chapter 17 energy in thermal processes: first law of thermodynamics
TRANSCRIPT
Chapter 17
Energy in Thermal Processes:
First Law of Thermodynamics
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17.1 Thermodynamics – Historical Background Until around 1850, thermodynamics and
mechanics were considered to be two distinct branches of science. Experiments by James Joule and others showed a
connection between the two branches of science. A connection was found to be between the
transfer of energy by heat in thermal processes and the transfer of energy by work in mechanical processes
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Internal Energy Internal energy is all the energy of a
system that is associated with its microscopic components These components are its atoms and
molecules The system is viewed from a reference
frame at rest with respect to the system
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Internal Energy and Other Energies Internal energy includes kinetic
energies due to Random translational motion Rotational motion Vibrational motion
Internal energy also includes intermolecular potential energy
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Heat Heat is a mechanism by which energy is
transferred between a system and its environment due to a temperature difference between them
Heat flows from the one at higher temperature to the one at lower temperature.
Heat is also represented by the amount of energy, Q, transferred by this mechanism
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Units of Heat Calorie is the unit used for heat
One calorie is the heat necessary to raise the temperature of 1 g of water from 14.5o C to 15.5o C
The Calorie used for food is actually 1 kcal
In the US Customary system, the unit is a BTU (British Thermal Unit)
One BTU is the heat required to raise the temperature of 1 lb of water from 63o F to 64o F
1 cal = 4.186 J
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17.2 Specific Heat Specific heat, c, is the heat amount of
energy per unit mass required to change the temperature of a substance by 1°C
If energy Q transfers to a sample of a substance of mass m and the temperature changes by T, then the specific heat is
Qc
m T
Q = m c TUnits of specific heat are J/kg°C
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Some Specific Heat Values
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More Specific Heat Values
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Sign Conventions If the temperature increases:
Q and T are positive Energy transfers into the system
If the temperature decreases: Q and T are negative Energy transfers out of the system
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Specific Heat of Water Water has a high specific heat relative to
most common materials Hydrogen and helium have higher specific heats
This is responsible for many weather phenomena Moderate temperatures near large bodies of water Global wind systems Land and sea breezes
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Calorimetry One technique for measuring specific
heat involves heating a material, adding it to a sample of water, and recording the final temperature
This technique is known as calorimetry A calorimeter is a device for the
measurement
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Calorimetry, cont The system of the sample and the water is
isolated The calorimeter allows no energy to enter or leave
the system Conservation of energy requires that the
amount of energy that leaves the sample equals the amount of energy that enters the water Conservation of Energy gives a mathematical
expression of this: Qcold= - Qhot
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Calorimetry, final The negative sign in the equation is critical for
consistency with the established sign convention
Since each Q = m c T, cunknown can be found
Technically, the mass of the water’s container should be included, but if mw>>mcontainer
it can be neglected
w w wx
x x
m c T Tc
m T T
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Phase Changes A phase change is when a substance
changes from one form to another Two common phase changes are
Solid to liquid (melting) Liquid to gas (boiling)
During a phase change, there is no change in temperature of the substance
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17.3 Latent Heat During phase changes, the energy added to a
substance modifies or breaks the bonds between the molecules or makes different molecular arrangements.
The amount of added energy also depends on the mass of the sample
If an amount of energy Q is required to change the phase of a sample of mass m,
Q mL
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Latent Heat, cont The quantity L, called the latent heat of
the substance, is the energy required for transforming the substance per unit mass from one phase to another phase
Latent means hidden The value of L depends on the substance
as well as the actual phase change
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Latent Heat, final The latent heat of fusion, Lf, is used during
melting or freezing The latent heat of vaporization, Lv, is used when
the phase change boiling or condensing The positive sign is used when the energy is
transferred into the system This will result in melting or boiling
The negative sign is used when energy is transferred out of the system This will result in freezing or condensation
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Sample Latent Heat Values
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Graph of Ice to Steam
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Warming Ice, Graph Part A Start with one gram of ice
at –30.0º C During A, the temperature
of the ice changes from –30.0º C to 0º C
Use Q = m cice ΔT Cice= 2090 J/kg º C In this case, 62.7 J of
energy are added
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Melting Ice, Graph Part B Once at 0º C, the phase
change (melting) starts The temperature stays
the same although energy is still being added
Use Q = m Lf The energy required is 333 J On the graph, the values move
from 62.7 J to 396 J
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Warming Water, Graph Part C Between 0º C and 100º C,
the material is liquid and no phase changes take place
Energy added increases the temperature
Use Q = m cwater ΔT 419 J are added The total is now 815 J
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Boiling Water, Graph Part D At 100º C, a phase
change occurs (boiling)
Temperature does not change
Use Q = m Lv This requires2260 J The total is now
3070 J
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Heating Steam After all the water is converted
to steam, the steam will heat up
No phase change occurs The added energy goes to
increasing the temperature Use Q = m csteam ΔT
In this case, 40.2 J are needed The temperature is going to 120o C The total is now 3110 J
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