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- Thermodynamics
- Applications of thermodynamics
Section summary
- An artifact of the second law of thermodynamics is the ability to heat an interior space using a heat pump. Heat pumps compress cold ambient air and, in so doing, heat it to room temperature without violation of conservation principles.
- To calculate the heat pump's coefficient of performance, use the equation
.
- A refrigerator is a heat pump; it takes warm ambient air and expands it to chill it.
Conceptual questions
In some Northern European nations, homes are being built without heating systems of any type. They are very well insulated and are kept warm by the body heat of the residents. However, when the residents are not at home, it is still warm in these houses. What is a possible explanation?
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Why do refrigerators, air conditioners, and heat pumps operate most cost-effectively for cycles with a small difference between
and
? (Note that the temperatures of the cycle employed are crucial to its
.)
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Grocery store managers contend that there is
less total energy consumption in the summer if the store is kept at a
low temperature. Make arguments to support or refute this claim, taking into account that there are numerous refrigerators and freezers in the store.
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Problem exercises
What is the coefficient of performance of an ideal heat pump that has heat transfer from a cold temperature of
to a hot temperature of
?
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Suppose you have an ideal refrigerator that cools an environment at
and has heat transfer to another environment at
. What is its coefficient of performance?
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What is the best coefficient of performance possible for a hypothetical refrigerator that could make liquid nitrogen at
and has heat transfer to the environment at
?
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(a) What is the best coefficient of performance for a heat pump that has a hot reservoir temperature of
and a cold reservoir temperature of
? (b) How much heat transfer occurs into the warm environment if
of work (
) is put into it? (c) If the cost of this work input is
, how does its cost compare with the direct heat transfer achieved by burning natural gas at a cost of 85.0 cents per therm. (A therm is a common unit of energy for natural gas and equals
.)
(a) 4.61
(b)
(c) To transfer
, heat pump costs $1.00, natural gas costs $1.34.
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(a) What is the best coefficient of performance for a refrigerator that cools an environment at
and has heat transfer to another environment at
? (b) How much work in joules must be done for a heat transfer of 4186 kJ from the cold environment? (c) What is the cost of doing this if the work costs 10.0 cents per
(a kilowatt-hour)? (d) How many kJ of heat transfer occurs into the warm environment? (e) Discuss what type of refrigerator might operate between these temperatures.
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Suppose you want to operate an ideal refrigerator with a cold temperature of
, and you would like it to have a coefficient of performance of 7.00. What is the hot reservoir temperature for such a refrigerator?
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An ideal heat pump is being considered for use in heating an environment with a temperature of
. What is the cold reservoir temperature if the pump is to have a coefficient of performance of 12.0?
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A 4-ton air conditioner removes
(48,000 British thermal units) from a cold environment in 1.00 h. (a) What energy input in joules is necessary to do this if the air conditioner has an energy efficiency rating (
) of 12.0? (b) What is the cost of doing this if the work costs 10.0 cents per
(one kilowatt-hour)? (c) Discuss whether this cost seems realistic. Note that the energy efficiency rating (
) of an air conditioner or refrigerator is defined to be the number of British thermal units of heat transfer from a cold environment per hour divided by the watts of power input.
(a)
(b) 40 cents
(c) This cost seems quite realistic; it says that running an air conditioner all day would cost $9.59 (if it ran continuously).
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Show that the coefficients of performance of refrigerators and heat pumps are related by
.
Start with the definitions of the
s and the conservation of energy relationship between
,
, and
.
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Questions & Answers
it is the relatively stable flow of income
what is circular flow of income
branches of macroeconomics
what is Flexible exchang rate?
is gdp a reliable measurement of wealth
introduction to econometrics
Why is unemployment rate never zero at full employment?
bcoz of existence of frictional unemployment in our economy.
Umashankar
what is flexible exchang rate?
poudel
due to existence of the pple with disabilities
Abdulraufu
the demand of a good rises, causing the demand for another good to fall
is it possible to leave every good at the same level
Joseph
I don't think so. because check it, if the demand for chicken increases, people will no longer consume fish like they used to causing a fall in the demand for fish
Anuolu
is not really possible to let the value of a goods to be same at the same time.....
Salome
Suppose the inflation rate is 6%, does it mean that all the goods you purchase will cost
6% more than previous year? Provide with reasoning.
Not necessarily. To measure the inflation rate economists normally use an averaged price index of a basket of certain goods. So if you purchase goods included in the basket, you will notice that you pay 6% more, otherwise not necessarily.
Waeth
discus major problems of macroeconomics
what is the problem of macroeconomics
Yoal
Economic growth
Stable prices
and low unemployment
Ephraim
explain inflationcause and itis degre
increase in general price levels
WEETO
Good day
How do I calculate this question: C= 100+5yd G= 2000 T= 2000 I(planned)=200.
Suppose the actual output is 3000. What is the level of planned expenditures at this level of output?
how to calculate actual output?
Chisomo
how to calculate the equilibrium income
Beshir
Criteria for determining money supply
who we can define macroeconomics in one line
Muhammad
Aggregate demand
Mohammed
C=k100 +9y and i=k50.calculate the equilibrium level of output
nawalparasi district
from belatari
Amisha
I am Camara from Guinea west Africa... happy to meet you guys here
Sekou
ahile becheclor ho
Amisha
hjr ktm bta ho
ani k kaam grnu hunxa tw
Amisha
money as unit of account means what?
Kalombe
A unit of account is something that can be used to value goods and services and make calculations
Jim
all of you please speak in English I can't understand you're language
Muhammad
I want to know how can we define macroeconomics in one line
Muhammad
it must be .9 or 0.9
no Mpc is greater than 1
Y=100+.9Y+50
Y-.9Y=150
0.1Y/0.1=150/0.1
Y=1500
Kalombe
Mercy is it clear?😋
Kalombe
hi can someone help me on this question
If a negative shocks shifts the IS curve to the left, what type of policy do you suggest so as to stabilize the level of output?
discuss your answer using appropriate graph.
if interest rate is increased this will will reduce the level of income shifting the curve to the left ◀️
Kalombe
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Source:
OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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