Title: Chapter 4: Matter and Heat
1Chapter 4 Matter and Heat
- Alyssa Jean-Mary
- Source The Physical Universe by Konrad B.
Krauskopf and Arthur Beiser
2Matter
- All matter is made up of billions of tiny
separate particles whether it is a solid, a
liquid, or a gas - These particles are in constant random motion
the kinetic energy of this motion is what
constitutes heat
3Temperature and Heat
- If something is at a higher temperature, it
contains more heat - However, if two objects are at different
temperatures, the object at the higher
temperature doesnt necessarily have more heat
than the object at the lower temperature just
because it is at a lower temperature - If there is more of an object with a lower
temperature, it has more heat than less of the
same object with a higher temperature - Also, if there are two objects with the same mass
(i.e. the same amount), even if they are at the
same temperature, they might have different
amounts of heat
4Temperature and Thermometers
- Temperature is a physical quantity that means
something in terms of sense impressions i.e.
temperature is something that gives rise to
sensations of hot and cold - A thermometer is a device used to measure
temperature - The thermometers that we use everyday are based
on the principle that when most substances are
heated, they expand, and when most substances are
cooled, they shrink in other words, they are
based on the fact that different materials react
differently to the same amount of temperature
change - Mercury-in-glass thermometers work because
mercury expands more than glass when heated to
the same temperature and it shrinks more than
glass when cooled to the same temperature. Thus,
the length of mercury in the thermometer
indicates the temperature i.e. a greater height
of mercury means a higher temperature, and a
lower height of mercury means a lower temperature - Bimetallic strip thermometers contain two
different kinds of metals that expand at
different rates. When these thermometers are
exposed to hot and cold, they bend in different
directions. At higher temperatures, the metal
that expands more is on the outside of the curve,
and at lower temperatures, the metal that shrinks
more is on the outside of the curve, thus bending
the strip in the opposite direction. The exact
amount of bending in either direction determines
the temperature. These strips are used in
thermostats that switch on and off heating
systems, refrigerators, and freezers when certain
preset temperatures are reached. - Thermometers can also be based on the principle
that the color and amount of light that is
emitted by an object varies with the temperature
of the object - For example, a poker that is thrust into a fire
first glows dull red. It then glows bright red,
orange, yellow, and finally, white, if it reaches
a high enough temperature. - This principle is what astronomers use to
determine the temperature of stars
5Temperature Scales
- On the Celsius scale, the freezing point of water
is 0C and the boiling point of water is 100C - On the Fahrenheit scale, the freezing point of
water is 32F and the boiling point of water is
212F - This scale is used in only a few English-speaking
countries (the same countries that still use the
British system of units) - To convert between Fahrenheit temperatures (TF)
to a Celsius temperatures (TC), use the following
equations - TF (9/5)TC 32 (to convert from TC to TF)
- TC (5/9)(TF 32) (to convert from TF to TC)
- Normal body temperature is 98.6F on the
Fahrenheit scale and 37.0C on the Celsius scale
6Example Calculations of Temperature Conversions
- Converting TF to TC Example What is 56F in
degrees Celsius? - Answer
- TC (5/9)(TF 32)
- TC (5/9)(56 32) 13.3C
- Converting TC to TF Example What is 324C in
degrees Fahrenheit? - Answer
- TF (9/5)TC 32
- TF (9/5)(324) 32 615.2F
7Heat
- The heat of a body of matter is the sum of the
kinetic energies of all the separate particles
that make up the body - The more kinetic energy the particles contain,
the more heat the body has, and the higher the
bodys temperature. - The amount of heat that a body contains is called
its internal energy - Since heat is a form of energy, the SI unit of
heat is the Joule - The nature of the substance determines how much
heat is needed to raise or lower the temperature
of 1kg of the substance by 1C. Liquid water
needs more heat to change its temperature than
almost any other substance. For water, it takes
4.2kJ of heat to raise the temperature of 1kg of
water by 1C. In contrast, gold only takes 0.13kJ
of heat to raise the temperature of 1kg of gold
by 1C. - The specific heat capacity or the specific heat
(c) of a substance is the amount of heat that
needs to be added to or removed from 1kg of the
substance in order to change its temperature by
1C - The SI unit for specific heat (c) is kJ/kgC
- Thus, the specific heat for water is 4.2kg/kgC
and the specific heat for gold is 0.13kJ/kgC. - Metals have fairly low specific heats. This means
that only a small amount of heat is needed to
change the temperature of 1kg of a metal by 1C. - The equation involving specific heat is
- Q mc?T
- where Q is the amount of heat that is added or
removed from the substance, m is the mass of the
substance, c is the specific gravity of the
substance, and ?T is the change in temperature
8Example Calculation using Specific Heat
- Example What is the specific heat of a substance
that required 453kJ to change the temperature
from 56C to 89C if the mass of the substance is
96kg? - Answer
- Given 453kJ, 56C, 89C, 96kg
- Looking for specific heat (c)
- Equation Q mc?T thus, c Q/(m?T) and ?T
Tf - Ti - Solution ?T 89C - 56C 33C c
453kJ/(96kg)(33C) 0.143kg/kgC
9Heat Transfer
- Three ways to transfer heat from one place to
another - Conduction is when heat flows through an object.
