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THE NATURE
of HEAT
As we know, heat is a form of energy. In the form of
infrared radiation, heat from the sun travels through space at the speed of
186,000 miles per second. Upon arriving on earth, much of the radiant heat is
absorbed by different kinds of matter and is converted into heat that we can
feel (sensible heat). When you sit in the sun for a period of time on a clear
spring day, you may find that your clothing and other objects around you
become warm. Similarly, when you walk barefoot across a beach on a summer day,
you may find the sand so hot that it burns your feet. In both cases, radiant
heat from the sun has been absorbed by matter, and has been converted into
heat that you can feel. In this chapter, we will study the effect of heat upon
matter.
HEAT AND THE MOTION OF
MOLECULES
Have you ever tried to drill a hole through a piece
of metal? Both the drill and the metal become very hot.
Around 1800, an English scientist named Count Rumford
noted that, when a drill was used to bore a cannon, the bit of the drill and
the cannon both became very hot. To keep the metals cool, he placed a cylinder
of water around the end of the cannon. As the boring continued, the water
became warmer and eventually boiled. Since the bit of the drill and the cannon
were cold at the start, Rumford concluded that the heat produced probably came
from the friction created by the particles of the metal of the
bit rubbing against the particles of the metal of the cannon. Further,
he theorized that the motions of the particles in the metals themselves (atoms
or molecules) generated the heat.
Recall that
various forms of energy can be converted into other forms: When electrical
energy passes through a thin wire, as in a toaster, the wire becomes hot
(electrical energy to heat energy). When you rub your hands together, heat is
produced from the friction between the rubbed surfaces (mechanical energy to
heat energy). In general, when any form of energy is absorbed by matter, the
energy is changed to heat. This may be explained by the kinetic-molecular
theory: The energy excites the molecules in the matter, causing them to move
faster and to collide more frequently. As more collisions take place, more
heat is; produced.
The effect of heat energy on the motion of molecules
can demonstrated by using a sealed tube containing a little mercury with some
glass beads floating on the surface of the mercury. At ordinary temperatures,
the glass beads merely float on the surface of the mercury. However, when the
tube of mercury is heated, the glass beads, bounce up and down in a violent
but random fashion. As still more heat is supplied to the sealed tube,
particles of mercury begin to move more swiftly. The glass beads are
repeatedly struck by many mercury particles at the same time; consequently,
the glass beads begin to move randomly themselves. Thus, Rumford's theory that
heat is related to the motions of molecules appears to be correct.
According
to the kinetic-molecular theory, heat energy
acquired by a body is transformed into increased kinetic energy of the
molecules of the body. We observe this increased kinetic energy whenever a
solid, a liquid, or a gas expands on heating. A further increase in
kinetic energy will eventually cause the particles of a
Recall that when an ice cube (a solid) is heated, it
melts and becomes liquid water. When the water is heated, it vaporizes and
becomes gaseous water. According to the kinetic-molecular theory, as
increasing amounts of heat are supplied to a piece of ice, the water
molecules move more rapidly until they gain sufficient energy to overcome
the attractive forces holding them together. This permits the ice to liquefy
and become water. Similarly, as still more energy is received, the water
molecules move at even greater speeds. The attractive forces in the liquid
are weakened and the water is converted into gaseous water. EXPANSION OF SOLIDS
Your laboratory experience with the ball and ring
apparatus indicated the effect of heat on volume. The increase in size is
not due to an increase in the size of the particles that make up the solid
ball, but rather to an increase in the average distance between the
particles. When an object is heated, its particles vibrate faster, collide
more violently, and consequently move farther apart, thereby increasing the
volume of the object.
When the object is cooled, the opposite change occurs
and the volume of the object decreases. This decrease in volume is called contraction.
The expansion of solids by heating may cause serious
practical problems. For example, the expansion of railroad tracks, bridges,
or the concrete in a roadbed can create dangerous situations. Thus,
allowance for the expansion of solids daring hot weather must he made in the
construction of rails, bridges, and roads. For example, when rails arc laid,
gaps between the ends of the rails provide for expansion. If this were not
done, consider what would happen to the railroad tracks on a very hot day.
