Heat, temperature, and thermodynamics Chapters 13-15 The - PowerPoint PPT Presentation

1 / 52
About This Presentation
Title:

Heat, temperature, and thermodynamics Chapters 13-15 The

Description:

Heat, temperature, and thermodynamics Chapters 13-15 The atomic theory of matter All matter in our everyday lives is made up of atoms. The motions of these atoms ... – PowerPoint PPT presentation

Number of Views:242
Avg rating:3.0/5.0
Slides: 53
Provided by: hsVersail
Category:

less

Transcript and Presenter's Notes

Title: Heat, temperature, and thermodynamics Chapters 13-15 The


1
Heat, temperature, and thermodynamics
  • Chapters 13-15

2
The atomic theory of matter
  • All matter in our everyday lives is made up of
    atoms.
  • The motions of these atoms determine what state
    of matter they exist in.
  • Relatively slow moving atoms in a solid have a
    balance of attractive and repulsive forces that
    hold them so that they move only slightly around
    a fixed position.

3
Continued
  • In a liquid, the chemical bonds of a solid have
    been overcome by increased atomic motion, leaving
    only Vanderwaals forces to keep the atoms close
    together, but not actually attached to each
    other.
  • In a gas, the atomic motion becomes great enough
    to overcome these electrostatic forces, and allow
    the atoms to separate completely.
  • The motion of these atoms is determined by the
    amount of thermal energy they have, specifically
    their kinetic energy.

4
Kinetic Theory of Temperature
  • The analysis of matter in terms of atoms in
    continuous random motion.
  • See page 368 to read the postulates. We will deal
    with these and the math more later.
  • In other words, measuring the temperature of
    something measures the average kinetic energy of
    its atoms and molecules.
  • However, the motions of the individual atoms and
    molecules may vary greatly within a substance of
    a certain temperature. Thats why we deal with
    average values.
  • A side note on absolute zero

5
Thermal expansion
  • Most materials expand when heated and contract
    when cooled.
  • Change in length of almost all solids is close to
    directly proportional to change in temperature
  • The proportionality can be modeled by this
    equation
  • ?LaL0?T
  • LL0(1a?T)

6
What does it all mean?!?!
  • L is final length
  • L0 is initial length
  • a is the coefficient of linear expansion, and
    differs from material to material
  • ?T is the change in temperature

7
Volumes of materials also undergo a change with
varying temperature
  • ?VßV0?T
  • ß is the coefficient of volume expansion
  • Calculations of these types need to be done in
    the design of any material that undergoes changes
    in temperature (roads and bridges, engine parts)
    as part of it use. If not taken into account,
    these stresses can cause failure of the
    materials.

8
A bridge is built of 2 slabs of concrete that are
9 m long at 20 degrees C. The ends are fixed with
an expansion joint between the slabs
  • How wide should the expansion joints between the
    slabs be (at 20 degrees C) to prevent buckling of
    the bridge if the temperature range is -20 to 45
    degrees Celsius?
  • What is the total range the joints must
    accommodate?

9
An ordinary glass is filled to the brim with
350.0 ml of water at 100 degrees Celsius. If the
temperature of the water was reduced to 15
degrees Celsius, how much extra water could now
be added to the glass?
10
The 70L steel gas tank of a car is filled to the
top with gasoline at 20 degrees C. The car sits
in the sun and the tank reaches a temperature of
40 degrees C. How much gas do you expect to
overflow from the tank?
11
Anomalous behavior of water between 0-4 degrees C
  • Due to the formation of ice crystals beginning at
    4 degrees Celsius, water expands when cooled
    between 0-4 degrees C.
  • As ice crystals collapse, water contracts when
    heated from 0-4 degrees C.
  • At what temperature is water most dense?
  • What is found inside an ice crystal?

12
Ideal gas law
  • Relationship of how P,V, or T will change.
  • Must hold two constant to study changes in the
    third.
  • The relationship of mass to the other three can
    be incorporated if the number of moles is used to
    represent the mass of the substance
  • n in molesmass in grams/molecular mass in g/mol
  • Ex CO2 has a molecular mass of 44g/mol. What is
    n for 132 g of CO2?
  • 3.0 mol

13
PVnRT
  • R is the universal gas constant with values found
    on page 364.
  • Gases follow this fairly accurately unless they
    are at very high pressures and/or close to
    boiling point
  • Ex 2L of nitrogen are at 00Celsius and 1 atm.
    Determine the number of moles of nitrogen in this
    container

14
STP-standard temperature and pressure
  • T273K
  • P1.00atm101.3kPa1.013x105 N/m2
  • Ex A given sample of CO2 contains 3.01x1023
    molecules at STP. What volume does this sample
    occupy?
  • 11.2L

15
A balloon has a volume of 1.0 m3. as it rises in
earths atmosphere, its volume expands. What will
be its new volume if its original temperature and
pressure are 20oC and 1.0 atm, and its final
temperature and pressure are -40oC and 0.10 atm?
16
Homework
  • Due Monday from page 379
  • Questions3,5,9,15
  • Problems 7,9,13,14,26,29,34