For example, when one end of a poker is placed in
a fire, the other end gets warm because heat is
flowing through the poker. This way to transfer
heat is inefficient. - Convection happens when a portion of a fluid
(either a gas or a liquid) is heated, since it
expands, it becomes lighter than the surrounding,
colder fluid, and thus rises upward. This is the
idea that heat rises. - Radiation consists of electromagnetic waves.
Light is one of the most common forms of
radiation. This type of heat transfer occurs in
space because, since space is virtually empty,
neither conduction nor convection is possible to
any real extent. This is how the earth receives
heat from the sun. The earth also radiates heat
back into space.
10Metabolic Energy
- Metabolism is a biochemical process by which the
energy content of the food an animal eats is
released - To express this energy content, the unit used is
kilocalorie (kcal). A kilocalorie is the amount
of heat needed to change the temperature of 1kg
of water by 1C. - 1 kcal 4.2kJ (kiloJoules)
- The kilocalorie is actually what is referred to
when a dietician uses the term calorie.
11Maximum Power Output from Metabolic Energy 1
- An animal converts about 10 to 20 percent of its
metabolic energy to mechanical energy through
muscular activity - The rest of its metabolic energy goes into heat
energy, most of which escapes through the skin of
the animal - The maximum power output that an animal is
capable of depends on its maximum metabolic rate.
The maximum metabolic rate depends on an animals
ability to release the resulting heat, which
depends on the animals surface area. - Thus, since a larger animal has a larger surface
area than a smaller animal, it has a higher
maximum power output because it has a greater
ability to release the resulting heat, which
means that it has a higher maximum metabolic
rate. But, since a larger animal also has more
mass than a smaller animal, its metabolic rate
per kilogram decreases because an animals mass
goes up faster with its size than its skin area
does. - African elephants partly overcome this limitation
of a small surface-area-to-mass ratio because of
their enormous ears. Their ears help them get rid
of metabolic heat. - Most birds are small because if the size of a
bird increases, its metabolic rate per kilogram
(along with its power output since they are
directly related) deceases even though the amount
of work needed to perform per kilogram to fly
stays the same. This is way large birds (i.e.
ostriches and emus) are not noted for their
flying ability
12Maximum Power Output from Metabolic Energy 2
- Some typical basal metabolic rates (i.e. rates
that correspond to an animal resting) are - For a person, 1.2W/kg
- For a cow, 0.67 W/kg
- For a pigeon, 5.2 W/kg
- When an animal is active, its metabolic rate is
much higher than when the animal is resting (i.e.
when it is at its basal rate) - For example, a 70-kg person has a basal rate of
about 80W. When performing light work while
sitting, the persons rate will be about 125W.
When the person is walking, the rate will be
about 300W, and when the person is running hard,
the rate could be as much as 1200W. - The highest metabolic rate per kilogram are in
the flight muscles of insects - In addition to the muscles using energy, the
brain of an animal also uses energy to function - A persons brain uses about 20 to 25 percent of
the metabolic energy the person has. - A monkeys brain uses about 9 percent of the
metabolic energy the monkey has. - A cats brain and a dogs brain use about 5
percent of their metabolic energy. - If the amount of energy from food a person has is
greater that a persons metabolic needs, then the
excess energy goes into additional tissue. If the
person is active, then the excess energy goes
into muscle. If not, then the excess energy goes
into fat. The energy that is stored in fat can be
used later if the amount of energy from food is
not enough for a persons metabolic needs.
13Fluids
- Solids have a definite size and shape. Their
particles vibrate around fixed positions. - Liquids have a definite size (i.e. volume), but
they dont have a definite shape because they
flow to fit the container they are in. Their
particles are about as far apart as those of
solids, but they are able to move about. - Gases dont have a definite size or shape they
fill whatever container they are in. Their
particles are able to move about freely. - Fluids are substances that can flow readily.
Liquids and gases are therefore called fluids.
14Density
- The density of a substance is its mass per unit
volume - density mass/volume OR d m/V
- When a metal is referred to as heavy as opposed
to light, what is meant is that the metal has a
high density - The proper SI unit of density is kg/m3 (kilograms
per cubic meter), but it is usually expressed in
g/cm3 (grams per cubic centimeter), where 1 g/cm3
1000 kg/m3
15Example Calculation of Density
- Example What is the density of an object that
measures 3.4 cm by 23.5 cm by 5.7 cm and has a
mass of 432 g? - Answer
- Given 3.4cm, 23.5cm, 5.7cm, 432g
- Looking for density (d)
- Equation d m/V, V LxWxH
- Solution V (3.4cm)(23.5cm)(5.7cm) 455.43cm3
and thus, d 432g/455.43cm3 0.949g/cm3
16Pressure
- Pressure is the ratio between the force acting
perpendicular on an object and the area of the
object - Pressure force/area OR P F/A
- The SI unit of pressure is the Pascal (Pa),
where, since force is expressed in Newton (N) and
area is expressed in m2, Pa N/m2 - The SI unit Pascal was named after the French
scientist and philosopher Blaise Pascal - Since the Pascal is a very small unit (i.e. the
amount of pressure that your thumb exerts by
pushing really hard on a table is about
1,000,000Pa), the unit kPa (kiloPascal) is
usually used, where 1 kPa 1000 Pa 0.145lb/in2
17Example Calculation of Pressure
- Example What is the pressure on an object that
has a force of 56N applied to it and an area of
32m2? - Answer
- Given 56N, 32m2
- Looking for pressure (P)
- Equation P F/A
- Solution P 56N/32m2 1.75Pa
18Pressure in a Fluid
- For fluids, pressure is useful because
- The forces that a fluid exerts on the walls of
its container and those that the walls exert on
the fluid always act perpendicular to each other - The force exerted by the pressure in a fluid is
the same in all directions at a given depth - An external pressure exerted on a fluid is
transmitted uniformly throughout the fluid - Because fluids have these properties, if we have
a tube with a fluid in it, and we apply a
pressure with a pump to the fluid at one end of
the tube, we can transmit the force to the fluid
at the other end of the tube that will then
transmit the force to push against a movable
piston. Thus, we can transmit a force from one
place to another. - If the fluid used in a machine to transmit forces
is a liquid, the machine is called hydraulic. - If the fluid used in a machine to transmit forces
is a gas, the machine is called pneumatic.