The metal would expand, making the tracks bend and buckle,
which might cause an oncoming
train to be derailed. In bridge construction, expansion joints allow for
changes in the length of the bridge. Concrete roadbeds are built with spaces
between the sections of concrete to allow for expansion.
The contraction of solids, by cooling, may also
present problems. Thus telephone and electrical wires are strung loosely to
prevent their snapping as contraction takes place during the colder times of
the year.
UNEQUAL
EXPANSION OF SOLIDS
Through extensive studies, scientists have found that
different metals expand at different rates when they are heated. For
example, when a piece of iron and a piece of aluminum of equal size are
heated together, we find that the aluminum expands more than twice as much
as the iron. When two strips of different metals are fastened together, they
form a compound bar, or bimetallic strip, which is employed in, useful
devices such as thermostats and metallic thermometers. In these devices the
two different metals, usually brass and steel, are welded together. When the
bar is heated, it bends because the brass expands more than the iron and
becomes longer than the iron.
Thus, the brass strip will be on the outside of the
bend. As it cools, the bar returns to its original shape. The thermostat is a device containing a compound liar
that regulates the heating systems of our homes. When the temperature in the
house falls below the setting on the thermostat, the compound bar, which
contracts as it cools, closes the circuit, turning on the heat. As
the room is
warmed, the compound bar in the thermostat expands, bends, and thereby
breaks the circuit, shutting off the heat.
The bimetallic thermometer is often used as an oven
thermometer to indicate the temperature within an oven, or within a piece of
meat that is cooking.
EXPANSION OF LIQUIDS
Liquids, like solids, expand when heated. In the
laboratory experience we demonstrated that when water is heated, it expands.
When the same water is cooled to its original temperature, the water
contracts to its original volume. Many other liquids, such as alcohol and
mercury, behave in the same way. At lower temperatures, however, the
behavior of water is an exception to this rule. As water is cooled from
100° C to 4° C, it contracts-like other liquids do. However, when water is
cooled below 4° C, the water expands-unlike other liquids. Water continues
to expand until it reaches 0° C, its freezing point. It has been found, as
shown in that the spaces between the water molecules in ice are larger than
the spaces between the water molecules in liquid water. Ice is therefore
said to have an open structure. Thus, as ice is formed, the need for
increased space between the molecules causes the volume of the ice to be
greater than that of the water from which it was formed. (This expansion in
volume begins as liquid water
is cooled below 4° C.) Since the volume of ice is greater than the
volume of water from which the ice is formed, the density of ice is less
than the density of water. (Recall that density equals weight divided by
volume.) This is why ice floats on water.
Like solids, different liquids expand at different
rates. As we will see later, the expansion of liquids is used in alcohol and
mercury thermometers.
The expansion of liquids must be considered in
certain heating systems. In a hot-water heating system, allowances
must be made for the expansion of heated water. As the furnace heats the
water in the heating system, the water expands. If the expansion continues,
pressure would build up in the pipes and could damage the entire system. To
avoid this difficulty, an expansion tank is provided. Excess heated water
enters the expansion tank and thereby reduces the pressure in the system.
EXPANSION OF GASES
Cases, like solids and liquids, expand when heated.
Our laboratory experience indicated that, as air is warmed, it expands.
Scientists have made similar observations with other gases which indicate
that gases confined in an elastic container expand when they are heated and
contract when they are cooled.
The expansion of gases by heat must be considered by
automobile tire. manufacturers, since tires may burst if allowed to remain
in the sun indefinitely. A less serious hazard caused by expanding gases is
that bottles of soda may crack or even explode if they are ex posed
to heat for a considerable length of time.
Products
such as whipped cream, shaving cream, deodorants, and insect repellents are
now supplied in aerosol cans. These cans contain the product itself
and a gas that forces the product
out of the can when the valve, is open. When the product is used up, the can
still contains unused gas. If this can is thrown into incinerator, the gas
becomes heated, expands, and may cause the can to explode. Such aerosol cans
should be discarded in a manner that does not involve heating.
Different solids and liquids expand at
different rates when heated. Gases,
however, generally expand at the same rate when heated to the same
temperature, at a given pressure.
