Temperature and Heat 1 Temperature and Heat by hcj

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									Temperature and Heat


   Chapter 12
Expectations

After this chapter, students will:
know what temperature is
be familiar with common temperature scales
calculate the changes in the linear dimensions of
  objects, and the volumes of substances, with
  temperature
understand heat as a form of energy
Expectations

After this chapter, students will:
Use the concept of specific heat capacity to relate
  temperature changes to gains and losses of heat
  energy
Calculate the energy transfers accompanying
  phase changes of materials
Analyze situations in which different phases of
  matter exist in equilibrium
Temperature

Temperature is a rational numerical
  characterization of the hotness or coldness of an
  object or a substance.

If a hotter (higher temperature) object touches a
   colder (lower temperature) one, heat energy tends
   to flow from the hotter to the colder.
Temperature Scales

Temperature scales are typically based on the phase
  changes of a very common material: water.

The ice point is the temperature at which liquid
  water and ice are in equilibrium, at one
  atmosphere of pressure.

The steam point is the equilibrium for liquid water
  and steam, at one atmosphere.
Temperature Scales: Celsius

Ice point: 0 °C      Steam point: 100°C

Anders Celsius
1701 – 1744

Swedish astronomer
Temperature Scales: Fahrenheit

Ice point: 32 °F            Steam point: 212°F

Daniel Gabriel Fahrenheit
1686 – 1736

German physicist
Temperature Scales: Absolute

Ice point: 273.15 K      Steam point: 373.15 K

William Thomson
(Lord Kelvin)
1824 – 1907

Scottish mathematician
and physicist
Temperature Scales: Rankine

Ice point: 491.67 °R   Steam point: 671.67 °R

William Rankine
1820 – 1872

Scottish engineer
Temperature Conversions

Fahrenheit / Celsius
        TC  TF  32
            5                 9
                          TF  TC  32
            9                 5


Celsius / Absolute

       TA  TC  273.15   TC  TA  273 .15 
Temperature Style and Grammar

Temperature differences or changes are expressed in
  “Celsius degrees” (C°).

The temperature of an object or substance is
  expressed in “degrees Celsius” (°C).

The unit of absolute temperature is the “kelvin” (K).
  There is no such thing as a “degree Kelvin” (°K).
Temperature Expansion – Linear

Most materials expand when they get hotter. As
 they expand, all their linear dimensions (length,
 width, height, diameter, etc.) expand
 proportionally.

The amount of the expansion depends on:
  The amount of temperature change
  The original size of the dimension
  A material property: coefficient of linear expansion
Temperature Expansion – Linear

               L  aL0 T

SI unit for coefficient of linear expansion: (C°)-1

Values of a for some materials are tabulated on
p. 342 of your textbook.
Temperature Expansion – Volume

              V  bV0 T

SI unit for coefficient of volume expansion: (C°)-1

Values of b for some materials are tabulated on
p. 342 of your textbook.
Temperature and Heat

Heat is the energy that moves from an object or
 substance at higher temperature to an object or
 substance at lower temperature because of their
 temperature difference.

SI unit of heat: the joule (J).
Temperature and Heat

When heat flows from a hotter object into a colder
 one, the internal energy of the hotter object
 decreases, and the internal energy of the colder
 object increases.

The internal energy consists of several forms of
  molecular kinetic and potential energy.
  Temperature is not a measure of an object’s total
  internal energy.
Temperature and Heat

Does the same amount of heat energy, flowing into
 or out of a variety of objects, change every
 object’s temperature by the same amount?

No. The change in temperature depends on:
the amount of heat lost or gained;
the mass of the object; and
the material that the object is made of.
Temperature and Heat

The material property is called the specific heat
  capacity:
                                     temperature
                  Q  cmT           change


    heat         specific
                            mass
    required     heat
                 capacity



SI units of specific heat capacity: J / (kg C°)
Heat: Other Units

Satan’s units* for heat:
calorie (cal): the amount of heat that increases the
  temperature of 1 gram of water by 1 C°
kilocalorie (kcal): increases 1 kg H2O by 1 C°
nutritional Calorie: = 1000 regular calories = 1 kcal
British Thermal Unit (BTU): increases the
  temperature of 1 pound of water by 1F°
(*Satan loves to spread misery and confusion.)
Heat and Phase Change

To change the phase of a material (melt ice, freeze
  water, boil water, condense steam) we must add
  heat, or remove heat.

At the phase-changing temperatures (melting point
  or boiling point), heat is added or removed
  without changing the temperature until the phase
  change is complete.
Heat and Phase Change
Heat and Phase Change

The amount of heat required to accomplish the
  phase change depends on the mass of material
  involved, what kind of material it is, and what
  phase change we are considering.
          heat          mass

In general: Q  mL
                                  latent heat


SI units of latent heat: J / kg
Heat and Phase Change
                   Phase change names

                      solid           liquid             gas


                                      melting        sublimation
from solid to:       -----
                                      (fusion)      (vaporization)

                                                      boiling or
                    freezing
from liquid to:                       -----          evaporation
                    (fusion)
                                                    (vaporization)

                  condensation     condensation
from gas to:                                           -----
                  (vaporization)   (vaporization)
Equilibrium Between Phases
                             most things
Fusion curves:               contract
                             when they
  temperatures and           freeze

  pressures at which
  the solid and liquid
  phases are in
  equilibrium.
                             water expands
                             when it freezes
Equilibrium Between Phases

Vaporization
 curves:
 temperatures and
 pressures for
 liquid – gas
 equilibrium.

                    water
Relative Humidity
Partial pressure: when a number of different
  molecular species are present in a mixture of
  gases, the total pressure of the mixture is the sum
  of the partial pressures due to each constituent.

If the partial pressure of water vapor in the
   atmosphere reaches the equilibrium pressure
   (from a vaporization curve), water leaves the
   atmosphere at the same rate it enters. (Fog or
   rain.)
 Relative Humidity

 Relative humidity tells how much water vapor is in
   the air, compared to how much can be in the air.

% relative humidity 


                                       or
            partial pressureof water vap in the air
                                                               100%
                                  or
equilibriu m pressureof water vap at the current te mperature
Relative Humidity

Given an existing partial pressure of water vapor in
  the air, if the air is cooled, it will reach a
  temperature for which the equilibrium vapor
  pressure of water decreases to be equal to the
  existing partial pressure of water vapor. At that
  temperature, the relative humidity is 100%, and
  water starts coming out of the atmosphere. This
  temperature is called the dew point.

								
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