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HEAT AND TEMPERATURE

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Title: HEAT AND TEMPERATURE


1
HEAT AND TEMPERATURE
  • Chapter 4 in Tillery

2
THE KINETIC MOLECULAR THEORY
  • In the 5th century B.C. the Greek philosopher
    Democritus proposed that matter was composed of
    small indivisible particles called atoms. Only a
    vacuum existed between the atoms.
  • Aristotle, 3rd century B.C., would not accept the
    concept of a vacuum and thus thought matter was
    continuous thus not composed of atoms.
  • Galileo and Newton believed in the idea of matter
    being composed of small particles, because it
    seemed to be consistent with the nature and
    behavior of matter.
  • During the late 1700s and into the early 1800s,
    strong evidence from chemical studies supported
    the idea of atoms and led to a series of
    inferences about atoms and their motion and
    behavior. These inferences are referred to as
    the Kinetic Molecular Theory.

3
ATOMS AND MOLECULES IN MOTION
  • The kinetic molecular theory is based on 3
    inferences 1) all matter is composed of small
    particles called atoms 2) the particles of
    matter are in constant motion 3) all collisions
    are perfectly elastic.
  • The atom is the basic building block of pure
    substances called elements. An atom cannot be
    divided into smaller particles and still keep the
    characteristics of a particular element.
  • There are also pure substances referred to as
    compounds. Some compounds are composed of tightly
    bound groups of atoms called molecules (example
    the water molecule).
  • A molecule may be defined as the smallest
    particle of a compound or a gaseous element that
    can exist and still retain the characheristic
    properties of that substance. (Tillery, 2007).
  • When a gas molecule collides with the wall of a
    container, it exerts a force called pressure on
    the container.

4
MOLECULES INTERACT AND ARE ATTRACTED TO EACH OTHER
  • Cohesion attractive forces of the same kind of
    molecules for each other.
  • Adhesion attractive forces between unlike
    molecules.

5
PHASES OF MATTER
  • The physical state (phase) of a substance depends
    mostly on the chemical bonding in the substance
  • Solids Solid particles are arranged in a
    definite pattern. Solids have both a definite
    shape and a definite volume
  • Liquids Liquid particles appear to vibrate
    around moving points, but actually travel in
    straight-line paths between collisions. Liquids
    have a definite volume but assume the shape of
    their containers
  • Gases Gas particles travel independently in
    straight-line paths. Gases assume the shape and
    volume of their container.
  • Plasmas - A plasma is composed of electrons and
    positive ions at very high temperatures.

6

http//abyss.uoregon.edu/js/21st_century_science/
lectures/lec05.html
7
TEMPERATURE AND MOLECULAR MOTION
  • Temperature is a measure of the average kinetic
    energy of the molecules making up a substance.
  • KE ½ mv2
  • See Figure 4.4, p. 88 in text.

8
THERMOMETERS AND THERMOMETER SCALES
  • Themomometers are really divices that measure the
    expansion or contraction of some material upon
    heating or cooling of the material
  • The unit we use for meausrement of temperature is
    usually "degrees" (). Actually, there are three
    temperature scales that are used today. The
    Kelvin(K) scale is used by scientists. The
    Celsius scale (C) is used in most of the world
    to measure air temperatures and other
    temperatures, but is also sometimes used by
    scientists. In the United States, the Fahrenheit
    scale (F) is used to measure air temperatures,
    body temperatures, etc.
  • The Fahrenheit Scale Developed by the German
    Physicist Fahrenheit in 1715. Originally a
    centigrade scale based on two reference points.
    An ice and salt water mixture for zero reference
    point and the temperature of the human body
    (taken to be 100) for the upper reference point.
    It was later found out that there is quite a
    range of human body temperature, with the average
    at 98.6 degrees Fahrenheit.
  • The Celsius Scale Invented by a Swedish
    astronomer, Celsius, in 1735. This scale is also
    based on two arbitrary reference points and was a
    centrigrade scale. The lower reference point,
    representing zero degrees Celsius, was the
    freezing/melting point of pure water. The upper
    reference point, representing 100 degrees
    Celsius, was the boiling point of pure water at
    Standard Barometric Pressure.
  • The Kelvin Scale Devised by William Thompson
    (Lord Kelvin) in 1848. This is not a relative
    scale with an arbitrary zero point, but has as
    its lowest point absolute zero the coldest
    possible temperature.