TEMPERATURE
Heat and temperature are two terms that are often confused.
We know that the temperature of a small sample of molten iron is
considerably higher that the temperature of the water in the ocean.
However, the total heat in a sample of molten iron is much less than
the total heat of the water in the ocean.
Scientist now accept Rumford’s theory that heat is related to the
motions of particles in matter. Thus,
heat depends on the total kinetic energy of the particles in a body.
Recall the equation relating kinetic energy with mass and velocity:
Because the water in the ocean is colder than the sample of molten
iron, the velocity of the particles in the water is less than the velocity
of the particles in the molten iron. However,
the much larger quantity (mass0 of water compensates for the smaller
velocity of the particles and thus the particles of water in the ocean
possess greater kinetic energy. This means that there is more heat in the water in the ocean
than in a small sample of molten iron.
But why is the temperature of molten iron higher?
Temperature, unlike heat, depends on the average kinetic energy of
the particles, that is, the kinetic energy per particle.
To find this average, we divide the total kinetic energy by the
number of particles. Thus, the
large mass of ocean water has a smaller average kinetic energy per particle
and consequently has a lower temperature than a small sample of molten iron.
MEASURING
TEMPERATURE
Instruments designed
to measure temperature are called thermometers.
Most thermometers are based on the principle that matter, on heating,
expands and, on cooling, contracts. In
general, matter expands and contacts regularly.
This means that the amount of expansion or contraction in length are
generally equal for the same increase or decrease in temperature.
This regular expansion and contraction has made it possible to
construct three different types of thermometers: gas (air), liquid (mercury
and alcohol), and solid (bimetallic) thermometers.
The gas (air) thermometer
In this thermometer, the glass bulb contains air.
When the bulb is warmed, the air in the bulb expands and forces some of the
colored water out of the tube. This changes the level of the liquid in the
tube. By placing a suitable scale alongside the tube, temperature changes
can be measured. Air thermometers of this type, while interesting, are not
very accurate because the volume of a gas is also influenced by the air
pressure around it. (Note that the flask contains a tube open at both ends.
Why?).
LIQUID THERMOMETERS
Thermometers containing liquids such as mercury and
alcohol are useful and accurate because these liquids usually expand and
contract uniformly (regularly).
Mercury thermometers are made by filling a thin glass
tube with mercury at a temperature greater than the maximum to be measured.
The tube is then cut and sealed at the top. When the mercury cools, it
contracts, leaving a partial vacuum above the mercury. (Liquids expand and
contract to a much greater extent than do solids. Thus, in the given
temperature range, the glass tube is scarcely affected by the temperature
change.) The partial vacuum eliminates the effect of air resistance to the
expansion of the mercury. The scale of the. thermometer is generally fixed
by locating the boiling and freezing points of water on the scale. The
distance between the boiling and freezing points is then divided into units
depending on the temperature scale used. This will be discussed in the next
section.
Since mercury freezes at -39° C, it cannot be used
to measure very low temperatures. However, mercury boils at 357° C, which
means that a mercury thermometer can be used to measure temperatures above
the boiling point of water. On the other hand, alcohol freezes at -114° C.
Accordingly, alcohol thermometers are used to measure low temperatures.
However, since alcohol boils at 78° C, alcohol thermometers cannot be used
to measure high temperatures.
SOLID (BIMETALLIC)
Recall that a bimetallic strip behaves as it does
because different metals expand at different
rates. Because most metals melt only at very high temperatures, a
thermometer that uses a bimetallic strip can measure temperatures as high as
1000° C. The dial thermometer, used in most homes as an oven thermometer,
is an example of a bimetallic thermometer. A curved bimetallic strip, with
the faster-expanding metal on the outside of the bend, is attached to a
pointer. Upon heating, the bimetallic strip moves, causing the pointer to
indicate the temperature on a circular scale.
Other metallic thermometers, called resistance
thermometers, use the principle that the resistance of a wire changes with
temperature. Such thermometers also measure high temperatures.