19Pressure and Depth
- Inside a pump, the air at the bottom of the
cylinder is under a greater amount of pressure
than the air at the top of the cylinder because
of the weight of the air in the cylinder. - In a tire pump, this pressure difference is never
small - At sea level on the earth, there is an average of
101 kPa (15 lb/in2) of pressure due to the weight
of air above us. This amount of pressure
corresponds to 10.1N on every square centimeter
of our bodies. We are not aware of this extreme
pressure because the pressures inside our bodies
are the same. - A barometer is used to measure atmospheric
pressure. - If the depth increases, the pressure also
increases. - Thus, most submarines cant go under the water
more than a few hundred meters without the danger
of collapsing. - At a depth of 10 km in the ocean, the amount of
pressure is about 100 times greater than the
amount of pressure at sea level, which is enough
pressure to compress water by about 3 percent of
its volume. The fish that live at these depths
survive just like we do at sea level i.e.
because their inside pressures are the same as
the outside pressure. - Scuba divers carry tanks of compressed air with
regulator valves, which allow the air to be at
the same pressure as the water around them. Scuba
stands for Self-Contained Underwater Breathing
Apparatus. As a diver is returning to the
surface, the diver must constantly breathe out to
allow the air pressure in the lungs to decrease
at the same rate as the pressure of the water is
decreasing. If this is not done, the pressure
difference between the lungs and the water might
burst the lungs. This is the same principle that
is in effect when deep-sea fish explode when they
are brought too fast to the surface of the water.
20Buoyancy
- If an object is in a fluid, there is an upward
force acting on it due to the fact that pressure
increases with increasing depth. Because of this
(i.e. that the pressure is greater at larger
depths), the upward force on the bottom of the
object is greater than the downward force on its
top. The difference between these two forces is
called the buoyant force. - An object floats when its buoyant force is
greater than its weight, and it sinks if its
buoyant force is less than its weight. - For example, balloons float in air and ships
float on the sea because their buoyant forces are
greater than that of their weights.
21Archimedes Principle
- A body of water is immersed in a tank of water.
This body of water is supported by a buoyant
force Fb that is equal to its weight, wwater
dVg. The buoyant force is the result of all the
forces from all the water that is in the tank
acting on this body of water. This buoyant force
is always upward because the pressure under the
body of water is always greater that the pressure
above it, while its weight is always a downward
force. The forces on the sides of the body of
water cancel each other out. - A solid object has a volume V and is in the same
size and shape tank as the tank of water that the
body of water is in. The forces on the object are
the same as the forces on the body of water.
Thus, Fb dVg, where d is the density of the
fluid, V is the volume of fluid that is displaced
by the object, and g is the acceleration of
gravity (9.8m/s2). - Archimedes Principle Buoyant force on an
object in a fluid weight of fluid displaced by
the object - Archimedes Principle holds whether an object
floats or sinks. An object floats if its weight
is less than the buoyant force, and it sinks if
its weight is greater than the buoyant force.
When an object is floating, only the part of the
object that is actually in the water is used to
calculate the volume. - If an object floats in a fluid, that means that
its average density is lower than the density of
the fluid. Even though the density of steel is
higher than the density of water, a steel ship
floats because it is a hollow shell, so
therefore, the average density of the ship is
actually lower than the density of water even
though it is made out of steel, and even when the
ship is loaded with cargo. If the ship gets a
leak and starts to take on water, the average
density of the ship starts to increase, and thus,
the ship starts to sink.
