The amount of heat Qvap needed to vaporize m grams of a substance is given by the following equation:
fusfus Lm∆Q ×
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Qvap = heat = [calories]m = mass = [grams]Lvap = Heat of Vaporization = [calories/gram]
of a substance is given by the following equation:
vapvap Lm∆Q ×=
Latent Heat and Phase Change
Gas
Tem
pera
ture
of
Subs
tanc
e
Phase
Liquid Phase
Solid
Tboil
Tmelt
40/67Increase in Heat Energy, ∆Q
Solid Phase
QmeltQboil
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Example - Heats of TransformationQuestion: How much heat energy is required for 10 grams of ice at 0°C to transition to liquid? To evaporate? The heat of fusion for water is 333 KJ/kg. The heat of vaporization is 2256 KJ/kg.Solution:Solution:
1) Calculate the heat energy required to melt 10 grams (0.01Kg) of ice.Qfus = Lfus * m = 333 KJ/kg * 0.01 Kg = 3.33 KJ
2) Calculate the heat energy needed to raised the temperature of the water from 0°C to 100°C.
Q = 0.00418 KJ/g*°C * 10.0 g * (100-0)°C = 4.18 kJ
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3) Calculate the heat energy required to vaporize 10g of water.Qvap = Lvap * m = 2256 KJ/kg * 0.01Kg = 22.56 KJ
4) The total heat required is the sum of 1-3.Qtot = 3.33 KJ + 4.18 KJ + 22.56 KJ = 30.07 KJ
Answer: Qtot = 30.07 KJ
Stop-and-think- Sweating
Question: Why is sweating an effective cooling mechanism?
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Answer: As sweat evaporates from your skin it transitions from the liquid to the gas phase. This phase transition requires a transfer of heat energy from the skin to the liquid, cooling the skin.
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Global Warming & Greenhouse Gases1) Heat energy radiates from the
sun to the earth
2) Some of the heat energy is reflected off greenhouse gasesreflected off greenhouse gases in the earth’s atmosphere.
3) Some of the heat energy is reflected of the earth’s surface.
4) Some of the energy reflected off the surface is trapped by greenhouse gases in the
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atmosphere.
5) As the concentration of greenhouse gases is increased due to emissions from cars and factories, the temperature of the earth rises.
The Laws of Thermodynamics
The Zeroth Law of ThermodynamicsWork in Thermodynamic Processes
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Work in Thermodynamic ProcessesThe First Law of ThermodynamicsHeat, Engines, and the Second Law of
ThermodynamicsEntropyHuman Metabolism
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Heat Energy and the Laws of Thermodynamics
The Zeroth Law of Thermodynamics states that objects in thermal
ilib i t th
The laws of thermodynamics govern energy exchanges:
equilibrium are at the same temperature.
The internal energy of a system is the sum of the heat energy of the system and the work done on or by the system.
The 1st Law of Thermodynamics states that energy can change state but cannot
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gy gbe created or destroyed.
The 2nd Law of Thermodynamics states that the total entropy, or disorder, of the universe can increase or stay the same but never decrease. The universe tends towards disorder.
Definition: Thermal Equilibrium
Thermal Equilibrium - Two objects are thermal equilibrium are at the same qtemperature. That is, when put in contact they remain at the same temperature and no longer exchange net heat energy.
A and B areIf A is placed in The temperature
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A and B are said to be in
thermal equilibrium.
pcontact with B,
they both remain at 10C°.
The temperature of A is 10C° and the temperature
of B is 10C°
Note that if objects are in thermal equilibrium, their molecules also have the same average kinetic energy
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Zeroth Law of ThermodynamicsZeroth Law - If object A is in thermal equilibrium with object B and object C, then objects B and C are also in thermalthen objects B and C are also in thermal equilibrium.
A10 degrees
Celsius
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B C10 degrees
Celsius10 degrees
Celsius
Note that if B and C are in thermal equilibrium with A, then they must also be at 10 degrees Celsius.
Work in Thermodynamics
As mentioned before:
U = q +U = q + w
U = system internal energyq = heat energyw = work done on or by the system
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Pressure-Volume Work
For constant pressure:
∆W = -P * ∆V
∆W = work done on or by the systemP = pressure∆V = volume change of the system
One common type of application of this equation is in calculating pressure-volume work done by
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engines.
Applied Pressure-Volume Work
If the system expands, work is done by the system on the surroundings. The internal energy of the systeminternal energy of the system decreases.
If the system contracts, work is done
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by the surroundings on the system. The internal energy of the system increases.
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The First Law of Thermodynamics
Energy in a reaction may be changed from one form to another but it is neither
t d d t d ti fcreated nor destroyed – conservation of energy.
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Application of The 1st Law
This implies that:
Q Q
∆U = ∆q + ∆w
Change in Heat Energy
W k d t b
Qinitial -Qfinal
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Change in System Internal Energy
Work done to or by the system
Ufinal - Uinitial Win -Wout
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Work and the 1st Law
There are 4 special applications of the 1st Law:
1) Adiabatic: Q = 0, ∆U = -W
2) Constant-volume: W = 0, ∆U = Q
3) Cyclical: ∆U = 0, Q = -W
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4) Free expansion processes: ∆U = Q = W
Adiabatic WorkAn example of adiabatic work is a closed system with a piston. If pressure is applied rapidly to depress the piston, the volume of the system will decrease. Work W is done on the system by the surroundings, but there is
∆V ≥ 0 Q = 0, ∆U = -W = -P*∆V
is done on the system by the surroundings, but there is no change in heat Q so the change in internal energy ∆U is equal to –W.
