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Unit 4 - Thermodynamics Chapters 9 and 10
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Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Dec 24, 2015

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Page 1: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Unit 4 - ThermodynamicsChapters 9 and 10

Page 2: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Chapter 9 - Heat

•Expectations:▫Learn the difference between

temperature and heat.▫Learn how different substances change

temperature or phase when energy is added to or removed from the substances.

Page 3: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Chapter 9 Preview• Section 1: Temperature & Thermal

Equilibrium▫Defining Temperature▫Measuring Temperature

• Section 2: Defining Heat▫Heat and Energy▫Thermal Conduction▫Heat and Work

• Section 3: Changes in Temperature and Phase▫Specific Heat Capacity▫Latent Heat

Page 4: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 1 – Temperature & Thermal Equilibrium•Objectives:

1. Relate temperature to the kinetic energy of atoms and molecules.

2. Describe the changes in the temperatures of two objects reaching thermal equilibrium.

3. Identify the various temperature scales, and convert from one scale to another.

Page 5: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Defining Temperature•Energy must either be added to or removed

from a substance in order to change its temperature.

•Temperature is proportional to the average kinetic energy of particles in the substance.

•The energies associated with atomic motion are referred to as internal energy, which is proportional to the substance’s temperature.▫It is due to both the random motions of its

particles and the potential energy that results from the distances and alignments between the particles.

Page 6: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Defining Temperature

•For an ideal gas, the internal energy depends only on the temperature of the gas.

•For nonideal gases, liquids, and solids, other properties contribute to the internal energy.

•U = internal energy•ΔU = change in internal energy•Thermal equilibrium = the state in

which two bodies in physical contact with each other have identical temperatures

Page 7: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Defining Temperature

•Thermal equilibrium is the basis for measuring temperature with thermometers.

•If the temperature of a substance increases, so does its volume = thermal expansion (ex. = bridges)

•Different substances undergo different amounts of expansion for a given temperature change.

•This is indicated by the coefficient of volume expansion, which is greatest in gases.

Page 8: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Measuring Temperature• In order for a device to be used as a

thermometer, it must make use of a change in some physical property that corresponds to changing temperature – the volume of a gas or liquid, the pressure of a gas at constant volume.

• Thermometers are calibrated using fixed temperatures.

• One reference point marks when the thermometer is in thermal equilibrium with a mixture of water and ice at one atmosphere of pressure.

Page 9: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Measuring Temperature

•This temperature is referred to as the ice point or melting point of water and is defined as 0ºC.

•The second reference point marks when the thermometer is in thermal equilibrium with a mixture of steam and water at one atmosphere of pressure.

•This temperature is referred to as the steam point or boiling point of water and is defined as 100ºC.

Page 10: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Measuring Temperature

•The remainder of the scale is divided into equally spaced units that represent the degrees between 0 and 100.

•The temperature scales most widely used today are the Fahrenheit, Celsius, and Kelvin scales.

•The U.S. still uses the Fahrenheit scale, but the Celsius scale is used by countries that utilize the metric system and within the sciences.

Page 11: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Measuring Temperature

•Celsius-Fahrenheit Temperature Conversion:

TF = TC + 32.0

•Celsius-Kelvin Temperature Conversion:

T = TC + 273.15

95

Page 12: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Measuring TemperatureScale Ice Point Steam Point

Fahrenheit 32ºF 212ºF

Celsius 0ºC 100ºC

Kelvin (absolute)

273.15 K 373.15 K

Practice A, p. 303

Page 13: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 2 – Defining Heat•Objectives:

1. Explain heat as the energy transferred between substances that are at different temperatures.

2. Relate heat and temperature change on the macroscopic level to particle motion on the microscopic level.

3. Apply the principle of energy conservation to calculate change in potential, kinetic, and internal energy.

Page 14: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy•To understand thermal processes we have to

consider the behavior of atoms and molecules.•Mechanics explains what is happening at the

molecular (microscopic) level and accounts for what we observe at the macroscopic level.

