Chapter 11: The 1 st Law of Thermodynamics Thermodynamics: the study of processes in which energy is transformed as heat and as work. (“movement of heat”) Recall: Work is done when energy is transferred from one body to another by mechanical means. (Ex: compressing a gas.) Heat is a transfer of energy due to a difference in temperature. (Ex: Heating a gas.)
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The 2nd Law of Thermodynamics · Chapter 11: The 1st Law of Thermodynamics Thermodynamics: the study of processes in which energy is transformed as heat and as work. (“movement
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Chapter 11: The 1st Law of
ThermodynamicsThermodynamics: the study of processes in
temperatures from a place called the hot reservoir
(the hot fuel).
Part of the heat input is used to perform work via the
working substance of the engine—which is the
material w/in the engine that actually does the
work. (Ex: gas/air mixture in combustion
chamber.)
The remainder of the input heat is rejected at a
temperature lower than the input temperature to a
place called the cold reservoir (or heat sink). (Ex:
radiator or exhaust). This is waste heat!
Schematic of Heat Engine, Fig. 15.12
This schematic
representation of a heat
engine shows the input heat
(QH) that originates from
the hot reservoir, the work
(W) that the engine does,
and the heat (QC) that the
engine rejects to the cold
reservoir.
Work is only done when
heat is transferred from
high temp. to low temp.Figure 15.12
Power plants: one example of a Heat
Engine
Power plants use the heat from the burning of coal, oil, gas, or heat from nuclear fission to produce energy that does work by turning electric generators.
Waste heat is also produced, sometimes referred to “heat pollution” because it pollutes the environment.
Waste heat discharged into waterways increases the temperature of that waterway.
Waste heat discharged into air can contribute to weather changes.
Efficiency (e) of a Heat Engine
Efficiency = ratio of work the heat engine does
to the heat input at high temp.
e = (work done) / (Input heat) = W / QH
can multiply this value by 100 to get percent
efficiency
The less heat needed to do the work = greater
efficiency
An engine that is 100% efficient would have
an e value of 1 (or 100%), meaning that QH =
W (non-existent)
Heat engines must obey Conservation of
Energy Law
(1st Law of Thermodynamics), so:
QH = W + QC (1st Law)
W = QH – QC
Substitute into efficiency
equation:
e=W/QH = (QH – QC)/QH
e = 1 – (QC / QH)
Or e = 1 – (Tc / Th )
The smaller QC is, the
greater the efficiency (less
heat wasted).
Figure 15.12
Operation of a Heat Engine
Slide 11-26
Answer
Consider your body as a system. Your body is “burning” energy in food, but staying at a constant temperature. This means that, for your body,
A. Q > 0.
B. Q = 0.
C. Q < 0.
Slide 11-40
The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency?
A. 300°C 30°C
B. 250°C 30°C
C. 200°C 20°C
D. 100°C 10°C
E. 90°C 0°C
Additional Questions
Slide 11-41
Answer
The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency?
Checking Understanding: Increasing Efficiency of a Heat PumpWhich of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.
A. All of the aboveB. 1 and 4C. 2 and 3
Slide 11-31
Answer
Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.
A. All of the aboveB. 1 and 4C. 2 and 3
Slide 11-32
The Theoretical Maximum Efficiency of aHeat Engine
Slide 11-27
Note: This equation does not depend on the type of fuel used in the heat engine nor is that important.
Slide 11-23
Closed system: ΔU = Q + W (Only nrg
exchanged)
Open system: ΔU = Q + W (but need to take
into account the change in internal nrg due to the
increase/decrease in the amount of matter)
Isolated system: ΔU = Q + W = 0 (since no nrg
in any form can leave or enter the system… i.e.,
ΔU=0.)
Energy is conserved in this process:
QH = W + QC
This process is reversible.
The inside of a refrigerator is the cold
reservoir, the outside is the hot reservoir. The
warm air you feel blowing out of your frig is
equal to the heat removed from inside your frig
plus the work done in removing it. QH = W +
QC
The same holds for air conditioner:
house=inside of frig, etc.
Refrigeration Process:
So, is it possible to cool your kitchen by leaving your
frig door open (or placing an air conditioning unit on
the floor in the middle of the room)?
NO!
Heat is removed from inside frig and exhausted into
kitchen, plus the work to remove this heat is also
exhausted… result?
Your kitchen gets warmer when you open the frig
door!
Heat is removed from room by A/C unit, but exhausted
back into room, in addition to the work to remove this
heat is also exhausted! Doesn’t cool the room; heats it
up and wastes electricity!!
Reading Quiz
3. A refrigerator is an example of a
A. reversible process.B. heat pump.C. cold reservoir.D. heat engine.E. hot reservoir.
Slide 11-10
Answer
3. A refrigerator is an example of a
A. reversible process.B. heat pump.C. cold reservoir.D. heat engine.E. hot reservoir.
Slide 11-11
Checking Understanding: Increasing Efficiency of a Heat PumpWhich of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.
A. All of the aboveB. 1 and 4C. 2 and 3
Slide 11-31
Answer
Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen.
A. All of the aboveB. 1 and 4C. 2 and 3
Slide 11-32
Refrigeration Cycle… how does a
refrigerator or A/C work anyway?1. Low pressure gas enters compressor where it becomes
high pressure, hot gas.
2. The gas goes into condenser coil (behind or under the frig) where heat* is removed (by coil) and cold (high pressure) fluid comes out (gas liquid: releasing heat in process). *Part of this heat is from what was extracted from inside frig.
3. The high pressure fluid goes through an expansion valve which allows the pressure (& temp.) to drop.
4. The low pressure (cold) fluid goes into evaporator coils in frig which turns into a gas as energy is absorbed from inside frig.
5. Back to the compressor… step 1.
Refrigeration Cycle
Heat Pumps
The air conditioner and heat pump do closely
related jobs. The air conditioner refrigerates
the room and heats up the outdoors, while the
heat pump refrigerates the outdoors and heats
up the inside room.
A heat pump can serve in a dual capacity—
equipped w/a switch that converts them from
A/C units in summer to heaters in winter…
cool!
Heat PumpsA heat pump takes heat
from QC and with the
addition of work, places it
in QH.
QH = QC + W Heat is removed from inside frig (or
outside of house where it’s cold) and
exhausted into kitchen (or inside your
house), plus the work to remove this
heat is also exhausted… result?
Your kitchen (inside of house) gets
warmer
(Remember… opening the frig door
causes the kitchen to heat up!)
What’s more efficient, a heat pump or
conventional electric heating system? A heat pump is very efficient
b/c if 1000J is used to do work of moving QC from outside to inside, you get heat totaling QH=QC+W, or QH=QC+1000J inside!
Your heat pumps 1000J + the Qc taken from the outdoors!
Versus, if you have an electric heater and 1000J of electric nrg is being delivered, then all you get is 1000J of heat, QH.
conventional electric heating system
Does it take more or less work the greater the
temperature difference when heating your
house with a heat pump?
More… it takes more work to pump heat
“uphill” from cold reservoir (outside) to hot
reservoir (inside). Remember, heat naturally
flows from hot to cold, so going from cold to
hot is an “uphill climb” for the heat. http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cf