THE
FAHRENHEIT AND CELSIUS
TEMPERATURE SCALES
Temperature markings on thermometers are indicated in
Fahrenheit degrees or Celsius degrees. The Fahrenheit and Celsius scales are
named after their originators, Gabriel
Fahrenheit and Anders
Celsius. (The Celsius scale is also called the centigrade
scale.) Both Fahrenheit and Celsius scales are calibrated by using the
boiling and freezing points of water. The Fahrenheit scale is used in the
English system of measurement and the Celsius scale in the metric system. In
the Fahrenheit scale, the freezing point of water is 32.° F, and the
boiling point of water is 212° F. The remainder of the scale between these
two points is marked off into 180 equal divisions (212 - 32 = 180) .
In the Celsius scale, the freezing point of water is 0° C and the boiling
point of water is 100° C. The remainder of the scale between these points
is divided into 100 equal divisions (100 ‑ 0 = 100). Note that there
are 180 divisions between the freezing and boiling points of water in the
Fahrenheit scale and 100 divisions between these points in the Celsius
scale. Thus, each Celsius division ( degree ) is 9/5 as large as
Fahrenheit division. This relationship, together with the fact that
there are 32 Fahrenheit divisions between 0 °F and 32° F, makes it
possible to convert one scale into the other by using the following
formulas:
°
C = 5/9(° F – 32)
°
F = 9/5 °C + 32
THE KELVIN SCALE
Confined gases, like most solids and liquids, expand
and contract uniformly. For this statement to be true, however, a gas must
be heated or cooled in such a way that the pressure remains constant. (
Recall that the air thermometer is inaccurate because it is affected by
surrounding air pressure.) If we start at 0° C, we find that, for every
Celsius degree rise in temperature, the volume of a gas increases 273 of its
original volume ( provided the pressure does not change). Similarly, if we
again start from 0° C, we find that for every Celsius degree drop in
temperature, the volume decreases 273 of its original volume. At -273° C,
the volume of a gas would shrink to zero and all molecular motion would
cease. This, in turn, means that the gas would contain no heat. (Actually, gases generally liquefy before this
temperature is reached.) Scientists refer to -273°C as absolute zero, a
temperature that has never been attained, although some scientists have come
very close to this point.
Absolute zero, -273°C, is also called 0 Kelvin ( 0 K
). The Kelvin scale, named after its originator, Lord Kelvin, is based on absolute temperatures. Since the Kelvin scale begins with
absolute zero (-273° C), we use the following formula to convert the
Celsius scale to the Kelvin scale:
degrees Kelvin = degrees Celsius
+ 273 degrees
This formula may be written as
K = ° C + 273
Let us find the freezing point of water (0° C) in
the Kelvin scale:
Thus, 0° C is equivalent to 273 K.
Now, let us find the boiling point of water:
Thus, 100° C is equivalent to 373 K. TRANSFER
of HEAT
When
a
metal spoon is placed in a bowl of hot soup, the entire spoon soon becomes
hot because the heat travels from the soup to the bowl-shaped part of the
spoon, and then to the handle. When ice is placed in warm water, the ice
soon melts. Both of these examples show that heat travels from one body to
another. Generally, when objects are at different temperatures, heat is
transferred from the warmer object to the cooler object until both objects
are at the same temperature. Heat transfer can occur through one of three
methods: conduction, convection, or radiation.
CONDUCTION
When one end of a metal rod is held in a flame, the
entire rod will become hot enough to burn the hand. The heat from the
flame reaches the hand by traveling through the rod. Substances that allow
heat to travel through them are called conductors. In general, as we
learned before, metals are good conductors. However, some metals conduct
heat more readily than others. This can be demonstrated by inserting rods
of aluminum, copper, iron, nickel, and brass into a brass sphere or disk
and then attaching a small ball of wax to the end of each rod. When the
center of the brass disk is heated, the wax at the tip of each metal melts
in the order in which the different metals conduct heat. The wax at the
tip of the copper melts first and the wax at the tip of the iron melts
last.
Conduction in most materials can be explained by the
kinetic-molecular theory. When one end of a rod is heated, the molecules
in that end of the rod vibrate faster and strike other nearby molecules,
causing them to vibrate faster. In this manner, the increased molecular
motion is transferred from one end of the rod to the other, permitting the
heat to travel through the rod.