22Example Calculation of Buoyant Force
- Example What is the buoyant force on an object
if it is in a fluid with a density of 2.3kg/m3
and displaces a volume of 45m3 of the fluid? - Answer
- Given 2.3kg/m3, 45m3
- Looking for buoyant force (Fb)
- Equation Fb dVg
- Solution Fb (2.3kg/m3)(45m3)(9.8m/s2)
1014.3N
23Boyles Law
- When the temperature of a gas is held constant,
if the pressure on the gas is doubled, the volume
of the gas is halved - in other words, the
pressure and the volume of a gas are inversely
proportional to each other (i.e. if one
decreases, the other one increases by the same
proportion), as long as the temperature is held
constant - As an equation, Boyles Law is
- p1/p2 V2/V1 (at constant temperature)
- where p1 is the initial pressure, p2 is the
final pressure, V1 is the initial volume, and V2
is the final volume - Another version of the equation is
- p1V1 p2V2
24Example Calculations using Boyles Law
- Example 1 What is the final volume of a gas that
started with a volume of 45L under a pressure of
2.3atm if 5.7atm is now applied to it? - Answer
- Given 45L, 2.3atm, 5.7atm
- Looking for final volume (V2)
- Equation p1V1 p2V2 - thus, V2 p1V1/p2
- Solution V2 (2.3atm)(45L)/(5.7atm) 18.2L
- Example 1 How much pressure is applied to a gas
that started with a volume of 900L under a
pressure of 24.3atm if it now has a volume of
48L? - Answer
- Given 900L, 24.3atm, 48L
- Looking for final pressure (p2)
- Equation p1V1 p2V2 thus, p2 p1V1/V2
- Solution p2 (24.3atm)(900L)/(48L) 455.6atm
25How Temperature Changes Affect Volume and Pressure
- When the pressure of a gas is held constant, if
the temperature of the gas is increased, the
volume of the gas is also increased AND if the
temperature of the gas is decreased, the volume
of the gas is also decreased. Thus, the
temperature and the volume of a gas are directly
proportional. - Instead, when the volume of a gas is held
constant, if the temperature of the gas is
increased, the pressure of the gas is also
increased AND if the temperature of the gas is
decreased, the pressure of the gas is also
decreased. Thus, the temperature and the pressure
of a gas are directly proportional.
26Absolute Zero
- On the Celsius scale, absolute zero is at -273C
- At absolute zero, if the volume of a gas was held
constant, the pressure of the gas would be zero.
Also, at absolute zero, if the pressure of a gas
was held constant, the volume of the gas would be
zero - It is unlikely that experiments will ever reach a
temperature of absolute zero for two reasons - 1. It is impossible to reach such a low
temperature. - 2. All known gases turn to liquids before they
reach this low temperature. - The absolute temperature scale is another scale
that is used to give temperatures (in addition to
the Fahrenheit scale and the Celsius scale). This
scale begins at absolute zero and is given as
degrees Celsius above absolute zero. Therefore, a
Celsius temperature (TC) can be converted into an
absolute temperature (TK) using TK TC 273 - On this scale, the freezing point of water is
273K (K Kelvin for English physicist Lord
Kelvin) and the boiling point of water is 373K
27Charless Law
- Charless Law says that the volume of a gas is
directly proportional to the absolute temperature
of the gas - As an equation, Charless Law is
- V1/V2 T1/T2 (at constant pressure)
- where V1 is the initial volume, V2 is the final
volume, T1 is the initial temperature in K
(absolute temperature), and T2 is the final
temperature in K (absolute temperature) - Another version of the equation is
- V1/T1 V2/T2
28Example Calculations using Charless Law
- Example 1 What is the final volume of a gas that
started with a volume of 97L at a temperature of
35C if it is heated to 56C? - Answer
- Given 97L, 35C, 56C
- Looking for final volume (V2)
- Equation V1/T1 V2/T2 - thus, V2 V1T2/T1 and
TK TC 273 - Solution TK 35C 273 308K TK 56C 273
329K thus, V2 (97L)(329K)/(308K) 103.6L
29The Ideal Gas Law
- Boyles Law and Charless Law are combined to
form the ideal gas law - p1V1/T1 p2V2/T2
- At constant temperature, T1 T2, and the
equation becomes Boyles Law - At constant pressure, p1 p2, and the equation
becomes Charless Law - Another version of the ideal gas law is
- pV/T constant
- This version of the ideal gas law shows that
these quantities as a whole dont change in value
for a gas sample even if the individual
quantities (i.e. p, V, or T) may change.
30Example Calculation using the Ideal Gas Law
- Example What is the final temperature of an
object that was initially at a pressure of
4.5atm, a volume of 345L, and a temperature of
90C, if it is now under a pressure of 3.2atm
with a volume of 65L? - Answer
- Given 4.5atm, 345L, 90C, 3.2atm, 65L
- Looking for final temperature (T2)
- Equation p1V1/T1 p2V2/T2 - thus, T2
p2V2T1/p1V1 and TK TC 273 - Solution TK 90C 273 363K thus, T2
(3.2atm)(65L)(363K)/(4.5atm)(345L) 48.6K
31The Kinetic Theory of Matter
- The kinetic theory of matter involves a simple
model that accounts for many physical and
chemical properties of matter. - This model says that all matter is composed of
tiny particles. - For a gas, these particles are referred to as
molecules, which are substances that consist of
two or more atoms. - For liquids and solids, the particles can be one
of three types molecules, atoms, or ions.
32The Kinetic Theory of Gases
- The sizes, speeds, and shapes of the molecules of
many kinds of matter are known today - For example, a molecule of Nitrogen, which is the
chief constituent of air, is about 1.8 x 10-10m
across. It has a mass of about 4.7 x 10-26kg. At
0C, its average speed is about 500m/s, which is
about the speed of a rifle bullet. Every second a
molecule of Nitrogen collides with more than a
billion other molecules. Most of the other
constituents of air have a similar size and speed
to that of Nitrogen. For every cubic centimeter
of air, there are 2.7 x 1019 other molecules
present. To get an idea of how many molecules
this is if all of the molecules that are present
in a cubic centimeter of air were divided equally
amount 6.3 billion people, each person would have
over 4 billion molecules of air.
