Transcript Physics: Ch. 10 Heat

Physics: Ch. 10 Heat
10-1 Temperature and
Thermal Equilibrium
We can hold a “hot” object and a
“cold” object with our hands and
describe the temperature as either
“hot” or “cold.”
 Our hands serve as qualitative
indicators of temperature. However,
this also depends on the
temperature of our hands. This
same object may feel hot or cold.
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Quick Lab pg. 358 Sensing
Temperature
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Determining an object’s temperature
with precision requires a standard
definition of temperature and a
procedure for making measurements
that establish how “hot” or “cold”
objects are.
Adding or removing energy usually
changes temperature
 Consider an electric range stove
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A substance’s temperature
increases as a direct result of
added energy among the particles
Table 10-1: Different forms of
energy
Translational
Rotational
Vibrational
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Internal Energy(U)-energy
associated with atomic motion and is
proportional to the substance’s
temperature. The energy of a
substance due to the random
motions of it component particles
and equal to the total energy of
those particles.
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For an ideal gas, internal energy
only depends on temperature. For
gases with 2 or more atoms, as well
as liquids and solids, other
properties contribute to the internal
energy.
Comparing Temperature
and Internal Energy
Compare the density, color,
temperature, and internal energy of
a glass of milk at 20°C with these
same properties of half a glass of
milk
 Do any of these properties change
from full to half a glass of milk?
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Demo 1: Temperature and
Internal Energy
Place one drop of food coloring in
each glass of cold and hot water.
 Describe what happens
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Thermal equilibrium
The state in which two bodies in
physical contact with each other
have identical temperatures
 Ex: a can of warm fruit juice in a
large beaker of cold water
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Thermal equilibrium is the basis for
measuring temperature with
thermometers. A thermometer in
contact with an object measures the
temperature when the mercury
stops rising or falling. They are in
thermal equilibrium with each other.
Conceptual Challenge pg.
360
Thermal expansion
For solids, liquids, or gases,
increasing the temperature also
increases its volume.
 Why are there gaps several
centimeters wide in concrete
segments of a bridge?

Coefficient of volume
expansion-indicates
expansion characteristics
Different substances have different
amounts of expansion for a
temperature.
 Gases have large values; liquids
have much smaller values; solids
have the smallest values
 Ex: liquids in solid containers
expand more than the container

Measuring Temperature
In order for a device to be used as a
thermometer, it must make use of a
change in some physical property
that corresponds to changing
temperature, such as volume of a
gas or liquid, or the pressure of a
gas at constant volume.
 Ex: expansion of volume of mercury
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Calibrating a thermometer
Thermometer (thin, unmarked glass
tube of a liquid)
 Equilibrium with ice and water=ice
point (0°C)
 Equilibrium with steam and
water=steam point (100°C)
 Divide the distance between the
points into equally spaced degrees
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Celsius-Fahrenheit
Temperature Conversion
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TF = 9/5 TC + 32.0
Celsius-Kelvin Temperature
Conversion
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T = TC + 273.15
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0 K=absolute zero
Temperature conversion
Sample problem 10A
 What are the equivalent Celsius and
Kelvin temperatures of 50°F?
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Ch. 10-2: Defining Heat
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Energy is transferred from warm objects
to colder objects because of a difference
in temperatures.
Ex: can of juice in a beaker of water (fig.
10-7)
The energy is heat.
Heat is transferred from the higher
energy object (hotter) to the less energy
object (colder)
How is the energy
transferred at the atomic
level?
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Fig. 10-8
The average kinetic energy of the juice
molecules is the highest. They transfer
energy to the can and then the can
transferes energy to the water molecules
surrounding it.
The water molecules’ energy will increase
while the juice molecules’ energy will
decrease until equilibrium is reached.
Energy can travel both ways.
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Thermal equilibrium may be understood
in terms of energy exchange between two
objects at equal temperature.
Fig. 10-9 Equal Temperature
Energy transferred from the can to the
water is the same as the energy
transferred from the water to the can.
The net energy transferred between the
objects = zero
The amount of energy transferred
depends on the difference of
temperatures.
 The greater the temperature
difference between two objects, the
greater the amount of energy that is
transferred between them as heat.
