Lessons 3 and 4 Thermodynamics

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Transcript Lessons 3 and 4 Thermodynamics

Thermodynamics
1
A few reminders
TEMPERATURE determines the
direction of flow of thermal energy
between two bodies in thermal
equilibrium
HOT
COLD
2
A few reminders
TEMPERATURE is also a measure
of the average kinetic energy of
particles in a substance
3
A few reminders
INTERNAL ENERGY is the sum of
the kinetic energy and potential
energies of particles in a
substance
K.E. + P.E.
4
Internal energy
The sum of the KE and PE of the
particles in a system
NOTE, THIS IS NOT THE SAME AS
THE TOTAL ENERGY.
5
A few reminders
In an ideal gas, the INTERNAL
ENERGY is all kinetic energy.
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What is
THERMODYNAMICS?
A study of the connection between
thermal energy entering or leaving
a system and the work done on or
by the system.
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A few words to consider
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Thermodynamic system
The system/machine that we are
considering the flow of heat energy
in/out of and work done on/by the
system.
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The surroundings
Everything else!
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Heat
The quantity of heat/thermal energy
(transferred by a temperature
difference).
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Work
The energy transferred (changed)
E.g. Work = Force x distance
or
Work = VIt
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Example
Finding the work done on or by a
gas when it expands at constant
pressure (i.e. a small change in
volume!)
(most of the systems we consider
will involve the compression or
expansion of gases under different
conditions)
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Work done by a gas
(constant pressure)
Work = force x distance
Work = force x Δx
A
(Pressure = F/A so F = PA)
Work = PAΔx
PP
(AΔx = ΔV)
Δx
Work = pΔV
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The 1st law of
thermodynamics
Q = ΔU + W
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The 1st law of
thermodynamics
Q = ΔU + W
Q = The thermal energy
given to a system (if this is
negative, thermal energy is
leaving the system)
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The 1st law of
thermodynamics
Q = ΔU + W
ΔU = The increase in
internal energy (if this is
negative the internal
energy is decreasing)
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The 1st law of
thermodynamics
Q = ΔU + W
W = The work done on the
surroundings (if this is
negative the surroundings
are doing work on the
system)
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The 1st law of
thermodynamics
Q = ΔU + W
This is really just
another form of the
principle of energy
conservation
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Ideal gas processes

In most cases we will be
considering changes to an ideal
gas (this will be the “system)
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pV diagrams and work
done
p
Changes that happen during a
thermodynamic process can
usefully be shown on a pV diagram
V
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pV diagrams and work
done
A
The area under the graph represents
the work done
p
B
This area represents
the work done by the
gas (on the
surroundings) when it
expands from state A
to state B
V
What happens if the gas is going from state B to A?
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ISOCHORIC
(isovolumetric) processes
These take place at constant volume
p
A
V = constant, so p/T = constant
B
V
Isochoric
decrease in
pressure
Q = negative
ΔU = negative
W = zero
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ISOBARIC processes
These take place at constant
pressure
p
A
B
p = constant, so V/T = constant
V
Isobaric
expansion
Q = positive
ΔU = positive
W = positive
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ISOTHERMAL processes
These take place at constant
temperature
A
p
B
V
Isothermal
expansion
T = constant, so pV = constant
Q = positive
ΔU = zero
W = positive
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ADIABATIC processes
A
No thermal energy transfer with the
surroundings (approximately a
rapid expansion or contraction)
p
B
V
Q = zero
ΔU = negative
W = positive
Adiabatic
expansion
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Heat engines and heat
pumps
A heat engine is any device that
uses a source of heat energy to do
work.
Examples include the internal
combustion engine of a car.
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“Reservoir”
implies a
constant heat
source
Heat engine
Below is a generalised diagram
showing the essential parts of any
heat engine.
Work done
ΔW
Hot
reservoir
Thot
Cold
reservoir
Thermal
energy
Qhot
Engine
Thermal
energy
Tcold
Qcold
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A simple example of using
an ideal gas in a heat
engine
Heat in
ΔU = (3/2)nRΔT
p
Heat in
A
Isobaric
expansion
B
Isovolumetric
decrease in
pressure
Area = work
done by gas
Isovolumetric
increase in
pressure
D
C
Isobaric
compression
Heat out
Heat out
V
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Let’s read!

Page 191 to 192 “An example of a
heat engine”
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Heat pump
Simply a heat engine run in
reverse! (Put work in, transfer heat
from cold reservoir to hot reservoir)
Input work
ΔW
Hot
reservoir
Thot
Cold
reservoir
Thermal
energy
Qhot
Engine
Thermal
energy
Tcold
Qcold
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Heat pump
Heat out
Heat out
p
Isovolumetric
decrease in
pressure
Isobaric
compression
Isovolumetric
increase in
pressure
Area = work
done on gas
Isobaric
expansion
Heat in
Heat in
V
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Questions

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Page 193
Questions 1 to 5
Page 194
Questions 10
33
2nd Law of
Thermodynamics and
entropy
There are many ways of stating the
2nd law, below is the Kelvin-Planck
formulation
This is possible
in a single
process
however
“No heat engine, operating over a
cycle, can take in heat from its
surroundings and totally convert it
totally into work” (some heat has to
be transferred to the cold reservoir)
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2nd Law of
Thermodynamics and
entropy
Other statements of the 2nd law
include
• No heat pump can transfer thermal
energy from a low temperature to a
higher temperature reservoir
without work being done on it
(Clausius)
• The entropy of the universe can
never decrease
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Entropy



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This is a measure of the
disorder of a system
Most systems, when left,
tend towards more disorder
(think of your bedroom!
This is why heat spreads
from hot to cold.
Entropy can decrease in a
small part of a system
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Entropy
Decrease in
entropy = Q/Thot
Thot
Increase in entropy
= Q/Tcold
ΔQ
Tcold
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1st and 2nd laws
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These laws MUST apply in all
situations
A refrigerator does transfer heat from
cold to hot, but work must be done
(electricity supplied and some
converted into heat) to do this
A boat could use the temperature
difference between the sea and
atmosphere to run, but eventually the
two reservoirs would reach the same
temperature
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Degradation
The more spread energy becomes,
the less useful it is. The heat
produced in the brakes of a car is
still energy, but not really in a
useful form. We call this energy
degradation
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That’s it!
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Now let’s try some
questions
Page 193
Questions 1 to 5
Page 194
Questions 10 to 13.
Let’s also have a
test on 4th
November
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