33The Three Basic Assumptions of the Kinetic Theory
of Gases
- There are three basic assumptions that apply to
the Kinetic Theory of gases - 1. Gas molecules are small compared to the
average distance between them. - 2. Gas molecules collide with each other without
losing kinetic energy. - 3. Gas molecules exert almost no force on one
another, except when they collide with each
other. - These assumptions have been verified by
experimentation - These assumptions show us that a gas is mainly
empty space in which isolated particles are all
moving around in different directions. Therefore,
we can compare a gas to a swarm of angry bees
that is in a closed room. Each molecule collides
with other molecules about billions of times a
second. Theses collisions change the speed and
direction of the molecule, but when they arent
colliding, they are unaffected by their
neighbors. There is no order to the motion of
these objects. They have no uniform speed or
direction. All that can be said about the
molecules is that they have an average speed and
that, at any given instant, there are as many
molecules moving in one direction as there are
molecules moving in the opposite direction. - If a molecule comes to a rest momentarily, it
will not stay that way long i.e. another
molecule will soon collide with it to send it
back into motion. - Also, if a molecule reaches a speed than the
average speed of the molecules, it will not stay
that way long either i.e. other collisions will
slow its speed.
34Properties of Gases from the Kinetic Theory of
Gases
- Gases can expand and even leak through a small
opening because of their rapid movement and the
fact that they dont have a strong attraction for
each other - Gases can be easily compressed because on
average, there molecules are far apart from each
other - One gas will mix with another gas because, since
the molecules are far apart from each other,
there is plenty of space in between them for
other molecules - The mass of a certain volume of a gas is much
less than the mass of the same volume of a liquid
or a solid because a gas is mainly empty space
35The Origin of Boyles Law
- A gas exerts a pressure on the walls of its
container because the billions and billions
molecules of the gas consistently hit the
container. When we measure these billions and
billions of tiny, separate hits of the molecules,
what we see is that a continuous force is
affecting the walls of the container. - The Kinetic Theory of Gases accounts for Boyles
Law, which states that p1V1 p2V2 when the gas
is at constant temperature - Think of the molecules of a gas in a cylinder as
some moving vertically (i.e. in between the
piston and the bottom of the cylinder) and as
some moving horizontally (i.e. in between the
walls of the cylinder) the molecules are moving
equally in either direction. Now, if the piston
is raised, which doubles the volume of the gas,
the molecules that are moving vertically are
going to have to travel further, which means that
they will not hit the piston or the bottom of the
container as much as they used to they will
actually hit the container half as much. The
molecules moving horizontally will also have to
change their bombardment of the walls of the
container because now they have more of the walls
to interact with. Since these molecules will need
to hit an area that is twice as big as before,
the number of hits on the walls of the container
will decrease just as those for the molecules
with vertical motion did. This shows that the
pressure in all parts of the cylinder (vertical
and horizontal) is cut in half when the volume is
doubled, which is what Boyles Law predicts. This
can be expanded to a real gas that has molecules
that are moving in random motion.
36Molecular Motion and Temperature
- A fourth assumption is added to the Kinetic
Theory of gases - 4. The absolute temperature of a gas is
proportional to the average kinetic energy of the
molecules of the gas - This assumption was added to account for the
behavior of a gas with a change in temperature - Since this shows that temperature is related to
the energy of the molecules, it also is related
to the speed of the molecules. Thus, if the
pressure inside a container increases, the
temperature inside that container also increases.
This is because when the pressure increases, the
molecules must be hitting the walls of the
container with more force, which means that they
are moving faster. - Earlier, we saw that if the temperature of a gas
is at 0K (or -273C), the pressure of the gas is
at zero. For this to occur, the bombardment of
the molecules must stop completely. So, at 0K
(i.e. absolute zero), the molecules of a gas
would lose all of their kinetic energy. (This is
a simplified idea since in reality, even at 0K,
there will be a small amount of KE that will
never be able to disappear.) The reason that
there is no temperature below 0K is that there is
simply no way to have less than no kinetic
energy. Thus, if there is constant volume, an
increase in the pressure of the gas will increase
the temperature of the gas, and if there is
constant pressure, an increase in the volume of
the gas will increase the temperature of the gas.
37The Origin of Charless Law
- When a gas is compressed, since the temperature
of the gas is the measure of the average kinetic
energy of the molecules of the gas, the
temperature in the cylinder should rise - Put a gas in a cylinder with a piston on top.
When the piston is moving down, thus increasing
the pressure inside the cylinder, the molecules
rebound from the piston with an increase in
energy, which causes an increase in the
temperature of the gas. This can be shown when
using a bicycle tire pump after you have used
the pump for awhile, you will notice that it gets
warmer because of the compression of the gas
inside as the pump is being used. Also, when the
piston is moving up, which decreases the pressure
inside the cylinder, the molecules will give up
some of their kinetic energy to the piston, which
will cause the temperature of the gas to
decrease. - Thus, as a gas expands, it cools. This can
explain the formation of clouds from rising moist
air. - As moist air is moving upward, since the
atmospheric pressure is decreasing, the water
vapor in the moist air is cooling, until it
condenses into the water droplets that constitute
clouds.