Substances that do not readily allow heat to pass
through them are called insulators. Gases and liquids are generally poor
conductors of heat because their molecules are farther apart than are the
molecules in solids. Therefore, neighboring molecules in a gas or in a
liquid are less affected by the increased motions of heated molecules, and
consequently heat is not conducted rapidly.
Substances like wood or plastic are poor conductors
of heat, so they are used to make handles for metallic objects that are to
be heated. The clothing we wear is also a poor conductor of heat, enabling
us to retain body warmth. Porous material is generally non-conducting
because it contains layers of trapped air which do not permit heat
transfer.
CONVECTION
Although gases and liquids are poor conductors of
heat, heat is transferred through them by the process of convection.
Convection is the transfer of heat due to the motion of the liquid or gas
itself. For example, when a beaker of water is heated the water layer closest to the heat source is warmed slowly by
conduction. As the water becomes warmer, it expands, becomes less dense,
and rises. This brings heat to the upper layer. At the same time, cooler water
from the upper portion
of the
beaker moves down, takes the place of the rising water, and becomes heated
itself. When warm enough, this water rises and carries heat upward. As
these processes continue, heat that enters the bottom of the beaker is
distributed throughout the beaker until all the water is at the same
temperature. The moving water in such a case is said to have set up a
convection current. Heat is also transferred through gases by convection.
It is by this means, in part, that a stove or a radiator heats a room.
Heat from the radiator warms the air above it, causing the air to expand,
become less dense, and rise. The cooler air that moves in to take the
place of the warmed air is also soon warmed. As this air rises, a
convection current is established. The convection current continues to
distribute heat throughout the room until the entire room is warmed.
The formation of a convection current in air is
demonstrated with a convection box apparatus. First the candle is lighted,
then smoking touch paper is placed over the
chimney, opposite the candle. The smoke, coloring the air, can be
seen to move down this chimney, across the box, and out through the other
chimney. This occurs because the air over the candle is heated, becomes
less dense, and rises, leaving a partial vacuum. Cooler, more dense air
from the first chimney moves in to fill the partial vacuum. This cycle
continues as long as heat is given off by the burning candle. RADIATION We know that light energy and heat energy travel from
the sun to the earth through space, which is an almost perfect
vacuum. These forms of energy, traveling without the aid of molecular
collisions, are transferred from the sun to the earth by radiation, that
is, by means of rays, or waves. You can understand this method of heat
transfer by standing a short distance from an open fire or by placing your
hand a little to one side of, but not touching, a hot radiator. Since
neither source of heat is being touched, you cannot receive heat by
conduction. Since warm air rises vertically from the heat source, the heat
cannot reach you by convection. The heat that is transferred to you from
the fire or radiator reaches you by radiation.
The heat radiated by one body ( the sun, for example)
is most rapidly absorbed by other bodies that are black in color and rough
in texture. In warm climates, white clothing Bodies that are shiny and smooth do not absorb heat
readily. Instead, these bodies reflect heat. Thus, aluminum used for
roofing keeps homes cool in the summer and warm in the winter. This
principle is utilized in the thermos (vacuum) bottle, which is so
constructed as to permit liquids to retain their temperatures for a long
time. A thermos bottle is double walled, with a partial vacuum between the
walls to prevent heat transfer by conduction or convection. A cork stopper
also prevents heat transfer by conduction. The inner glass walls are
silvered to reflect radiant heat back into the liquid, thereby minimizing
heat loss by radiation. Thus, a hot liquid remains hot because heat is
lost very slowly. A cold liquid remains cold in thermos bottles because
outside heat enters very slowly by conduction, convection, or radiation. MEASURING
HEAT We learned that temperature is a measure of the average kinetic
energy of the molecules of a substance. This is the same as saying that
temperature represents the average intensity of the motion of the
molecules, or the degree of hotness of a substance. Average kinetic energy
means the total kinetic energy divided by the total number of particles.