W
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W
Vinitial Vfinal
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Constant-Volume WorkAn example of constant-volume work is heating a sealed cylinder. There is no volume change so the change in internal energy is equal to the change in heat energyenergy.
Tinitial Tfinal
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∆V = 0 so W = 0∆T ≥ 0 so Q ≥ 0
∆U = Q
Cyclical Work and EnginesIn an engine, heat is used to increase pressure leading to an increase in volume. The change in volume does work on the surroundings. The work energy is transferred back from the surroundings to compress the cylinder back to its original size and the cycle repeatsand the cycle repeats.
∆U = 0 Q W
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Q = -W
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Free Expansion WorkConnecting a pressurized system to a vacuum is an example of free-expansion work. The gas from the pressurized system will immediately and irreversibly expand to fill the vacuum The process occurs rapidlyexpand to fill the vacuum. The process occurs rapidly, but unlike adiabatic work the process is irreversible.
∆U = Q = W = 0
Heating an open beaker may be considered free-i k Th t th t t f th
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expansion work. The system, the contents of the beaker, are open to the surroundings so there can be no volume changes or heat exchanges.
Energy Calculations: Example
Question: If the volume of a sealed balloon increases from 10mL to 100mL when heated and then shrinks back to 10mL as it cools, what
f tapplication of the 1st Law is this?
Constant-volume
Adiabatic
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Cyclical
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Energy Calculations: Answer
Question: If the volume of a sealed balloon increases from 10mL to 100mL when heated and then shrinks back to 10mL as it cools, what
f tapplication of the 1st Law is this?
Constant-volume
Adiabatic
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Cyclical
Definition - Entropy
Entropy (S) - n. Entropy is a measure of the disorder of a system or its surroundings.surroundings.
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A landfill possesses lots of entropy!
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The 2nd Law of ThermodynamicsThe 2nd Law of Thermodynamics states that the total entropy of the universe can never decrease, it can only increase or stay the same.
∆Stotal ≥ 0
Note that the entropy of the system may decrease, so long as the entropy of the s rro ndings increases b an eq al or greater
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surroundings increases by an equal or greater amount.
Thus, ∆Ssys + ∆Ssur ≥ 0
Relative Entropies
Low entropy states are more organized than high entropy states.
Lower Entropy Higher Entropy
Your clean bedroom Your room after finals
Solids Liquids
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Liquids
Salt tablets in water
Gases
Dissolved salt in water
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Definition - Engine EfficiencyIn a real combustion engine such as your car, some of the work energy is lost to the surroundings as heat rather than transferred to pressure-volume work which moves a piston.
The efficiency of an engine is the ratio of the heat energy put in (for example by combustion) to the work output by the engine (such as movement of the wheels of a car).
ε = Wout / Qin
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If the efficiency of an engine is 0.7, the 30% of the heat from combustion is lost as heat to the surroundings. In the real world, this would be a very efficient engine!!!
Entropy and Human Metabolism
Question: Metabolic processes in the body, such as the building of proteins and DNA, lead to increased order. How does this not violate the 2nd Law?
Answer: Although the building of structural proteins in the body lead to increased order in the system, which is the body, they lead to increased disorder in the surroundings through loss of heat.
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surroundings through loss of heat. Metabolic processes are usually linked to a catabolic process such as the combustion of glucose or the flow of ions down a concentration gradient.
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Entropy: Stop-and-Think QuestionQuestion: When an ice cube melts, does system entropy increase, decrease, or remain the same?
Pick the best answerPick the best answer.A. Remain the same because there is no reaction, only a state change.
B. Increase because the molecules are changing
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g gfrom an ordered crystalline state to a disordered liquid state.C. Decrease because heat is absorbed to melt the ice.
Entropy: Stop-and-Think AnswerQuestion: When an ice cube melts, does system entropy increase, decrease, or remain the same?
Pick the best answerPick the best answer.A. Remains the same because there is no reaction, only a state change.
B. Increases because the molecules are changing
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g gfrom an ordered crystalline state to a disordered liquid state.C. Decreases because heat is absorbed to melt the ice.
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Entropy is a Entropy is a
Expansion coefficients
measure how
Expansion coefficients
measure how
The 2nd Law states that net
entrop can
The 2nd Law states that net
entrop can
Learning Summary
Heat energy mayHeat energy may
measure of disorder.
measure of disorder.
measure how much objects expand when
heated.
measure how much objects expand when
heated.
entropy can never
decrease.
entropy can never
decrease.
The 1st LawThe 1st Law
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Heat energy may be gained or lost
to the surroundings during work.
Heat energy may be gained or lost
to the surroundings during work.
The 1 Law states that
energy is never created or destroyed.
The 1 Law states that
energy is never created or destroyed.
CongratulationsY h f ll l t dYou have successfully completed
the tutorial
Heat and Thermodynamics
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