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http//go.hrw.com/resources/go_sc/hst/HP1PE730.PDF
13
HEAT AND INTERNAL ENERGY
  • Temperature and heat are not the same concepts.
  • As we have already stated, temperature is a
    measure of the average kinetic energy of the
    molecules of a substance.
  • The internal energy of a substance is the total
    kinetic and potential energy of the molecules of
    a substance.
  • Heat is a measure of the internal energy that has
    been absorbed or transferred from one body (or
    substance) to another.
  • Heat energy may be transferred from a body at
    higher temperature to a body at a lower
    temperature, or heat energy may be gained by a
    body during an energy form conversion. An energy
    form conversion actually does work on the
    molecules in a substance to speed them up.

14
INTERNAL ENERGY
http//hyperphysics.phy-astr.gsu.edu/hbase/thermo/
inteng.html
15
HEAT AS ENERGY TRANSFER
16
MEASURES OF HEAT
  • Heat is a form of energy. We have already stated
    that the Joule is the SI unit for energy.
    Therefore, the SI unit for heat energy is the
    Joule.
  • However, scientists often use the calorie to
    measure heat. A calorie is the heat required to
    raise the temperature of 1 gram of water by 1
    degree Celsius. The nutritionists Calorie is
    really a kilocalorie, which is the heat required
    to raise the temperature of 1 kg of water by 1
    degree Celsius.
  • One calorie is equivalent to 4.184 J.
  • In the English system, the BTU (British Thermal
    Unit) is used to measure heat. A BTU is the heat
    required to increase the temperature of 1 pound
    of water by one degree Fahrenheit.

17
SPECIFIC HEAT
  • When heat is transferred, the amount of heat lost
    or gained by a substance is directly proportional
    to the change in temperature.
  • Also when heat is transferred, the amount of heat
    lost or gained by a substance is directly
    proportional to the mass of the substance.
  • And certainly different substances require
    different amounts of heat to go through the same
    temperature change. This is referred to as the
    specific heat of the substance.
  • The specific heat of a substance may be defined
    as the amount of heat required to increase the
    temperature of one gram of the substance by one
    degree Celsius.
  • See Table 4.2 in your text for specific heats of
    materials.

18
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19
PHASE TRANSITIONS NO TEMPERATURE CHANGE
Latent Heat Of Fusion (solid - liquid
transitions) Qf m Lf
Latent Heat Of Vaporization ((liquid - gas
transitions) Qv m Lv
20
Heat flow
  • Three mechanisms for heat transfer due to a
    temperature difference
  • Conduction
  • Convection
  • Radiation
  • Natural flow is always from higher temperature
    regions to cooler ones

21
Conduction
  • Heat flowing through matter
  • Mechanism
  • Hotter atoms collide with cooler ones,
    transferring some of their energy
  • Direct physical contact required cannot occur in
    a vacuum
  • Poor conductors insulators (Styrofoam, wool,
    air)

22
Sample conductivities
23
Convection
  • Energy transfer through the bulk motion of hot
    material
  • Examples
  • Space heater
  • Gas furnace (forced)
  • Natural convection mechanism - hot air rises

24
Radiation
  • Radiant energy - energy associated with
    electromagnetic waves
  • Can operate through a vacuum
  • All objects emit and absorb radiation
  • Temperature determines
  • Emission rate
  • Intensity of emitted light
  • Type of radiation given off
  • Temperature determined by balance between rates
    of emission and absorption
  • Example Global warming

25
Energy, heat, and molecular theory
  • Two responses of matter to heat
  • Temperature increase within a given phase
  • Heat goes mostly into internal kinetic energy
  • Specific heat
  • Phase change at constant temperature
  • Related to changes in internal potential energy
  • Latent heat

26
Evaporation and condensation
  • Individual molecules can change phase any time
  • Evaporation
  • Energy required to overcome phase cohesion
  • Higher energy molecules near the surface can then
    escape
  • Condensation
  • Gas molecules near the surface lose KE to liquid
    molecules and merge

27
Thermodynamics
  • The study of heat and its relationship to
    mechanical and other forms of energy
  • Thermodynamic analysis includes
  • System
  • Surroundings (everything else)
  • Internal energy (the total internal potential and
    kinetic energy of the object in question)
  • Energy conversion
  • Friction - converts mechanical energy into heat
  • Heat engines - devices converting heat into
    mechanical energy
  • Other applications heat pumps, refrigerators,
    organisms, hurricanes, stars, black holes, ,
    virtually any system with energy inputs and
    outputs

28
The first law of thermodynamics
  • Conservation of energy
  • Components
  • Internal energy
  • Heat
  • Work
  • Stated in terms of changes in internal energy
  • Application heat engines

29
The second law of thermodynamics
  • Equivalent statements
  • No process can solely convert a quantity of heat
    to work (heat engines)
  • Heat never flows spontaneously from a cold object
    to a hot object
  • Natural processes tend toward a greater state of
    disorder (entropy)

30
END
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