•Heat = the energy transferred between objects because of a difference in their temperature

•Energy transferred as heat tends to move from an object at higher temperature to an object at lower temperature.

Page 15: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

•The direction in which energy travels as heat can be explained at the atomic level.

Page 16: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

Page 17: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

•Thermal equilibrium may be understood in terms of energy exchange between two objects at equal temperature.

•At the same temperature, the net energy transferred between two objects is zero.

Page 18: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

•So, the difference between temperature and heat:1) The atoms of all objects are in

continuous motion, so all objects have some internal energy

= Temperature is a measure of that energy

= All objects have some temperature

Page 19: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

2) The energy transferred from one object to another because of the temperature difference between them is heat

= No temperature difference means no net energy transferred as heat

Page 20: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Energy

•Heat, like work, is energy in transit.•Heat units can be converted into joules,

the SI unit for energy.

Page 21: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermal Conduction

•Thermal conduction occurs when energy transfer increases the temperature of an object.

•The rate of thermal conduction depends on the properties of the substance being heated.

•Substances that rapidly transfer energy as heat = thermal conductors

•Substances that slowly transfer energy as heat = thermal insulators

Page 22: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermal Conduction

•Convection is the transfer of energy involving the movement of cold and hot matter.▫It involves the combined effects of heat,

pressure differences, conduction, and buoyancy.

•Electromagnetic radiation occurs when objects reduce their internal energy by giving off radiation of particular wavelengths or when objects are heated by the radiation.

Page 23: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Work

•When objects collide in-elastically, not all of their initial kinetic energy remains as kinetic energy after the collision.

•Some of the energy is absorbed as internal energy by the objects.

•If changes in internal energy are taken into account along with changes in mechanical energy, the total energy is a universally conserved property.

Page 24: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat and Work

•Conservation of Energy:ΔPE + ΔKE + ΔU = 0

Practice B, p 311

Page 25: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 3 – Changes in Temperature & Phase•Objectives:

1. Perform calculations with specific heat capacity.

2. Interpret the various sections of a heating curve.

Page 26: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Specific Heat Capacity

•Specific heat capacity = the energy required to change the temperature of 1 kg of a substance by 1ºC

specific heat capacity =

cp =

energy transferred as heatmass x change in temp

Q

mΔT

Page 27: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Specific Heat Capacity

•The p indicates that the specific heat capacity is measured at constant pressure.

•The equation for specific heat capacity applies to both substances that absorb energy from their surroundings and those that transfer energy to their surroundings.

Page 28: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Specific Heat Capacity

•To measure the specific heat capacity of a substance, it is necessary to measure mass, temperature change, and energy transferred as heat.

Qw = -Qx

Qw + Qx = 0

cp,wmwΔTw = -cp,xmxΔTx

Page 29: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Specific Heat Capacity

•The w indicates water.•Calorimetry = experimental procedure

used to measure the energy transferred from one substance to another as heat

•Calorimeter = device that contains a thermometer for measuring temperature and a stirrer to ensure a uniform mixture of energy throughout water

•Practice C, p 315

Page 30: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Latent Heat•When substances melt, freeze, boil,

condense, or sublime, the energy added or removed changes the internal energy of the substance without changing the substance’s temperature.

•These changes in matter are called phase changes.

•Sublime = change from a solid to a vapor or from vapor to a solid

Page 31: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Latent Heat

•Phase changes result from a change in the potential energy between particles of a substance.

•When energy is added to or removed from a substance that is undergoing a phase change, the particles of the substance rearrange themselves to make up for their change of energy.

•This rearrangement occurs without a change in the average kinetic energy of the particles.

Page 32: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Latent Heat

•The energy that is added or removed per unit mass is called latent heat (L).

Q = mL

•During melting, the energy that is added to a substance equals the difference between the total potential energies for particles in the solid and liquid phases = heat of fusion (Lf)

Page 33: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Latent Heat

•During vaporization, the energy that is added to a substance equals the difference in the potential energy of attraction between the liquid particles and between the gas particles = heat of vaporization (Lv)

Page 34: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Chapter 10 - Thermodynamics

•Expectations:▫Learn how two types of energy transfer

– work and heat – serve to change a system’s internal energy.

