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Chapter 9 Heat

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Chapter 9 Heat Pages 297 320 Coach Kelsoe A small decrease in entropy = more order or less disorder A large increase in entropy = less order or more disorder DID ... – PowerPoint PPT presentation

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Title: Chapter 9 Heat


1
Chapter 9 Heat
  • Pages 297 320
  • Coach Kelsoe

2
Chapter 9 Heat
  • Section One
  • Temperature and Thermal Equilibrium
  • Pages 298 - 304

3
Section One Objectives
  • Relate temperature to the kinetic energy of atoms
    and molecules
  • Describe the changes in the temperatures of two
    objects reaching thermal equilibrium
  • Identify the various temperature scales, and
    convert from one scale to another

4
Section One Vocabulary
  • Temperature a measure of the average kinetic
    energy of the particles in a substance
  • Internal energy the energy of a substance due
    to both the random motions of its particles and
    to the potential energy that results from the
    distances and alignments between the particles
  • Thermal equilibrium the state in which two
    bodies in physical contact with each other have
    identical temperatures

5
Section One Formulas
  • Celsius Fahrenheit Temperature Conversion
  • Fahrenheit temperature (9/5 x Celsius
    temperature) 32.0
  • Celsius Kelvin Temperature Conversion
  • T TC 273.15

6
Temperature Thermal Equilibrium
  • The temperature of a substance is proportional to
    the average kinetic energy of particles in the
    substance.
  • A substances temperature increases as a direct
    result of added energy being distributed among
    the particles of the substance.
  • The energies associated with atomic motion are
    referred to as internal energy, which is
    proportional to the substances temperature.

7
Temperature Thermal Equilibrium
  • Thermal equilibrium is the basis for measuring
    temperature with thermometers.
  • By placing a thermometer in contact with an
    object and waiting until the column of liquid in
    the thermometer stops rising or falling, you can
    find the temperature of the object.
  • The reason is that the thermometer is in thermal
    equilibrium with the object.
  • Remember that thermal equilibrium is the state in
    which two bodies in physical contact with each
    other have identical temperatures.

8
Practice Problem
  • What is the equivalent Celsius temperature of
    50.0F?
  • Given TF 50.0F
  • Unknown TC ?
  • Use the Celsius-Fahrenheit equation to convert
    Fahrenheit into Celsius.
  • TF 9/5TC 32.0
  • TC 5/9 (TF 32.0)
  • TC 5/9 (50.0 32.0)C 10.0C

9
Practice Problem
  • What is the equivalent Kelvin temperature of
    50.0F?
  • Use the Celsius-Kelvin equation to convert
    Celsius into Kelvin.
  • T TC 273.15
  • T (10.0 273.15)K 283.2 K

10
Chapter 9 Heat
  • Section Two
  • Defining Heat
  • Pages 305 - 312

11
Section Two Objectives
  • Explain heat as the energy transferred between
    substances that are at different temperatures
  • Relate heat and temperature change on the
    macroscopic level to particle motion on the
    microscopic level
  • Apply the principle of energy conservation to
    calculate changes in potential, kinetic, and
    internal energy

12
Section Two Vocabulary
  • Heat the energy transferred between objects
    because of a difference in their temperatures

13
Section Two Formula
  • Conservation of Energy
  • ?PE ?KE ?U 0

14
Defining Heat
  • Heat is the energy transferred between objects
    because of a difference in their temperatures.
  • The word heat is sometimes used to refer to the
    process by which energy is transferred between
    objects because of a difference in their
    temperatures.

15
Heat
  • Heat- the energy transferred between objects
    because of a difference in their temperatures
  • Energy is transferred between substances as heat.
  • The symbol for heat is Q and is measured in
    joules (J).

16
Internal Energy
  • When objects collide inelastically, 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.
  • Ex. If you pull out a nail after hammering it
    into wood, it feels warm. The nail encounters
    friction as it is pulled out the energy required
    to overcome the friction is transformed into
    internal energy. The increased internal energy
    raises the nails temperature.
  • The symbol for a change in internal energy is ?U.

