general physics-i 18
TRANSCRIPT
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Chapter 18
Temperature, Heat, and the First Law
of Thermodynamics
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Things to learnWe will learn concepts useful in thermodynamics (temperature, heat, work, and heat capacity) and relations among them.
We will learn laws of thermodynamics and their applications:
-Zeroth law: T equal at thermal equilibrium
-First law: energy conservation- Adiabatic, constant-volume, cyclical, free-
expansion processes.
We will learn effects of heat:
-Change in T
-Change of phase (heat of transformation)
-Thermal expansion
We will learn modes of heat transfer.
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18-2 Temperature
Thermodynamics=Thermal physics
TemperatureHot? Cold?
Scale?Kelvin (K) in SI unit
[273 + Celsius]
Universe was very hot right after Big Bang(1039 K)
Now about (3K)
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18-3 The zeroth Law of Thermodynamics
If bodies A and B are each in thermal equilibrium with a
third body T, then they are in thermal equilibrium with each
other.
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18-4 Measuring temperature
Practical standard0 degrees Celsius : freezing point of water100 degrees Celsius : boiling point of water
Easily reproducible; these may vary!
Scientific reference scaleT of Triple-point of water is defined by
273.16 K
International agreementreproducible
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Calibrating thermometers
Triple-point of water = vapor, water, and
ice are in equilibrium(273.16 K)
Make a gas thermometer in
equilibrium with a triple-point water
cell which has a definite temperature
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Constant-volume gas thermometer
Use the fact: T= C pT: temperature
C: constant if the volume of a gas is fixed
P: pressure
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Constant-volume gas thermometergives correct value as p3 0
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18-5 Celsius & Fahrenheit
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18-6 Thermal expansion
Thermal expansions
-Expansion slots for sections of the bridge
-Dental cavity filling: thermal expansion property should be matched.
-Bimetal strip
Linear expansion
Volume expansion
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SP 18-2
A oil trucker loaded 37,000 L of diesel fuel in Las Vegas and delivered his entire load in Payson, UT (23 oC lower).
β of diesel fuel = 9.5 × 10-4 /oC
α of steel truck tank = 11 × 10-6 /oC
How many liters did he deliver?
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18-7 Temperature and Heat
Heat is the energy transferred between system and environment
because of the temperature difference
a) System is hotter than environmentSystem loses heat (Q<0)
b) Same temperature (in equilibrium)No heat flows (Q=0)
c) System is colder than environmentSystem gains heat (Q>0)
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18-8 The absorption of heat by solids and liquids
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Heat Capacity C
Specific Heat (heat capacity per unit mass)
Molar Specific Heat (heat capacity per one mole)
• 1 mol = 6.02 X 1023
An important point:
• constant pressure vs. constant volume
Heat of Transformation
-Heat of vaporization
- Lv=539 cal/g = 40.7 kJ/mol = 2256 kJ/kg (water)
-Heat of fusion
- LF=79.5 cal/g = 6.01 kJ/mol = 333 kJ/kg (water)
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Molar Specific Heat
Problem 23
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SP 18-3
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18-9 A closer look at Heat and Work
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P-V plot
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Problem 43
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18-10 The first law of Thermodynamics18-11 Special cases
Adiabatic process: no heat transfer
Constant volume process: no work
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Cyclical process = [Q=W]
Returns to the same state:If the system received a
certain amount of heat, the same amount of work
has been done by the system.
Internal energy remains the same after a cycle.
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Problem 48 cyclic process
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Free expansion = [pext=0:W=0]
No heat transfer.No external pressure.
No work in expansion.No equilibrium during
the expansion
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SP 18-5Boiling water steam
Constant pressure (1atm)Work done by the gas?
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18-12 Heat Transfer Mechanisms: conduction
Problem 61
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Conduction through a composite slabP must be invariant
(energy conservation)
Parallel combination:P=P1+P2 energy is transferred through two channels
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Convection
Difference in temperature
Difference in pressure
Fluid can flow(from high pressure to low pressure)
Heat is transferred
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Radiation
Electromagnetic wavee.g. radiation from the Sun
Any object with T > 0 K radiates!
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SP 18-7
Ts = -23 oC; Ta = + 6.0 oC
Water: m = 4.5 g, A = 9.0 cm2, d = 5.0 mm
ε = 0.90
Find the time required for the water to freeze via radiation. Can the freezing be accomplished during one night?