Lecture #4 Chapter 3 & 4

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Oct 27, 2013 (3 years and 1 month ago)

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Lecture #4 Chapter 3 & 4


2
st

Law of Thermodynamics


Law of Conservation of Energy


No work or heat added to system (W + Q = 0)


“Isolated” or closed


The change in total energy is always zero


Δ(KE + PE + TE) = 0


Say that the energy in the isolated system is


conserved


Example: chemical converted to mechanical and


thermal


X = Y + Z

2

Energy Conversion


Energy into a system equal energy out plus the


increase of energy in system



Fossil fuel steam power generating station


E
in

= E
out

because no energy is stored


E
fuel

+ E
air

+ E
water in

= E
electricity

+ E
water out

+E
combustion gases



Efficiency =
useful energy or work out/ total energy in * 100


Unusable forms such as waste heat

3

4

5

6

Automobile Efficiency




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Fig. 4
-
20, p. 118

8


2
nd

Law of Thermodynamics
(Natural decrease in usefulness)

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Work and First Law



E = KE + PE + TE + chemical + electrical


W
on

+ Q
to

=
Δ
(KE+PE+TE) =
Δ
E


W
on

=
-

W
by


Q
to

=
Δ
E + W
by


For the bicycle pump example


Won =
Δ
E =
Δ
TE

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Heat and Temperature


Q = mc
Δ
T or
Δ
T = Q/mc


m = mass, c = specific heat, Q = amount of heat
added or removed,
Δ
T = change in temperature


Table 4.2 has specific heat of some substances


Phase change


Heat of vaporization


Amount of heat needed to convert to a gas


2260 kJ/kg for water


Heat of fusion


Amount of heat needed to convert to a liquid


335 kJ/kg for water

11

Heat Transfer


Conduction


Q
cond
/t = k x A x (T
2



T
1
) /
δ


k = thermal conductivity, A = area,
δ

= thickness of
material


Convection


Q
conv

= h x A x (T
s



T
air
)


h = thermal convectivity


Radiation


v (m/s) =
λ

(m) x
f

(per second)

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Observations

The 1
st

Law of Thermodynamics does
not

determine what direction a process will
naturally occur



For example the 1
st

law is not violated if a hot
object gains more energy from the cold
surroundings as long as the energy gained by
hot object is equal to the energy lost by cold
object

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Observations (cont’d)

We need laws of thermodynamics to predict:



The direction a process will naturally take


Simple processes are easy to predict


Complicated processes are more difficult to predict



The amount of work the naturally occurring
process could have made

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The 2
nd

Law of Thermodynamics


The 2
nd

Law of Thermodynamics predicts in
what direction processes will naturally occur


The direction that creates more energy at ambient
conditions and less ability to produce work


This is useful for both:


Simple processes where intuitively we know the
direction


For complex processes where we may not know the
final outcome

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The 2
nd

Law of Thermodynamics

The 2
nd

Law of Thermodynamics determines:



The maximum possible amount of work that
can be produced from a process


The amount of disorder the process has
caused

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Summary of the 2
nd

Law of
Thermodynamics


The flow of heat naturally is from hotter to colder


Naturally occurring processes result in more
disorder


Energy has quality as well as quantity


All energy in the form of heat cannot be
converted to work


A portion must be transferred to a low temperature
sink


Entropy is a rating of disorder and randomness


High quality
≈ low entropy


Low quality ≈ high entropy

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Heat Engines (power cycle)


Energy from a high temperature source is
transferred to the heat engine


A portion of the high temperature energy is
converted to work


The remaining energy is transferred to low
temperature sink


The efficiency = (work out)/(heat in) is always
less than 100%


Electrical power plants, and automobile and jet
engines all operate by these principles

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Heat Engines (power cycle)


Usually the heat engine
consists of turbines,
pistons, etc.


For now it will be
modeled as a circle or
box with:


Energy at high
temperature going in


Work and energy at low
temperature coming out

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Heat Engines


Power Cycles
(cont’d)


Nicolas Leonard Sadi Carnot (1796


1832)
determined maximum possible efficiency for a
heat engine


Biographical comment:



A quiet, unassuming Frenchman who lived during
the turbulent Napoleonic years and had an
unspectacular life



One of Carnot’s mottos



Speak little of what you know, and not at all of what
you do not know


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Carnot Efficiency


Max Efficiency = (T
h



T
c
) / T
h


For % efficiency multiply by 100


T
h

= High temperature source


T
c

= Low temperature sink


All temperatures must be in absolute (K)

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Example of Maximum Efficiency of
Heat Engine

Heat engine receives heat from steam at
300
°
C and exhausts heat to air at 100
°
C



What is the maximum efficiency?


T
h

= 300
°
C + 273 = 573 K


T
c

= 100
°
C + 273 = 373 K


Max Efficiency = (573 K


373 K) / 573 K =
0.35 = 35%

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Actual Efficiencies of Heat Engines


Power Cycles


The efficiencies of all heat engines are less than
100% because:


All heat engines cannot operate greater than Carnot
Efficiency even if they were constructed perfectly (no
friction, etc.)


Losses such as friction even decrease the efficiency
to a value 1/2 to 2/3 the Carnot efficiency


Less efficient processes are used because:


Cheaper


Easier to use

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To Increase Efficiency of Heat
Engine


Power Cycle:


Increase temperature of heat source


Decrease temperature of heat sink

C

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