38Liquids and Solids Intermolecular Forces
- If you compare a gas to a swarm of angry bees,
then a liquid is bees in their hive, crawling
constantly over one another. - The molecules in a liquid slide past one another
easily, which is why liquids flow. Liquids flow
less readily than gases do because of the
intermolecular attractions that act only over
short distances. - The molecules in a solid are held together with a
stronger force than those that hold together
liquids. Actually, this force is so strong that
the molecules of solids are not free to move
about. The molecules of solids still move,
however they vibrate back and forth rapidly
between the particles that they are in between,
as if they were on a spring. This spring
represents the bond that is between two
molecules. This bond is electrical in nature. - The reason why a solid is elastic is because
after the molecules have been pulled apart or
pushed together by some force, the molecules
return to their original positions, with the
normal amount of space between the molecules
instead of too much or too less. A force that is
too great may deform the solid permanently. When
this occurs, the molecules move to new normal
positions and find new molecules to bond with. A
solid can actually break apart if too much force
is applied.
39Evaporation Changing a Liquid into a Gas
- A liquid is placed in an open container. The
molecules of the liquid are moving in all
directions in the dish, some moving faster than
others. Some of the molecules are moving fast
enough upward to escape into the air. They escape
into the air even though they have an attraction
to their neighbor molecules because the
attraction is not enough to stop them from
escaping. This loss of molecules to the air is
referred to as evaporation. Since it is the
faster molecules, and thus, the warmer molecules
that escape into the air, the slower molecules
are left behind in the liquid, which makes the
liquid cool. - If you compare the evaporation of water to
alcohol, you see that alcohol evaporates more
quickly than water, and thus, cools more quickly
than water. This is because the attraction liquid
alcohol molecules have for one another is less
than the attraction liquid water molecules have
for one another, and thus a greater number of
alcohol molecules can escape.
40Boiling Changing a Liquid into a Gas
- When a liquid is heated, at a certain
temperature, even molecules that are traveling at
average speed (i.e. not only the molecules that
are traveling at high speeds) can overcome the
attraction between their neighbor molecules and
escape into the air. At this temperature, there
are bubbles of gas throughout the liquid, and
thus, the liquid is boiling. Therefore, this
temperature is referred to the boiling point of
the liquid. - The boiling point of water is 100C, which is
higher than the boiling point of alcohol, which
is 78C. This reinforces the idea that alcohol
evaporates more quickly than water. - Evaporation and boiling differ in the following
two ways - 1. Evaporation occurs only at the surface of the
liquid, whereas boiling occurs throughout the
entire liquid. - 2. Evaporation occurs at all temperatures,
whereas boiling only occurs at the boiling point
or temperatures above the boiling point.
41Heat of Vaporization
- To change a liquid to a gas, whether by
evaporation or boiling, energy is needed - For evaporation, the energy is supplied from the
heat content of the liquid itself, which is why
the liquid that is left behind is cooler - For boiling, the energy is supplied from heat
from an outside source - The heat of vaporization of a substance is the
amount of energy that is needed to change each
kilogram of liquid into gas at its boiling point - For water at its boiling point, 100C, the heat
of vaporization is 2260kJ - The temperature of a liquid and its gas are not
different. Because of this, the kinetic energy
that the liquid has is the same amount of kinetic
energy that its gas has. Thus, the extra energy
that is supplied to the liquid to turn it into a
gas does not go into the kinetic energy of the
gas. Because the molecules in a liquid are closer
together, the intermolecular forces in a liquid
are stronger than those in a gas. In order to
change a liquid into a gas, the molecules of the
liquid have to be broken apart and moved so that
they are in positions that are far apart from
each other, and thus have smaller attractions for
each other. This requires that the strong forces
between molecules in a liquid need to be
overcome. The molecules of the liquid that are
moving apart to become gas molecules are gaining
potential energy, just like a stone that is
thrown upward against the earths gravity gains
potential energy, except this is potential energy
with respect to intermolecular forces. Thus, the
extra energy that is supplied to the liquid to
turn it into a gas becomes potential energy of
the gas. - When the reverse occurs, i.e. when a gas becomes
a liquid, instead of the liquid molecules
escaping from the liquid into the air, the gas
molecules are falling toward one another because
of their attraction to one another. When this
occurs, the potential energy that the gas
molecules are losing is taken up as heat by the
surroundings.
42Melting Changing a Solid into a Liquid
- Heat is needed to change a solid into a liquid at
its melting point, just like heat is needed to
change a liquid to a gas at its boiling point - The heat of fusion of a substance is the amount
of heat that is needed to change each kilogram of
solid into liquid at its melting point - For water at its melting point, the heat of
fusion is 335kJ/kg - Most other substances have a lower heat of fusion
than water - The same amount of heat that is needed to change
one kilogram of a solid to a liquid has to be
released in order to change one kilogram of a
liquid into a solid - The heat of fusion of a substance is always much
smaller than the heat of vaporization of the
substance - The molecules of a solid are arranged in such a
way that they have the maximum amount of force
between themselves and their neighbors. To become
a liquid, the molecules of a solid have to become
more random, to be able to move about more. To do
this, energy needs to be added so that the forces
between the molecules of a solid are overcome.