17
Conservation of Energy
  • ?PE ?KE ?U 0 (change in potential energy
    change in kinetic energy change in internal
    energy 0)

18
Specific Heat Capacity
  • Defined as the energy required to change the
    temperature of 1 kg of that substance by 1 C
  • Every substance has a unique specific heat
    capacity.
  • c(p) Q/m?T (specific heat capacity energy
    transferred as heat / mass change in
    temperature)

19
Calorimetry
  • Calorimetry is used to determine specific heat
    capacity.
  • To measure the specific heat capacity of a
    substance you must measure the mass, temperature
    change, and energy transferred as heat.
  • You can find the mass and ?T directly, but to
    find the measurement of heat you have to measure
    the heat transferred between the object and a
    known quantity of water.
  • cp,wmw?Tw -cp,xmx?Tx (Specific heat
    capacity of water mass of water change of
    temperature of water - Specific heat capacity
    of substance mass of substance change of
    temperature of substance)

20
Latent Heat
  • When substances melt, freeze, boil, or condense,
    the energy added or removed changes the internal
    energy without changing the substances
    temperature. These changes are called phase
    changes.
  • Latent heat is energy transferred during phase
    changes.

21
Practice Problem
  • A 61.4 kg roller skater on level ground brakes
    from 20.5 m/s to 0 m/s. What is the total change
    in the internal energy of the system?
  • Given m61.4 kg
  • vi20.5 m/s vf0 m/s
  • Find potential and kinetic energy using the
    givens.

22
Practice Problem
  • There is no potential energy since the roller
    skater is in motion.
  • To find kinetic energy use the formula
  • KE ½mv2
  • Kinetic energy 12,870.98 J
  • ?PE ?KE ?U 0
  • 0 12,870.98J ?U 0
  • ?U -12,870.98J

23
The First Law of Thermodynamics!
24
Symbols to Know
  • Quantities
    Units
  • U change in internal energy
    J joules
  • Q heat
    J joules
  • W work
    J joules
  • eff efficiency
    (unit less)

25
Positive or Negative?
  • Sign Convention for heat, q
  • Heat is transferred into the system ? q gt 0
  • Heat is transferred out of the system ? q lt 0
  • Sign Convention for work, w
  • Work is done upon the system by the surroundings
    ? w lt 0
  • Work is done by the system on the surroundings ?
    w gt 0

26
UQ-W Change in systems internal energyenergy
transferred to or from system as heat-energy
transferred to or from as work
  • The principle of energy conservation that takes
    into account a systems internal energy as well
    as work and heat is called the first law of
    thermodynamics.

27
Sample Problem
  • A total of 135J of work is done on a gaseous
    refrigerant as it undergoes compression. If the
    internal energy of the gas increases by 114J
    during the process, what is the total amount of
    energy transferred as heat? Has energy been
    added to or removed from the refrigerant as heat?

28
Sample Problem
  • Given W -135J U 114J
  • Unknown Q ?
  • (since the work is done on the gas, W is a
    negative value)
  • First rearrange the equation UQ-W to Q
    UW
  • Now plug in the values
  • Q114J(-135J)
  • Q -21J (sign for Q is negative, which indicates
    energy is transferred as heat from the
    refrigerant)

29
The Second Law of Thermodynamics!!
30
The Basis for the Law
  • The requirement that a heat engine give up some
    energy at a lower temperature in order to do work
    does not follow the first law.
  • Therefore this requirement is the basis of what
    is called the second law of thermodynamics.
  • The second law states that no machine can
    transfer all of its absorbed energy as work.

31
When you shuffle a deck of cards, it is highly
improbable that the cards would end up separated
by suit and in numerical sequence.
Example
  • Such a highly ordered arrangement can be formed
    in only a few ways, but there are more than 8 x
    1067 ways to arrange 52 cards.

32
Definition
  • A measure of the randomness or disorder of a
    system

The greater the entropy of a system is, the
greater a systems disorder!!!!
33
  • Once a system has reached a state of greatest
    disorder, it will tend to remain in that state
    and have maximum entropy.

34
  • A small decrease in entropy more order or less
    disorder
  • A large increase in entropy less order or more
    disorder

35
DID YOU KNOW?
  • Entropy decreases in many systems on Earth. For
    example, atoms and molecules become incorporated
    into complex and orderly biological structures
    such as cells and tissues. These appear to be
    spontaneous because we think of the Earth itself
    as a closed system. So much energy comes from the
    sun that the disorder in chemical and biological
    systems is reduced, while the total entropy of
    the Earth, sun, and intervening space increases.

36
The entropy of the universe increases in all
natural processes.
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