17
Heat-the energy transferred from one object to
another due to a difference in temperature
  • Heat spontaneously flows from an area of higher
    temperature to lower temperature
  • Measured in joules or calories
  • The direction of heat flow depends upon
    temperature, the amount of heat flow depends on
    thermal energy.
  • Thermal energy is the internal energy of all the
    molecules of a substance

18
What determines how much an objects temperature
will change when heat flows into it?
  • Qmc?T
  • Q is heat
  • m is mass
  • C is specific heat capacity-a characteristic of a
    material due to its bonds and electrostatic
    forces that effects how much heat it can absorb
    before changing temperature
  • Page 387-low specific heats mean little energy is
    needed to change and objects temperature

19
Waters high specific heat has a huge effect on
the earths climate
  • West and east coast of US
  • Bermuda vs. N. Carolina

20
Specific heat calculations
  • Imagine you have a 1kg copper pan on the
    stovetop. How much heat would be required to
    raise its temperature from 10 degrees C to 90
    degrees C?
  • What if it was filled with 1kg of water, then how
    much heat would it take to raise the temperature
    of the entire system from 10 to 90 degrees C?

21
Consider an iron block of 0.5 kg at 75 degrees C
dropped into a container of water at 15 degrees
C. Assume no heat transfer outside the system.
Which of the following statements are true?
  • The decrease in iron temperature is equal to the
    increase in water temperature.
  • The quantity of heat lost by the iron is equal to
    the quantity of heat gained by the water
  • The iron and water will both reach the same
    temperature
  • The final temperature of the iron and water is
    half way between the initial temperature of both.

22
Consider an iron block of 0.5 kg at 75 degrees C
dropped into a container of water at 15 degrees
C. Assume no heat transfer outside the system. If
there is 1.2 kg of water present, solve the final
temperature of each.
23
Latent heat
  • Latent means hidden
  • Latent heat is the name given to the heat used to
    change the state of a substance
  • Temperature does not change during phase change,
    even though additional heat is being added. Why?
  • 335 J/g for latent heat of fusion of water
  • 2255J/g for latent heat of vaporization of water
  • 4.18 J/g to raise the temperature 1 degree C in
    liquid state

24
A note on evaporation
  • Evaporation can take place below 100 degrees C,
    but requires slightly more energy at lower
    temperatures (2450J/g at 20 degrees )
  • Evaporation is a cooling process
  • Condensation is warming process
  • Basketball players and coolie cups

25
Latent heat calculations
  • If you put 0.2 kg of ice at -8 degrees C into a
    glass of water at 20 degrees C, can the ice cool
    the water to 0 degrees C?

26
At what temperature will the water end up?
27
0.05 kg of water vapor at 100 degrees C condenses
on your skin and cools to 80 degrees before you
can remove it. How much heat is transferred to
your skin?
28
Homework due Friday
  • Questions 1,2
  • Problems 1,2,8,9,11,12,21,28,30,31

29
Heat transfer by conduction
  • Transfer of kinetic energy by collisions of
    atoms, molecules, or electrons of a material.
  • Must be a difference in temperature for heat to
    flow by conduction
  • Ice on plates
  • Some materials are better conductors than others
    due to their grasp on electrons
  • See page 396

30
Insulative properties of air
  • Windows and goosebumps
  • Thermal resistivity of building materials defined
    by R values, higher R values mean a better
    insulator
  • Also depends on thickness of the material

31
Heat transfer by convection
  • Movement of large numbers of molecules over large
    distances
  • Only occurs in fluids
  • Forced convection vs. natural convection
  • Heating and cooling vents, fan direction
  • Convection and natural processes of the earth
    plate movement, weather patterns, sea breezes

32
Heat transfer by radiation
  • Requires no material for heat transfer, occurs by
    electromagnetic radiation emitted by one object
    and absorbed by another
  • Heat from sun or fire, infrared radiation
  • How does color affect emission and absorption of
    radiant energy?
  • Mr. Stewart wants to drink his coffee in 10
    minutes. If he wants his coffee to cool fastest,
    should he add room temperature cream right away,
    or wait until he is ready to drink it?