But, the amount of energy that is needed to do
this is not as much as for a liquid to become a
gas since a liquid still has a definite volume,
even if it doesnt have a definite shape, unlike
a gas, which has no definite shape or volume.
Since the molecules of a gas are so far apart,
they move more freely and thus, can expand. In a
vacuum, the molecules of a gas could expand
indefinitely.
43Water
- If we start with 1kg of ice at -50C, and we add
heat, the ice will increase in temperature until
0C (i.e. the melting point of water), which is
when it will start to melt. The temperature will
remain at 0C until all of the ice has melting.
After all of the ice has melted, and thus, has
become water, the temperature of the water will
increase until 100C (i.e. the boiling point of
water) is reached, which is when it will start
boiling. The amount of energy that is required at
this point (i.e. to turn the water into steam) is
a lot more than the amount of energy that was
required to melt the ice. The temperature will
remain at 100C until all the water has become
steam, and then, the temperature of the steam
will rise since more heat is still being added.
44Sublimation
- Sublimation occurs when a solid turns directly
into a gas, without first turning into a liquid. - Most substances will sublime as long as the right
conditions of temperature and pressure are
present. - Usually pressures well under atmospheric pressure
are needed in order for sublimation to occur. - One example of an exception is solid carbon
dioxide, which is also referred to as dry ice.
Solid carbon dioxide sublimes (i.e. turns into a
gas) at temperatures above -79C, even if it is
at atmospheric pressure - Instant coffee can be made using sublimation.
Coffee is first brewed, and then it is frozen.
Following that, it is put into a vacuum chamber.
The ice that is in the frozen coffee sublimes to
water vapor, which is pumped away. Freeze drying
coffee like this doesnt affect the flavor of the
coffee as much as when you dry it by heating. The
process of freeze drying is also used to preserve
many other materials, including blood plasma.
45Changes of State
46Energy Transformations
- Remember that any form of energy can be converted
into another form of energy. This applies to
heat, since it is a form of energy. The
conversion of heat to another form of energy does
not occur efficiently. - For example, mechanical energy is obtained by
heat that is given off from burning coal and oil
in various types of engines. A large amount of
the heat that is given off does not get changed
into mechanical energy it is wasted. In an
electric power station, about two-thirds of the
heat is wasted. This is a serious situation since
these loses occur on the raw energy that is
available to us. - This inefficient conversion of heat in engines
was discovered in the nineteenth century, at the
start of the Industrial Revolution. The loss of
heat is not due to poor design or construction of
the engines it is just because heat cant be
converted to another form of energy without these
losses. The reasons for this inefficient
conversion was studied by engineers to get as
much mechanical energy as they could out of a
given amount of fuel and by scientists to study
the properties of heat. What was learned was for
the idea that heat is actually the kinetic energy
of random molecular motion.
47Heat Engines
- Since all that is needed to obtain heat is to
burn a fuel, heat is an easy and a cheap form of
energy to obtain - A heat engine is a device that turns heat into
mechanical energy - Some examples of heat engines are the gasoline
and diesel engines of cars, the jet engines of
aircraft, and the steam turbines of ships and
power stations. - All engines operate in the same basic way a gas
is heated and then it expands against a piston or
the blades of a turbine - When a gas in a cylinder on top of which is a
piston is heated, since the temperature of the
gas is increasing, the pressure of the gas is
increasing, which makes the piston move upward.
This upward movement is what is used to make use
of an engine. When the piston reaches the top of
the cylinder, the conversion of heat into
mechanical energy stops since the piston stops
moving. If we want to continue to make use of an
engine, we need to push the piston back down
again. Then, we can start another cycle to expand
the gas. - If the piston is pushed back down to continue the
cycle when the gas is still hot, the amount of
work that needs to be done is the same as the
amount of energy that was produced by the
expansion of the gas. This means that if it is
pushed back down when the gas is still hot, no
net work will be done. Thus, for some work to be
done, the gas first must be cooled so that there
is less work required for the piston to be pushed
back down. This is where the heat is lost in an
engine. Thus, if you want an engine to continue
to work, there is no way to prevent this heat
loss. The heat that is lost usually ends up in
the atmosphere around the engine, in the water of
a nearby river, or in the ocean.
48The Complete Cycle of Heat Engines
- In the compete cycle of an engine, heat flows
into and out of the engine. During this process,
some of the heat is converted into mechanical
energy. - In order for an engine to operate, both a hot
reservoir and a cold reservoir are needed. A gas
flows naturally from the hot reservoir to the
cold reservoir. - In a gasoline or diesel engine, the hot reservoir
is the burning gases of the power stroke, and the
cold reservoir is the atmosphere. - Even though a vast amount of heat is contained in
the molecular motions of the atmosphere, the
oceans, and the earth itself, it is only rarely
used because a colder reservoir is needed for the
heat to flow into. - A refrigerator is the reserve of a heat engine.