Recall that the ocean contains much more heat
than does a
small amount of molten iron. However, since the ocean contains many more
particles than the molten iron, the temperature
of the ocean
(that is, the total kinetic energy divided by the total number of
particles) is lower than that of the molten iron. Temperature, therefore, does not tell us the quantity
of heat present. The quantity of heat represents the total kinetic energy
contained by all of
the particles of the substance. In the metric system, we measure the quantity of heat
by a unit called the calorie. The calorie is the amount of heat needed to
raise the temperature of 1 gram of water 1 Celsius degree. In the English
system, heat is measured by a unit called the British Thermal Unit (BTU).
A BTU is the amount of heat needed to raise the temperature of 1 pound of
water 1 Fahrenheit degree. The amount of heat energy present in a
substance cannot be measured directly with simple measuring devices.
Instead, it is measured by observing its effect on a given quantity of
water in a device called a calorimeter.
One type of calorimeter, consists of two polished metal cups
surrounded by air, a poor conductor of heat.
An insulating cover, holding a thermometer, makes up the top of the
calorimeter. The polished cups reflect heat, thus maintaining the
temperature of the liquid in the container. To determine the amount of heat energy absorbed (or lost) by a given quantity of water, we multiply the weight of the water in grams by the change in temperature of the water in Celsius degrees. Thus: amount of heat = weight of water X change in temperature In a calorimeter, when 20 grams of water at 20°C are
heated to a temperature of 30°C, how much heat is absorbed? The temperature change = the final temperature - the
initial temperature = 30°C - 20°C = 10 Celsius degrees. Substituting,
We conclude that 200 calories have been absorbed.
(We assume that no heat has escaped from the calorimeter.)
HEAT
EXCHANGE IN WATER
In a calorimeter, when a quantity of water at a given temperature is
mixed with a quantity of water at a different temperature, the amount of
heat lost by the "hot" water is equal to the amount of heat
gained by the "cold" water. Suppose
we mix 100 grams of water at 90° C with 100 grams of water at 40° C and
find that the final temperature of the mixture is 65° C. Let us calculate
the number of calories lost. by the hot water and gained by the cold
water. The
temperature of the hot water dropped from 90° C to 65° C, a decrease of
25 Celsius degrees. Since we began with 100 grams of hot water that
underwent a temperature change of 25° C, we determine the amount of heat
lost:
amount of heat =
weight of water X change in temperature
amount of heat = 100 grams X 25° C amount of heat = 2500 calories
The minus sign in the answer indicates that 2500
calories of heat have been lost.
The temperature of the cold water increased from 40° C to 65° C,
an increase of 25 Celsius degrees. Since we began with 100 grams of cold
water that underwent a temperature
change of 25° C, we determine the amount of heat gained: amount
of heat = weight of water X change in temperature
amount of heat = 100 grams X 25° C amount of heat = 2500 calories
Note that the amount of heat lost by the hot water
(2500 calories) is the same as the amount of heat gained by the cold water
(2500 calories). We assume that the heat exchange was "perfect"
and that no heat escaped from the calorimeter. The
quantity of heat needed to raise the temperature of 1 gram of a substance
1 Celsius degree is called the specific heat of the substance. For
water, the specific heat is 1. This means that 1 calorie of heat will
raise the temperature of 1 gram of water 1° C. Water is the only
substance for which this is true. Other substances vary in the quantity of
heat needed to raise 1 gram of the substance 1 Celsius degree.
Consequently the formula
amount of heat =weight of water X change in
temperature applies only to
water.
CALORIES
AND FOOD
Your body requires energy in order to perform its daily tasks.
Most of this energy comes from energy‑rich foods such as
carbohydrates and fats. This energy is released when the body utilizes
these foods. Using special calorimeters, scientists have measured the
energy content, or the number of calories present, in fixed quantities of
certain foods. For example, a slice of white bread contains about 60 000
calories; a typical chocolate bar may contain about 300 000 calories. Nutritionists use a special kind of notation when discussing the calorie content of foods. They define a food Calorie ( written with a capital letter) as 1000 calories. On a calorie table, therefore, we would read that a slice of white bread contains about 60 Calories and that a chocolate bar contains about 300 Calories. This
information comes from "Physical Science Workbook", 1961 |
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Page Last Updated: Friday March 02, 2007 Webmaster: Larry Jones Pickens County School District |