Download Thermodynamics

Thermodynamics
Physics H
Mr. Padilla
Thermodynamics
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The study of heat and its transformation into
mechanical energy.
Foundation
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–
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Conservation of energy
Heat flows from hot to cold
Until 1840 heat was thought to be an invisible
fluid called caloric.
Q, W, & U
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Work can transfer energy to a substance. That
energy could then leave as heat.
–
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The reverse is also possible
A change in internal energy is apparent as a
change in temperature or phase.
This change in energy can take place on a
substance or a combination of substances
called a system.
Work done on/by a gas
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Previously: W=Fd
When work is done
on an enclosed gas:
–
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W=Δ(PV)
If P is held constant,
–
W = PΔV
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ΔV is + work is done by
the gas (W is +)
ΔV is - work is done on
the gas (W is -)
Work is only done if
volume changes
Heat, Work, and Internal Energy
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If the gas expands, as
shown in the figure, DV is
positive, and the work done
by the gas on the piston is
positive.
If the gas is compressed, DV
is negative, and the work
done by the gas on the
piston is negative. (In other
words, the piston does work
on the gas.)
Knowledge about Pressure and
Volume

P = F/A (force/area)

ΔV = Ad (area multiplied by the displacement)

1 Pa = 1 N/m2
Sample Problem 11A

An engine cylinder has a cross-sectional area
of 0.010 m2. How much work can be done by a
gas in the cylinder if the gas exerts a constant
pressure of 7.5 x 105 Pa on the piston and
moves the piston a distance of 0.040m?
Solution

Choose the equation:
W  PDV  PAd

Plug and Chug:

W=(7.5x105 N/m2)(0.010 m2)(0.040 m)
W= 3.0x102 J
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Thermodynamic Processes
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No Work is done in a
constant volume
process.
–
–
Any changes to
internal energy would
be as a result of heat
Called:
isovolumetric
process

Internal energy is
constant in a
constant-temp
process.
–
–
Work can still be
done if volume
changes
Called isothermal
process
Isovolumetric Processes

A bomb calorimeter is a
device where a small
quantity of a substance
undergoes a combustion
reaction. The reaction
causes a change in
temperature and pressure,
but because of the thick
walls there is no volume
change. Energy can only be
transferred as heat.
Isothermal Processes

Work is done inside by the
molecules hitting the inside of
the balloon (energy is lost as
work). Energy from the
outside is added as heat.
Adiabatic Process
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Process in which no heat enters or leaves a
system.
Changes of volume can be done rapidly so
heat has little time to leave or can be insulated
Changes in air temperature ~ pressure change
–
Temp of dry air drops 10°C for each 1km increase in
altitude
1st Law of Thermodynamics

Whenever heat is added to a system, it
transforms to an equal amount of some other
form of energy
–
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Energy, including heat cannot be created or
destroyed
Heat added does 1 or both of 2 things..
–
–
1) increases the internal energy of the system if it
remains in the system
2) does external work if it leaves the system
Energy Conservation
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If friction is taken into account, mechanical
energy is not conserved.
Consider the example of a roller coaster:
–
–
A steady decrease in the car’s total mechanical energy
occurs because of work being done against the friction
between the car’s axles and its bearings and between
the car’s wheels and the coaster track.
If the internal energy for the roller coaster (the system)
and the energy dissipated to the surrounding air (the
environment) are taken into account, then the total
energy will be constant.
Energy Conservation

Notice that in the presence of friction the internal
energy (U) of the roller coaster increases as KE
+ PE decreases.
Heat added = increase in internal energy +
external work done by system

If 10J of energy is added
to a system that does no
external work, by how
much will the internal
energy of that system be
raised?

If 10J of energy is added
to a system that does 4J
of external work, by how
much will the internal
energy of that system be
raised?

10J

10J – 4J = 6J
Energy Conservation
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The principle of energy conservation that takes
into account a system’s internal energy as well
as work and heat is called the first law of
thermodynamics.

