Title: HEAT AND TEMPERATURE
1HEAT AND TEMPERATURE
2THE 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.
3ATOMS 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.
4MOLECULES 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.
5PHASES 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
7TEMPERATURE 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.
8THERMOMETERS 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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12http//go.hrw.com/resources/go_sc/hst/HP1PE730.PDF
13HEAT 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.
14INTERNAL ENERGY
http//hyperphysics.phy-astr.gsu.edu/hbase/thermo/
inteng.html
15HEAT AS ENERGY TRANSFER
16MEASURES 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.
17SPECIFIC 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.
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19PHASE TRANSITIONS NO TEMPERATURE CHANGE
Latent Heat Of Fusion (solid - liquid
transitions) Qf m Lf
Latent Heat Of Vaporization ((liquid - gas
transitions) Qv m Lv
20Heat 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
21Conduction
- 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)
22Sample conductivities
23Convection
- Energy transfer through the bulk motion of hot
material - Examples
- Space heater
- Gas furnace (forced)
- Natural convection mechanism - hot air rises
24Radiation
- 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
25Energy, 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
26Evaporation 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
27Thermodynamics
- 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
28The first law of thermodynamics
- Conservation of energy
- Components
- Internal energy
- Heat
- Work
- Stated in terms of changes in internal energy
-
- Application heat engines
29The 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)
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