Heat Engine Example (22.5):
A particular heat engine has a mechanical power
output of 5.00 kW and an efficiency of 25.0%. The
engine expels 8.00 x 103 J of exhaust energy in
each cycle. Find:
a) The energy taken in during each cycle.
b) The time interval of each cycle.
In one complete cycle, a heat engine extracts heat
Qh from a thermal reservoir, does work W and
does not eject any heat into the environment. This
heat engine is impossible because its operation
violates
1. Conservation of energy (1st Law) only
2. 2nd Law only
3. Both 1st Law and 2nd Law.
The energy input to an engine is 3.00 times
greater than the work it performs. What is
the thermal efficiency?
1. 3.00
2. 1.00
3. 0.33
4. Impossible to determine
The energy input to an engine is 3.00 times
greater than the work it performs. What
fraction of the energy input is expelled to
the cold reservoir?
1. 0.333
2. 0.667
3. 1.00
4. Impossible to determine
To increase the efficiency of a heat engine, it would be better
1. to increase the temperature of the hot reservoir while
keeping the temperature of the cold reservoir constant.
2. to decrease the temperature of the cold reservoir while
keeping the temperature of the hot reservoir constant.
3. to decrease the temperature of the hot reservoir while
keeping the temperature of the cold reservoir constant.
4. to increase the temperature of the cold reservoir while
keeping the temperature of the hot reservoir constant.
5. none of these.
A device containing an ideal gas executes
the cycle shown. This is a
1. heat engine
2. refrigerator
3. Neither because the net work done
by/on the device is zero
P
Start/finish
V
The outdoor portion of a central air-conditioning
unit has a fan that blows air across the
condenser coils. If this fan breaks, why won’t the
air conditioner cool the house properly?
If you left the door to your refrigerator open,
after a few hours your kitchen would feel
1. Warmer.
2. Cooler.
3. Cozy.
4. Roomier.
5. It depends on the efficiency of your
refrigerator.
By what factor does the cost of heating your
home change when you replace your electric
heater with an electric heat pump that has a
COP of 4.00? Assume the heat pump motor is
100% efficient.
1. 4.00
2. 2.00
3. 0.500
4. 0.250
It is impossible to transfer a given
quantity of heat from a cold reservoir
to a hot reservoir.
1. true.
2. false.
An air conditioner is brought into the center of
a dorm room and turned on. After several
minutes, the temperature of the room
1. Increases
2. Decreases
3. remains constant
Carnot Engine Pre-Question:
Draw a P-V diagram for the following cyclic
process:
1)
2)
3)
4)
Isothermal expansion
Adiabatic expansion
Isothermal compression
Adiabatic compression
Carnot Example (22.3):
A 35%-efficient Carnot engine is run in reverse
(i.e. as a fridge). What is the COP?
It is not difficult to imagine a process that
produces more work than the Carnot cycle. For
example:
P
W
V
How then, is the Carnot engine “ideal”?
What must be true for a Carnot engine to have
an efficiency of 1?
You wish to maximize the COP of a Carnot heat
pump. You should:
1. Maximize the temperature of the hot reservoir.
2. Maximize the temperature of the cold reservoir.
3. Minimize the temperature of the hot reservoir.
4. Minimize the temperature of the cold reservoir.
Problem 1 (22.12):
A heat pump has a COP of 4.20 and requires a power of 1.75 kW to
operate.
a) How much energy does the heat pump add to your home in one
hour?
b) What is the pump is doing? Where does the energy come from?
c) If the heat pump were reversed to act as an air conditioner, what
would the COP be?
Problem 2 (22.27):
Argon enters a turbine at a rate of 80 kg/min, a temperature of 800
C, and a pressure of 1.5MPa. It expands adiabatically as it pushes on
the turbine blades and exits at pressure 300 kPa.
a) Calculate its temperature at the exit.
b) Calculate the (maximum) power output of the turning turbine.
c) The turbine is one component of a model closed-cycle gas
turbine engine. Calculate the maximum efficiency of this engine.