AP Physics
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Could all the air in a box rush to one side of the
box leaving a vacuum in the other side?
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Will heat flow spontaneously from a cold object
to a hot object?
Nature’s processes have preferred directions
Direction is important in Thermodynamic
processes
REVERSIBLE
IRREVERSIBLE
A reversible process is a transition from one state
of a thermodynamic system to another,
during which the system is always very close
to a state of mechanical and thermal
equilibrium (including uniform temperature
and pressure)
A process that despite any
small change in conditions
cannot be reversed.
In other words…a small change in the system
that can be reversed.
Ex. a gas that expands slowly and adiabatically
can be compressed slowly and adiabatically by
a small increase in pressure
Ex. Conversion of work to heat
by friction
A device that converts heat partly into work or mechanical energy.
In a heat engine a quantity of matter, called the working substance,
inside the engine undergoes addition and subtraction of heat,
expansion and compression, and sometimes, a phase change.
Ex. of Heat Engines
Working Substance
The simplest engines to analyze are those that have a working substance
which undergoes a cyclic process.
Cyclic process
 a sequence of processes that eventually leaves the substance in the same state as it
started
 when a cycle is complete the system starts and ends with the same internal energy
Cycle
Absorb Heat at High Temp.  Do Work  Discard Heat at Low Temp.
Internal Energy doesn’t change after a cycle.
U  0  Q  W
Q  W
Q   W   W
Q  QH  QC  W
A gasoline engine in a large truck takes in 2500 J of heat and delivers 500 J of
mechanical work per cycle. The heat is obtained by burning gasoline
with heat of combustion Lc = 5.0 x 104 J/g.
a) What is the thermal efficiency of this engine?
b) How much heat is discarded in each cycle?
c)
How much gasoline is burned during each cycle?
d) If the engine goes through 100 cycles per second, what is its power
output in watts? In horsepower? [1hp = 746 W]
e) How much gasoline is burned per second? per hour?
idealized model of the thermodynamic processes in a gasoline engine
idealized model of the thermodynamic processes in a gasoline engine
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idealized model of the thermodynamic
processes in a diesel engine
there are no spark plugs for ignition in a
diesel engine
the fuel is injected just before the
power stroke
the fuel ignites spontaneously from the
high temperature developed during the
adiabatic compression
Pros
Cons
heat engine operating in reverse
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take heat from a cold place (inside
the fridge) and expel it to a warmer
place (outside the fridge)
I thought heat didn’t flow from cold to
hot?!?!...well it doesn’t…to move
heat from cold to hot requires some
work!
Don’t Write This
1.
2.
3.
4.
5.
The compressor takes in the refrigerant fluid (working substance) and
compresses it adiabatically (this requires work)
The refrigerant is then sent to the condenser coil at high temperature
Heat transfers from the hot refrigerant to the surroundings (outside the
fridge) which partially condenses the fluid
Refrigerant fluid expands
adiabatically into the
evaporator (the expansion
valve controls this). The
expansion cools the fluid
enough so that it is colder
than the inside of the fridge.
The fluid is then transferred
into the fridge where it
absorbs the heat from the
inside of the fridge and
partially vaporizes
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Air conditioners operate the same as
a refrigerator
Difference – inside of room/building
is like the inside of fridge;
outside of fridge is now outside of
room/building
Heat Pumps are similar to an
air conditioner except
they’re backwards.
2LT for Engines
It is impossible for any system to
undergo a process in which it
absorbs heat from a reservoir at a
single temperature and converts
the heat completely into
mechanical work with the system
ending in the same state in which
it began.
A 100% efficient engine
does not exist.
2LT for Refrigerators
It is impossible for any process to
have as its sole result the transfer
of heat from a cooler object to a
hotter object.
It is impossible to have a
workless refrigerator.
2LT – No engine is 100% efficient
In 1824 Sadi Carnot (a French engineer) determined the hypothetical
maximum possible efficiency for an engine based on the 2LT.
Carnot Cycle
To maximize efficiency we must avoid irreversible processes.
Heat flow as a result of a temperature difference is an irreversible
process.
To avoid this irreversible process the only time that heat can flow must
be during an isothermal process.
W is the work done by the system
1. Isothermal
Expansion (a b)
(Heat Absorbed)
2. Adiabatic Expansion
(b  c)
3. Isothermal
Compression
(c  d)
(Heat Released)
4. Adiabatic
Compression
(d  a)
The thermal efficiency of a Carnot Engine is dependent only on
the absolute temperature (temperature in Kelvin) of the heat
reservoirs.
A Carnot engine takes 200o J of heat from a reservoir at 500 K, does
some work, and discards some heat to a reservoir at 350 K.
a) What is the engine’s efficiency?
b) How much work does it do?
c) How much heat is discarded by the engine?
A quantitative measure of disorder.
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Nature (irreversible processes) tends towards randomness (disorder).
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No process is possible in which the total entropy decreases when all
systems taking part in the process are included.
Reversible Processes
Change in Entropy is the heat transferred divided by the
absolute temperature (Kelvin).
Thermo Process
Heat (Q)
Isothermal
Q = -W
Isobaric
Q = ΔU - W
Isochoric
Q = ΔU
Adiabatic
Q=0
Entropy