# Lecture #4 Chapter 3 & 4

Mechanics

Oct 27, 2013 (4 years and 6 months 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

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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

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4

5

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Automobile Efficiency

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Fig. 4
-
20, p. 118

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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
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

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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

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

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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)

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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