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Quick Lab Explanation
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The nerves in our hands detect the
energy change passing through the skin
of our hands.
If you place a hand in cold water, energy
is transferred from the hand to the cold
water. When the hand is placed in a
higher temperature water (warm), energy
is transferred from the water to the cool
hand. The energy into the skin causes
the water to feel hot.
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Likewise, the hand that has been in
hot water gains energy from the
water. The loss of energy to the
lukewarm water makes that water
feel cold.
Units of Heat-Table 10-3
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Because heat is a form of energy, all heat
units can be converted to joules, the SI
unit for energy.
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PE=Potential Energy
KE=kinetic energy
U=Internal Energy
W=Work
Q=Heat
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Heat and Work
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Hammer a nail into wood. Pry the nail
loose from the wood and touch the side of
the nail. It should feel warm.
Work is done to the nail to pull it out of
the wood. The nail encounters friction
with the wood and most energy to
overcome this friction is transferred to
internal energy. The internal energy
increase raises the temperature of the
nail.
Increasing Internal Energy
Friction
 In solids, deforming their structure
Ex: stretching a rubber band or
bending a piece of metal
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Demo 2-Show the
conversion of work into
internal energy
QuickLab pg. 368
Total Energy is conserved
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Not all energy from work is transferred
into mechanical energy and not all kinetic
energy in inelastic collisions remains
kinetic energy.
Some energy is absorbed by objects as
internal energy (the nail in the wood)
If internal energy is taken into account,
the total energy is a universally
conserved property.
Conservation of Energy
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ΔPE + ΔKE + ΔU = 0
Sample ProblemConservation of Energy pg.
369
Conservation of Energy
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A 0.10 kg ball falls 10.0 m onto a
hard floor and then bounces back up
to a 9.0 m. How much of its
mechanical energy is transformed to
the internal energy of the ball and
the floor?
10-3 changes in
temperature and phase
We will explore a property of all
substances that causes their
temperatures to vary by different
amounts when equal amounts of
energy are added to or removed
from them.
 Ex: why the water in a swimming
pool is cool on a hot, sunny day
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This property affects the motion of
atoms and molecules in a substance,
which determines how much the
substances’ temperature changes for
a given amount of energy added or
removed
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Specific heat capacity-the energy
required to raise the temperature of
1 kg of a substance by 1ºC at
constant pressure
Specific Heat Capacity at
constant pressure
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Cp = Q
mΔT
When temperature increases, T and
Q are taken to be positive, which
corresponds to energy transferred
into the substance.
 When the temperature decreases, T
and Q are negative and energy is
transferred from the substance.
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Table 10-4 Specific Heat
Capacities
Specific heat capacities depends on
phase
 Ex: water, ice, and steam all have
different heat capacities
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Calorimetry
By the equation, we need to find the
mass, temperature change, and
energy transferred as heat to find
the specific heat capacity of a
substance
 The measurement of heat is difficult
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If a hot substance is placed in an
insulated container of cool water,
energy conservation requires that
the energy that substance gives up
must equal the energy absorbed by
the water.
 Qw = Qx
 cpwmwΔTw = cpxmxΔTx
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Calorimetry-a procedure used to measure
the energy transferred from one
substance to another as heat
Calorimeter-device used for making this
measurement; a thermometer is also
used for measuring the final temperature
when the substances are at thermal
equilibrium and a stirrer to ensure the
uniform mixture of energy throughout the
water
Fig. 10-12
Sample problem 10C
Calorimetry practice
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You are preparing to take a bath.
The cold water faucet supplies water
at 20°C, and the water from the hot
water faucet is 60°C. Each faucet
has poured 25.0 kg of water into the
tub. What is the temperature of the
bath?
Latent Heat
Look at the graph and chart on pg.
376
 These show the temperature change
of 10 g of ice as it is heated from –
25°C in the ice phase to steam
above 125°C at atmospheric
pressure
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When substances melt, freeze, boil,
condense, or sublime, the energy
added or removed changes the
internal energy of the substance
without changing its temperature.
 These changes in matter are called
phase changes.
