# 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
Temperature Expansion – Volume

V  bV0 T

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

Values of b for some materials are tabulated on
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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