33
Angle of radiation can have effect on heat
transferred
  • Seasons are not caused by the closeness of the
    earth to the sun, but rather by the angle of the
    suns rays on the earth
  • Earth is closer to N Hemisphere during December
    than in July, but the rays in December are less
    direct than in July
  • See 14-13 on page 402

34
Thermodynamics
  • The study of the processes of the transfer of
    energy as heat and work
  • Law 1 the change of the internal energy of a
    closed system is equal to the energy added to the
    system by heat minus the work done by the system
    on the surroundings
  • ?UQ-W
  • KEPE?UQ-W

35
This is a restatement of the law of conservation
of energy
  • A systems internal energy is U and is a state
    variable. Other state variables include
    P,V,T,m,and n.
  • Q and W are not state variables as they are not
    characteristics of the material or state, rather
    something that is added or done to that material

36
Other important first law vocab
  • Isothermal process- takes place at a constant
    temperature. See page 410, 15-2
  • Since the temperature is kept constant, the U
    does not change. Therefore, in an isothermal
    process, WQ
  • Heat added to the gas equals the work done
    (output) by the gas

37
Adiabatic process- no heat is allowed to flow
into or out of the system. Therefore, Q0
  • ?U-W
  • Internal energy and also temperature decrease if
    the gas expands
  • Adiabatic processes can either occur in a well
    insulated system, or in one where expansion or
    contraction happens very quickly so that heat
    flow cannot take place during the process

38
Isobaric process- pressure is kept constant as
volume changes
  • For an isobaric process, work can be solved by
    WP?V
  • Isovolumetric process-volume is kept constant as
    pressure changes
  • For isovolumetric, no volume change means no work
    done
  • See table 15-1 on page 412

39
Second law- heat only spontaneously flows from
areas of higher temperature to lower temperature.
  • Entropy is another state variable of a system.
  • Entropy is a measure of disorder of a system.
    More about this later.
  • Heat is the graveyard of all other forms of
    energy
  • To get work from heat energy is difficult.

40
Third law- no system can reach absolute zero
  • Would require a sample of matter to exist at zero
    volume
  • Bose-Einstein condensates are closest
    experimenters have come
  • In a Bose-Einstein condensate, the atoms all
    exist in the same quantum state, and act as a
    single super atom
  • Displays bizarre properties, such as being able
    to slow light down to the speed of a bicycle.

41
Heat and refrigeration cycles
  • Pair up for a small research project
  • Heat engines, any device that transforms thermal
    energy into mechanical work
  • Steam engine/Rankine cycle, internal combustion
    engine/otto cycle, Carnot Engine,
  • Air Conditioner/refrigeration cycle, Heat pump
  • Diesel Cycle, Stirling Cycle, Brayton Cycle

42
Requirements
  • Classify as either a power cycle or a heat pump
    cycle
  • When applicable, explain the design features and
    what it is used for
  • Describe the processes that take place within it
    and what it is designed to do
  • Include the thermodynamic processes that take
    place (ieisothermal, isobaric, isometric) and
    explain what they mean
  • Give a brief historical perspective on your
    cycle, including any impact its evolution has
    had on modern society
  • Will be presented as a powerpoint.

43
Thermo calculations
  • Keep in mind if heat is added to a system Q is
    positive
  • If heat is lost by a system, Q is negative
  • If work is done by a system, W is positive
  • If work is done on a system, W is negative

44
2200 J of heat is added to a system and 1450 J of
work is done on the system. What is the change in
internal energy of the system?
  • ?UQ-W
  • 3650 J

45
An ideal gas expands isothermally performing
1900J of work in the process. Solve the change in
internal energy of the gas and the heat absorbed
during expansion.
  • ?U0 since no change in temp
  • Heat absorbed must be equal to work done1900J

46
At constant pressure of 1 atmosphere (101000
N/m2), 45,800J of heat is added to a gas in a
container fitted with a frictionless movable
piston. Solve the work done by the gas if the
volume changes by 0.3 m3. Also solve the change
in internal energy of the gas.
  • For an isobaric process, work can be solved by
    WP?V
  • 30300 J
  • 15500 J

47
The pressure in an ideal gas is cut in half
slowly while being kept in a container with rigid
walls. In this process, 24,000J of heat left the
gas. How much work was done during this process
and what was the change in internal energy of the
gas?
  • Isovolumetric, so W0
  • ?Q-24,000J

48
In an engine, an almost ideal gas is compressed
adiabatically to half its volume. In doing so,
2000 J of work is done on the gas. How much heat
flows into or out of the gas, and what is its
change in internal energy?
  • None, it is adiabatic
  • 2000 J

49
Heat cycles and engines
  • It is easy to turn work into heat
  • It is difficult to get usable work from heat
  • Power cycles using heat require much more energy
    input in the form of heat than they give output
    work
  • They also require a temperature difference from
    one part of the process to the other

50
Heat must be able to flow from an area of high
temp to low temp for work to be done by the system
  • In all thermo cycles, the ?U0 because it returns
    to its starting state at the end of each cycle
  • Heat input (QH) at a high temperature (TH) is
    partly transformed to work and partly exhausted
    as unused heat (QL)
  • QH W x QL
  • TH and TL are operating temperatures

51
Heat engines and efficiency
  • Why must we have a difference in temperature to
    run a heat engine?
  • Without a temperature gradient, the pressure
    would be the same in all parts of the engine and
    equal amounts of work would be needed during
    intake and exhaust and would give no net work

52
See page 418 for efficiency equations
Write a Comment
User Comments (0)
About PowerShow.com