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Thermodynamics1
Relationships Between Heat and Work
Internal energy can do work
Heat/work transferred to/from a system
Imagine removing a nail that has been
hammered into a piece of wood.
System – a substance or combination of
substances to which energy is added or
removed
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Work is done by friction.
Work increases the internal energy of
the iron atoms in the nail.
Increase in internal energy translates
into an increase in temperature.
Energy is transferred as heat from the
nail to the surrounding air until the
nail and air are at the same
temperature.
Internal energy decreases through the
transfer of energy as heat.
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System is rarely isolated from its
surroundings
Must account for all the
interactions between the system
and its environment that could
affect the system’s internal
energy
Work done on/by a gas
Energy can also be transferred as heat,
which is then available to do work.
In thermodynamic systems, work is
defined in terms of pressure and change
in volume.
A flask of water heated until boiling with a
balloon placed over the mouth of the flask.
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Energy transferred as heat from the
burner to the water.
Internal energy of the water is
increased until the water reaches its
boiling point and changes phase.
Volume of steam increases; expansion
provides a force that expands the
balloon and does work on the
atmosphere (by pushing it back).
o Steam does work
o Steam’s internal energy
decreases (conservation of
energy)
Heat and work
Energy transferred to/from a substance
which changes a substance’s internal
energy
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Energy in transit
Pressure = F/A
ΔV = Ad (area x displacement)
Recall … W = Fd
W = Fd(A) =
A
F(Ad) =
A
P ΔV
Work = pressure x volume change
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Note: definition of work assume
pressure is constant
Gas expands, volume is positive
and work is done by the gas
Gas is compressed, ΔV is negative,
and work is done ON the gas
Gas volume remains constant = no
displacement and no work
Pressure can change
during a process, but
work is only done if
the volume changes.
Thermodynamics2
Practice 10A
Thermodynamic processes
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
0.040 m?
3 processes are related to each other:
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Generally, energy is transferred as both
heat and work.
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Constant-volume process
When a gas has a change in temperature,
but no change in volume, no work is done
on or by the system.
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Isovolumetric process
Bomb calorimeter
Internal energy (U)
Heat (Q)
Work (W)
If one process of energy transfer is
dominant, the other type is
negligible
Approximate an ideal process (one
of the three types of
thermodynamic processes)
Ideal processes in gases
o All objects have internal
energy (sum of kinetic and
potential energy)
o Monatomic gases only have
kinetic energy, and so are
simpler to study
Thermodynamics3
Internal energy in a constant-temperature
process
Energy is not transferred as heat in an
adiabatic process
Isothermal process – a system’s temperature
remains constant and internal energy does not
change when energy is transferred to/from the
system as heat or work.
Adiabatic process – changes occur but
no energy is transferred to or from a
system as heat.
Example: inflated balloon inside a temperaturecontrolled building. Approaching storm leads to
a decrease in atmospheric pressure
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Lower pressure leads to expansion of
balloon (balloon does work on the
atmosphere)
Decrease of internal energy leads to a
decrease in internal temperature of the
balloon (decrease in kinetic energy inside
the balloon)
Increase in atmospheric temperature (due
to the work done on the atmosphere by the
balloon) is transferred into the balloon as
heat.
Net result = equilibrium; amount of energy
transferred out of the balloon equals the
amount of heat transferred into the
balloon.
Occurs slowly
A tank of compressed gas is used to fill a
balloon. Inflation occurs rapidly. In the
tank of gas:
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Pressure decreases
Internal energy decreases
Temperature decreases
If the tank and balloon are thermally
insulated, so no energy is transferred as
heat:
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Decrease in internal energy must
equal the energy transferred
from the gas as work
Adiabatic process must occur
rapidly
These three processes rarely occur
ideally, but many situations can be
approximated by one of the 3 processes,
allowing you to make predictions.
Examples: internal-combustion engines,
refrigerators.
Thermodynamics4
The First Law of Thermodynamics
Energy conservation
In the absence of friction,
mechanical energy (KE + PE) is
conserved.
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When friction is accounted for,
mechanical energy is no longer
conserved
Mechanical energy decreases as
internal energy increases
Most of the internal energy is
dissipated to the surrounding air
as heat
When internal energy for the
system and energy dissipated as
heat to the atmosphere are
included, then total energy will
be constant.
First law of thermodynamics
The principle of energy
conservation that takes into
account a system’s internal energy,
as well as work and heat
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Any change in the energy of
an object can only come
about by either a transfer of
heat or the performance of
work
Consider a balloon that is squeezed rapidly:
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Process isn’t isothermal
Work is done on the system
o Balloon and the inside air (system) are compressed
o Air’s internal energy and temperature increase
o Work is negative
Squeezing = adiabatic , so Q = 0 and ΔU = —W
After the squeezing,
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Energy is transferred from the system as heat
Some internal energy inside the balloon is transferred outside the balloon.