▫Learn a new form of the law of energy conservation and see how machine efficiency is limited.

Page 35: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Chapter 10 Preview• Section 1: Relationships Between Heat &

Work▫Heat, Work, and Internal Energy▫Thermodynamic Processes

• Section 2: The First Law of Thermodynamics▫Energy Conservation▫Cyclic Processes

• Section 3: Changes in Temperature & Phase▫Efficiency of Heat Engines▫Entropy

Page 36: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 1 – Relationships Between Heat & Work• Objectives:

1. Recognize that a system can absorb or release energy as heat in order for work to be done on or by the system and that work done on or by a system can result in the transfer of energy as heat.

2. Compute the amount of work done during a thermodynamic process.

3. Distinguish between isovolumetric, isothermal, and adiabatic thermodynamic processes.

Page 37: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat, Work, & Internal Energy

•Work can increase the internal energy of a substance.

•The internal energy can then decrease through the transfer of energy as heat.

•Energy can be transferred to a substance as heat and then be used to do work.

•On a microscopic scale, heat and work are similar – they refer to energy in transit.

Page 38: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat, Work, & Internal Energy

•This energy exists in systems – sets of particles or interacting components considered to be a distinct physical entity for the purpose of study.

•Systems are rarely isolated from their surroundings, so we have to account for interactions with the environment as well.

•Environment = combination of conditions and influences outside a system that affect the behavior of the system

Page 39: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat, Work, & Internal Energy

•In thermodynamic systems, work is defined in terms of pressure and volume change.

•Work done by a gas:

W = PΔV

work = pressure x volume change

Page 40: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat, Work, & Internal Energy

•If a gas expands:▫ΔV is positive▫Work done by the gas is positive

•If a gas is compressed:▫ΔV is negative▫Work done by the gas is negative

•When the gas volume remains constant, there is no displacement and no work is done on or by the system.

Page 41: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Heat, Work, & Internal Energy

•Practice A, p 338

Page 42: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermodynamic Processes

•Most processes transfer energy as both heat and work.

•In many processes, one type of energy transfer is dominant and the other type is negligible.

•In these instances, the real process can be approximated with an ideal process.

Page 43: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermodynamic Processes

•In general, when a gas undergoes a change in temperature but no change in volume, no work is done on or by the system.

•This type of process is called a constant-volume process, or isovolumetric process.

Page 44: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermodynamic Processes•Isothermal process = system’s temperature

remains constant and internal energy doesn’t change when energy is transferred to or from the system as heat or work

•These processes must happen slowly.

Page 45: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermodynamic ProcessesIn an

isothermal

process, small

amounts of energy

are removed as work.

Energy is added as

heat.

Thermal equilibrium is

restored.

Page 46: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Thermodynamic Processes

•Adiabatic process = process in which changes occur but no energy is transferred to or from a system as heat

•The decrease in internal energy must be equal to the energy transferred from the gas as work.

•This process must happen rapidly.http://www.youtube.com/watch?v=dQeCEqkE9eE&safe=active

Page 47: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 2 – The First Law of Thermodynamics•Objectives:

1. Illustrate how the first law of thermodynamics is a statement of energy conservation.

2. Calculate heat, work, and the change in internal energy by applying the first law of thermodynamics.

3. Apply the first law of thermodynamics to describe cyclic processes.

Page 48: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation

Page 49: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation•On a roller coaster that experiences friction,

mechanical energy isn’t conserved.•A steady decrease in the car’s total

mechanical energy occurs because of friction.•Mechanical energy is transferred to the atoms

and molecules throughout the entire roller coaster (cars and track).

•The roller coaster’s internal energy increases by an amount equal to the decrease in the mechanical energy.

Page 50: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation

•Most of this energy is gradually dissipated to the air surrounding the roller coaster as heat.

•The principle of energy conservation that takes into account a system’s internal energy as well as work and heat is called the first law of thermodynamics.