The first law of thermodynamics can be
expressed mathematically as follows:
DU = Q – W
Change in system’s internal energy = energy
transferred to or from system as heat – energy
transferred to or from system as work
Signs of Q and W for a system
1St Law of Thermodynamics for
Special Processes
Sample Problem 11B
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A total of 135 J of work is done
on a gaseous refrigerant as it
undergoes compression. If the
internal energy of the gas
increases by 114 J during the
process, what is the total
amount of energy transferred as
heat? Has energy been added
to or removed from the
refrigerant as heat?
Solution

Choose your equation:
DU = Q – W

Rearrange the equation:
Q = DU + W
Solution

Plug and Chug:
Q = 114 J + (–135 J)
Q = –21 J
The sign for the value of Q is negative. This
indicates that energy is transferred as heat
from the refrigerant.
Absolute Temperature
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Absolute Zero:
Lowest possible
temperature, 273
degrees C below
zero
Other absolute
temperatures…
Cyclic Process
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A thermodynamic process in which a system
returns to the same conditions under which it
started.
–
The change in internal energy of a system is zero in
a cyclic process
DUnet = 0 and Qnet = Wnet
The Steps of a Gasoline Engine
Cycle
The Steps of a Refrigeration
Cycle
Heat Engines use Heat to do Work
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A heat engine is able to do work by transferring
energy from a high-temperature substance at Th to
a substance at a lower temperature (the air
surrounding the engine) at Tc.
Wnet = Qh – Qc
The larger the difference between the amount of
energy transferred as heat into the engine and out
of the engine, the more work the engine can do.
2nd Law of Thermodynamics
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Heat will never of itself flow from a cold object
to a hot object.
Heat can be made to flow the other way, but
only by external effort.
–
Ex: Air Conditioner, Refrigerator
Heat Engines
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Changing heat completely
into work can never be
done.
It is possible to change
some.
A heat engine is any
device that changes
external energy into
mechanical work.
Amount of work done
–
Wnet=Qh-Qc
Heat Engines
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Every heat engine will
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1) absorb heat from a reservoir of higher temp.
increasing its internal energy
2) convert some of this energy into mechanical work
3) expel the remaining energy as heat to some
lower-temp. reservoir, usually called a sink
There is always heat exhaust
Thermodynamic Efficiency
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The ideal efficiency, or Carnot Efficiency, of a
heat engine can be found using the formula…
Ideal efficiency = Wnet/Qhot =
Qhot  Qcold
Qhot
Ideal Efficiency = Thot - Tcold
Thot

Heat converted to useful work depends on the
temp difference between the hot reservoir and
the cold sink.
Carnot Efficiency
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What is the ideal
efficiency of an engine
having a hot reservoir at
300K and a cold
reservoir at 150K?
What is the ideal
efficiency of an engine
having a hot reservoir at
400K and a cold
reservoir at 0K?
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(300K-150K)/300K =.5
50%
(400K-0K)/400K = 1
100%
Sample Problem 11C
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Find the efficiency of a gasoline engine that,
during one cycle, receives 204 J of energy
from combustion and loses 153 J as heat to
the exhaust.
Disorder
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Natural systems tend to proceed toward a state
of greater disorder.
This idea is called entropy
Ex: Gas molecules in a bottle. What will
happen when the bottle is opened?
Entropy normally increases in natural systems.
When work is input, entropy can decrease
Entropy
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Greater disorder means there is less energy to
do work.
If all gas particles moved toward the piston, all of
the internal energy could be used to do work.
This extremely well ordered system is highly
improbable.
Entropy
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Because of the connection between a
system’s entropy, its ability to do work, and
the direction of energy transfer, the second
law of thermodynamics can also be
expressed in terms of entropy change:
The entropy of the universe increases in all
natural processes.

Entropy can decrease for parts of systems,
provided this decrease is offset by a greater
increase in entropy elsewhere in the universe.