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Heat is the energy that is exchanged
between two objects at different
temperatures or between two
objects at the same temperature
when one of them is undergoing a
phase change
Phase changes involve potential
energy between particles
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If the particles are far enough apart,
the bonds between them can break.
The work needed to increase
potential energy and break a bond is
provided by collisions with energetic
atoms or molecules as shown in fig.
10-14
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New bonds can be formed if atoms
or molecules are brought close
together. This involves the
collection of particles going from a
high potential energy to a lower
potential energy. This decrease
involves a release of energy in the
form of increasing kinetic energy of
nearby particles.
Energy required to melt a substance
goes into rearranging the molecules
Phase changes result from a change
in the potential energy between
particles of a substance.
 Energy is added or removed from a
substance undergoing a phase
change and so the particles
rearrange themselves to make up
for the change in energy.
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This occurs without a change in the
average kinetic energy of the
particles
 For example, energy is absorbed if
ice is melting and breaking the
bonds of the solid. However, new
bonds will form between the liquid
molecules and release some (not all)
energy again.
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Fig. 10-15
The difference between the PE of the
broken bonds and the newly formed
bonds is equal to the net energy
added to the ice. (no energy is
available to increase the kinetic
energy of the molecules)
 As a result, there is no increase in
temperature of the ice-and-water
mixture
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Net energy=heat of fusion
 The energy per unit mass
transferred in order to change a
substance from solid to liquid or
from liquid to solid at constant
temperature and pressure
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The energy required to vaporize a
substance mostly goes into
separating the molecules, not
increasing the kinetic energy of the
molecules.
Fig. 10-16
The net energy added to the liquid
to vaporize it equals the difference
in the potential energy of attraction
between the particles of a liquid and
the potential energy of attraction
between the gas particles.
 This is the heat of
vaporization=energy per unit mass
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The energy needed to vaporize a
substance is higher than to melt the
substance
 Therefore, the heat of vaporization
values are higher than the heat of
fusion values
 Latent heat=heat of fusion and the
heat of vaporization
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Latent heat = the energy per unit
mass that is transferred during a
phase change of a substance
 Q = mL
 Table 10-6
 Lf = heat of fusion
 Lv = heat of vaporization
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Sample problem 10D
Heat of phase change
 How much energy is removed when
10 g of water is cooled from steam
at 133°C to liquid at 53°C?
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How much energy is needed to melt
a 100 g sample of aluminum whose
initial temperature is 20°C?
10-4 Controlling Heat
Thermal conduction-the process by
which energy is transferred as heat
through a material between two
points at different temperatures.
 Ex: heating a metal skillet
 The atoms gain energy and pass
energy along to their neighbors by
collisions
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Thermal conductors-substances that
rapidly transfer energy as heat
 Ex: metals
 Thermal insulators-substances that
slowly transfer energy as heat
 Ex: cork, ceramic, cardboard,
fiberglass, gases
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Convection & Radiation
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Convection-displacement of cold matter
by hot matter, such as when hot air over
a flame rises upward (also involves
conduction & buoyancy)
The air particles are heated(conduction),
causing expansion and the density to
decrease. The warm air is then displaced
by cold air.
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Electromagnetic radiation-no
transfer of matter in this type;
objects radiate energy by
wavelengths
Body Temperature
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Body Temp is around 37°C
Body must work to maintain this temp in
very cold or very hot surrounding air
temperatures
Proper insulation is needed to keep the
cold out
Hypothermia slows down pulse, blood
pressure, and respiration and could be
fatal at 25.6°C
Insulating Materials for Cold
The energy given off by the body
must be kept in to prevent
hypothermia
 An insulating material surrounds the
body, such as air.
 Clothing is made to provide two
layers of clothing with air trapped in
between
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Evaporation aids in hot
temperatures
In hot climates, clothing is worn to
protect the skin from direct sunlight
and prevent excessive loss of body
water from evaporation while also
cooling the wearer.
 Heat exhaustion or heat stroke can
result of body temperature too high
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Allowing air flow over the skin allows any
perspiration on the skin to evaporate,
causing a great deal of energy to be
given off to the gas phase
This causes the skin to cool
Head wraps can help evaporate
perspiration during hot climates or help
insulate during cold temperatures