Thermodynamics5
Still with the balloon …
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Internal energy decreases
o ΔU is negative
Energy is removed from the
system as heat
o ΔQ is negative
Change in internal energy now is
—ΔU = —Q or ΔU = Q.
Thinking about changes in internal energy,
visualize a circle.
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Work done on the system, OR
energy is transferred as heat into
the system – the arrow points
INTO the circle
o Internal energy increases
Work done by the system, OR
energy is transferred as heat out
of the system – the arrow points
OUT OF the circle
o Internal energy decreases
Practice 10B
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?
1st law mathematically
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Energy is conserved
Change in internal energy = Uf - Ui
Change in internal energy must equal
the net transfer of energy as both heat
and work
∆U = Q−W
All quantities have the same energy units,
joule.
A system’s internal energy can be changed
by transferring energy as either work,
heat, or a combination of the two.
Note: table 2, page 344, shows the first law
of thermodynamics for special processes.
Cyclic processes
Cyclic processes: the system’s properties
at the end of the process are identical to
the system’s properties before the process
took place.
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Final and initial values of internal
energy are the same
Change in internal energy = 0
∆Unet = 0 and Qnet = Wnet
Similar to an isothermal process,
but has no net change in internal
energy
Ex: Refrigerator, heat engine
Thermodynamics6
Heat engines
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Uses heat to do mechanical work
Does work by transferring energy from a high-temperature substance to a lowertemperature substance
For each cycle of the heat engine, net work done equals the difference between
energy transferred as heat (Qh) from the high-temperature substance to the engine
and the energy transferred as heat from the engine to a lower-temperature
substance (Qc).
Larger the difference in temperature, the more work that can be done.
o Internal combustion engine
o No heat engine works perfectly. Only part of the available internal energy
leaves the engine as work done on the environment; most of the energy is
removed as heat.
Wnet = Qnet = Qh―Qc
Thermodynamics7
The Second Law of Thermodynamics
Conclusion of First Law of Thermodynamics
You can’t win!
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The total amount of work you can get out of any device is exactly equal to the
heat flow (change in kinetic energy) that drives the machine.
It’s impossible to get more work out of a system than you put in.
But, all energy entering and leaving the system is accounted for and is conserved.
Second Law of Thermodynamics
You can’t even break even!
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Based solely on the 1st law of thermodynamics, we should be able to expect
machines to be 100% efficient
o Energy cannot be gained or lost, just transformed
It is impossible to construct a heat engine operating in a cycle that absorbs
energy from a hot reservoir and does an equivalent amount of work.
Second Law of Thermodynamics: no cyclic process that converts heat entirely into
work is possible.
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W can never equal Qh; some energy must always be transferred as heat to the
system’s surroundings (Qc>0).
Efficiency of an engine
Efficiency Equation
1. A cyclic process cannot completely
convert energy transferred as heat into
work.
2. A cyclic process cannot transfer energy
as heat from a low-temperature body to
a high-temperature body without work
being done.
Eff = Wnet = Qh―Qc = 1 ―Qc
Qh
Qh
Qh
Efficiency is a measure of the useful
energy taken out of a process relative to
the total energy that is put into the
process.
= 1 ― energy removed as heat
energy added as heat
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Unitless quantity
Only use the magnitudes for the
energies added to and taken from the
engine
A heat engine is 100% efficient (eff = 1)
if there is no energy transferred away
as heat (Qc = 0)
Efficiency = net work done by engine
energy added to engine as heat
= energy added as heat ―energy removed as heat
energy added as heat
No such engine.
All engines are <100% efficient.
Smaller the fraction of usable energy that
can be provided, the lower its efficiency is.
Efficiency equation is the maximum value:
friction, thermal conduction, inertia lower
actual efficiency.
Thermodynamics8
Practice 10C
Entropy
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.
Have you ever noticed that once you open a
new deck of cards and shuffle them, that the
cards never revert back to their suit and
numerical sequence ever again, just by
shuffling? Why?
There are 8 x 1067 ways to arrange 52
cards (52!), but only a few ways to arrange
the cards by suit and numerical sequence.
Entropy: the measure of a system’s
disorder.
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In thermodynamics, a system left to
itself tends to go from a state of
order to disorder.
The greater the entropy, the greater
the disorder
Entropy of a system tends to
increase until it reaches a state of
maximum entropy.
Greater disorder means less work
Not all gas particles move in an orderly fashion towards the piston to do useful work.
Instead, some move in all different directions; they transfer energy through collisions
with the cylinder walls and with each other – not just with the piston.
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The motion of the particles of a system is not well-ordered, and consequently is
less useful for doing work.
Second law in terms of entropy change: the entropy of the universe increases in all
natural processes.
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Entropy can decrease for parts of systems, but the decrease is offset by a greater
increase in entropy elsewhere in the universe.