•This principle is often applied to systems, so values for work and heat provide information.

Page 51: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation

Page 52: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation

•The total change in the internal energy is:ΔU = Uf – Ui

•Uf = final internal energy

•Ui = initial internal energy

Page 53: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Energy Conservation

•The first law of thermodynamics:ΔU = Q – W

•ΔU = change in internal energy•Q = energy transferred to or from a

system as heat•W = energy transferred to or from system

as workPractice B, p 345

Page 54: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Cyclic Processes

•In a cyclic process, the system’s properties at the end of the process are identical to the system’s properties before the process took place.

•The final and initial values of internal energy are the same, and the change in internal energy is zero.

ΔUnet = 0 and Qnet = Wnet

Page 55: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Cyclic Processes

•These processes resemble isothermal ones in that all energy is transferred as work and heat.

•Heat engines use heat to do work as part of a cyclic process.

•They do work by transferring energy from a high-temperature substance to a lower-temperature substance.

Page 56: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

•For each complete cycle, the net work done will equal the difference between the energy transferred as heat from a high-temperature substance to the engine (Qh) and the energy transferred as heat from the engine to a lower-temperature substance (Qc).

Wnet = Qh - Qc

Cyclic Processes

Page 57: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Cyclic Processes

•The larger the difference between the energy transferred as heat into the engine and out of the engine, the more work the engine can do in each cycle.

•No heat engine works perfectly – only part of the available internal energy leaves the engine as work.

•Most of the energy is removed as heat.

Page 58: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Section 3 – The Second Law of Thermodynamics•Objectives:

1. Recognize why the second law of thermodynamics requires two bodies at different temperatures for work to be done.

2. Calculate the efficiency of a heat engine.

3. Relate the disorder of a system to its ability to do work or transfer energy as heat.

Page 59: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Efficiency of Heat Engines• It is impossible to construct a heat engine

that, operating in a cycle, absorbs energy from a hot reservoir and does an equivalent amount of work.

•This is the basis for the second law of thermodynamics: no cyclic process that converts heat entirely into work is possible.

•This tells us that some energy must always be transferred as heat to the system’s surroundings (Qc > 0).

Page 60: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Efficiency of Heat Engines

•Cyclic processes can be made to approach ideal situations.

•A measure of how well an engine operates is given the engine’s efficiency (eff).

•Efficiency = a measure of the useful energy taken out of a process relative to the total energy put into the process.

Page 61: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Efficiency of Heat Engines

•Equation for the efficiency of a heat engine:

eff = = = 1 –

•Wnet = net work done by engine

•Qh = energy added as heat

•Qc = energy removed as heat

Wnet Qh - QcQh Qh Qh

Qc

Page 62: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Efficiency of Heat Engines•Efficiency is a unitless quantity that can be

calculated using only the magnitudes for the engines added to and taken away from the engine.

•The efficiencies of all engines are less than 1.0.•The smaller the fraction of usable energy that

an engine can provide, the lower its efficiency is.

•The efficiency equation gives only a maximum value for an engine’s efficiency.

•Practice C, p. 355

Page 63: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Entropy• In thermodynamics, a system left to itself

tends to go from a state with a very ordered set of energies to one in which there is less order.

•Entropy = the measure of a system’s disorder•The greater the entropy of a system is, the

greater, the system’s disorder.•The entropy of systems tends to increase;

once a system has reached a state of greatest disorder, it will tend to remain in that state and have maximum entropy.

Page 64: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Entropy

•The motion of the particles of a system is not well ordered and is less useful for doing work.

Page 65: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Entropy

•Because of the connection between a system’s entropy, its ability to do work, and the direction of energy transfer, the second law of thermodynamics can also be stated as: The entropy of the universe increases

in all natural processes.

Page 66: Unit 4 - Thermodynamics Chapters 9 and 10. Chapter 9 - Heat Expectations: ▫L▫Learn the difference between temperature and heat. ▫L▫Learn how different.

Entropy•Entropy can decrease for parts of systems

provided that the decrease is offset by a greater increase in entropy elsewhere in the universe.