P1258776022HCtIi - PowerPoint PPT Presentation

Title: P1258776022HCtIi


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Chapter 11 Thermal Physics
Temperature A quantitative measure of hot or
cold Operational definition Temperature is
what is measured by a thermometer. mercury
thermometer constant volume gas
thermometer bimetallic strip All involve
measurable physical changes in response to
temperature changes. Temperature Scales water
freezes water boils Fahrenheit 32º
212º Celsius 0º 100º TF 9/5 TC
32 Examples What Fahrenheit temperature
corresponds to 37.0 ºC? to 20.0 ºC? What Celsius
temperature corresponds to 86 ºF?
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Gas thermometer and absolute temperature
scales Constant volume gas thermometer pressure
changes indicate temperature changes At low
densities Dp µDT All gases have same
x-intercept p 0 at TC -273ºC TF -460ºF
Absolute temperature scales T 0 when p
0 Kelvin TK TC 273 Rankine TR TF 460
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Thermal Expansion A change in temperature causes
a change in length in a solid object. Usually,
change in length DL is proportional to change in
temperature, DT is proportional to original
length, L0 depends upon type of material
L0
DL
a Coefficient of Linear Expansion (sometime
a) Determine a experimentally
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Example The center span of the
Verrazano-Narrows Bridge is 1300 m long.
Assuming the bridge is made of steel, how much
expansion will there be for a temperature range
of 120ºC?
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Example A copper hot-water pipe is 10.0 m long
when installed on a day when the temperature is
10ºC. How long is the pipe when it carries hot
water at 60ºC if the pipe is free to expand?
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Volume expansion (of solids and liquids) linear
expansion in 3-D! A change in temperature causes
a change in volume. The change in volume DV is
proportional to change in temperature, DT is
proportional to original length, V0 depends upon
type of material
b Coefficient of volume expansion for solids,
b 3a
How are voids (holes) in materials affected by
changing temperatures?
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Voids in materials expand and contract as if they
were made of the material.
Example How much water overflows from a 1 liter
glass beaker filled to the brim with water at
20C when beaker and contents are both heated to
95 C? Use average value of b 525E-6/ºC
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What is heat? One thing which is transferred
between two systems initially at different
temperatures as they come to thermal
equilibrium. Heat is energy which is transferred
solely because of temperature differences. Heat
addition to a system can cause temperature
changes. Mechanical Equivalent of Heat 1
calorie 4.187 joules Heat Units, defined in
terms of the properties of water calorie the
energy required to raise the temperature of 1
gram of water by 1 ºC (4.187 J) kilocalorie
a.k.a. Calorie a.k.a. dietetic calorie 1000
calories (4187 J) British Thermal Unit the
energy required to raise the temperature of 1
pound of water by 1 ºF (1060 J)
Example A candy bar has 200 dietetic Calories.
How many time must a 10 kg weight be lifted
through a height of 2m to work off the candy bar
if the human body is 10 efficient at converting
calories to mechanical energy?
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Calorimetry For a sample of a substance that does
not undergo a change of state Heat added Q is
proportional to DT Tf- Ti , the change in
temperature m, the mass of the sample Q mc
DT c is the materials specific heat In a
closed system (hot object interacts with cool
object) heat gained by cool object() heat lost
by hot object(-) 0
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Example A Styrofoam cup of negligible heat
capacity contains 150 g of water at 10ºC. If you
add 100 g of water at 85ºC, what is the final
temperature of the mixture?
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Example A 74 g metal block is heated to 90ºC.
It is then submerged in 300 g of water at 10ºC.
The final temperature is 14ºC. What is the
specific heat of the material?
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Phase Changes Phase of Matter solid, liquid, gas
etc. Phase Change (Phase Transition) solid -gt
liquid (melting), liquid -gt gas (boiling) heat
added (or removed) without a change in
temperature (reversible process) Two states of
matter coexist (in equilibrium) only at the
transition temperature amount of heat
proportional to mass of substance Q mL L
is the Latent heat Water Latent heat of fusion
is the energy cost to convert a unit mass of ice
at the melting temperature to a unit mass of
water at the melting temperature. Lf 80
kcal/kg 334 kJ/kg Latent heat of vaporization
is the energy cost to convert a unit mass of
water at the boiling temperature to a unit mass
of steam at the boiling temperature. Lv 540
kcal/kg 2260 kJ/kg
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Water Latent heat of fusion is the energy cost
to convert a unit mass of ice at the melting
temperature to a unit mass of water at the
melting temperature. Lf 80 kcal/kg 334
kJ/kg Latent heat of vaporization is the energy
cost to convert a unit mass of water at the
boiling temperature to a unit mass of steam at
the boiling temperature. Lf 540 kcal/kg
2260 kJ/kg
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Example A 105 g copper calorimeter contains 307
g of water at 23ºC. If 52 g of ice at 0ºC is
added to the calorimeter, what is the final
temperature of the system?
Example A 105 g copper calorimeter contains 307
g of water at 23ºC. If 95 g of ice at 0ºC is
added to the calorimeter, what is the final
temperature of the system?
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Mechanisms of Heat Transfer Conduction Convection
(Natural vs Forced) Radiation (Electromagnetic
Waves) Conduction No bulk motion of
matter Microscopic transfer of kinetic energy to
adjacent regions Because KE of electrons in a
conductor easily transported from one region to
another, good electrical conductors are generally
good thermal conductors Rate of heat flow
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Example A Styrofoam cooler has a surface area
of 0.50 m2 and an average thickness of 2.0 cm.
How long will it take for 1.5 kg of ice to melt
if the outside temperature is 30ºC? use K0.030
W/(m ºC)
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Convection Bulk motion of fluid (forced or
natural) chief mechanism of heat lost under most
situations COMPLEX!!! Depends upon fluid and
geometry h skin coefficient depends upon
temperatures, fluid, geometry, etc.
Radiation (electromagnetic waves) Blackbody
Radiation Stephan- Boltsmann Law
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Radiation (electromagnetic waves) Blackbody
Radiation Stephan- Boltsmann Law
I
l
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Example A patient waiting to be seen by his
physician is asked to remove all his clothes in a
room that is at 16ºC. Calculate the rate of heat
loss by radiation given that the patient has a
temperature of 34ºC and his surface area is 1.6
m2. Assume an emissivity of 0.80.