It uses mechanical energy to push heat from a
cold reservoir to a warm reservoir. Energy is
required for this movement because heat naturally
flows from a warm reservoir to a cold one. Since
there is a large amount of energy that is needed
to drive a refrigerator, it is not a good cold
reservoir for an engine to use.
49Thermodynamics
- Thermodynamics is the study of heat
transformation - There are two fundamental laws of thermodynamics
- 1. Energy cannot be created or destroyed, but it
can be converted from one form to another. - 2. It is impossible to take heat from a source
and change all of it to mechanical energy or
work some heat must be wasted. - As you can see, the first law of thermodynamics
is actually the law of conservation of energy. It
basically is saying that we cannot obtain
anything from nothing. - The second law of thermodynamics referrers only
to heat. It says that the conversion of heat into
another form of energy is inefficient. - Thermodynamics specifies the maximum efficiency
of a heat engine only by ignoring the losses to
friction and some other practical difficulties.
The maximum efficiency depends only on the
absolute temperatures of the hot reservoir and
the cold reservoir by which the engine operates - Maximum efficiency (work output/energy
input)maximum - OR
- Eff(max) 1 (Tcold/Thot)
- where Eff(max) is the maximum efficiency, Tcold
is the temperature of the cold reservoir, and
Thot is the temperature of the hot reservoir - This equation shows that the greater the ratio
between the two temperatures, the less heat is
wasted, and therefore, the more efficient the
engine is.
50A Steam Turbine
- A steam turbine is what is used in a power
station. The steam comes from a boiler that is
heated by either a coal furnace, a oil furnace, a
gas furnace, or a nuclear reactor. The turbine
shaft is connected to an electric generator. In a
typical power station, the steam enters the
turbine at about 570C (843K) and exits at about
95C (368K) into a partial vacuum. The maximum
efficiency of a turbine like this is equal to 1
(368K/843K) or 0.56, so the maximum efficiency
is 56 percent. The actual efficiency is less than
40 percent due to friction and other sources of
energy loss.
51Why a Heat Engine Must Be Inefficient
- When a gas in a heat engine is heated, its
molecules increase their average speed, and thus
their average kinetic energy - The problem is that the engine can only use this
increased energy if the molecules of the gas are
moving in approximately the same direction as the
piston or turbine blades, but the gas molecules
are moving in random directions - To make the gas molecules move in the same
direction, a lot of the heat that was added to
the gas would need to be converted to mechanical
energy - It is impossible to do this conversion in a heat
engine, so only a small amount of the heat that
is in the gas can be extracted as energy of
orderly motion
52The Fate of the Universe
- Even though not all heat energy can be converted
to another form of energy, other forms of energy
can entirely be converted to heat - Because of this, there is an overall tendency
toward an increase in the heat energy of the
universe, while the other forms of energy are
thus decreased - Some examples
- When coal or oil is burned, chemical energy
becomes heat - When a machine is operated, friction turns some
of its energy into heat - An electric light bulb emits heat in addition to
its light - On earth, most of the lost heat is released into
the atmosphere, the oceans, and the earth, where
it is largely unavailable for recovery - In the universe, from a thermodynamics
standpoint, the stars, including the sun, are the
hot reservoir, and everything else, including the
earth and the moon, etc., is the cold reservoir.
As time goes on, the stars get colder, and the
rest of the universe will grow warmer, which
means that there will be less and less energy
available for the further evolution of the
universe. If seen on a molecular level, the order
will become disorder. If this continues,
everything in the universe will have the same
temperature and thus the same amount of average
energy, which is a condition referred to as heat
death. This is the only possible fate of the
universe.
53Entropy
- Entropy is the measure of the disorder of the
molecules that make up any body of matter - For example, liquid water has more entropy that
ice since its molecules are more randomly
arranged, and the entropy of steam is greater
than that of liquid water for the same reason - If the second law of thermodynamics is rewritten
in terms of entropy The entropy of a system of
some kind isolated from the rest of the universe
cannot decrease. - Writing the second law like this allows it to be
applied in an exact way to a variety of systems - For example, while it is being turned into ice,
if liquid water would rise from the ground by
itself, energy would be conserved because the
heat that is lost by the liquid water to change
it to ice would be converted to kinetic energy.
This would conserve energy, and thus obey the
first law of thermodynamics. This doesnt occur
because it violates the second law of
thermodynamics If this was to occur, the entropy
of the liquid water would decrease, which is
impossible. - In some cases, it might seem that the entropy of
something is decreasing, but the entropy of the
universe is still increasing - For example, a plant that is taking carbon
dioxide and water and converting them to leaves
and flowers appears to be losing entropy. But, it
needs sunlight to do this, and in order to
produce this sunlight, the entropy of the sun is
increasing. If this increase in entropy of the
sun is taken into account with the lost entropy
of the plant, overall the universe in actually
increasing its entropy - The second law expressed this way is an unusual
physical principle - It applies to assemblies of many particles, not
to individual particles - It tells what cannot occur, not what can occur
- It is closely tied to the direction of time
- If an event involves only a few individual
particles, it is reversible, but if it involves
many particles, it is not always reversible
i.e. time never run backwards since it is always
going in the direction of entropy increase - For example, a bouncing ball can be played
forward and backward, and you wouldnt be able to
tell which is which, but if an egg is broken and
it is played backward, you would be able to tell
the difference between backward and forward