-
Home
- College physics for ap® courses
- 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?
Got questions? Get instant answers now!
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
.)
Got questions? Get instant answers now!
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.
Got questions? Get instant answers now!
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
?
Got questions? Get instant answers now!
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?
Got questions? Get instant answers now!
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
?
Got questions? Get instant answers now!
(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.
Got questions? Get instant answers now!
(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.
Got questions? Get instant answers now!
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?
Got questions? Get instant answers now!
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?
Got questions? Get instant answers now!
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).
Got questions? Get instant answers now!
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
.
Got questions? Get instant answers now!
Questions & Answers
If auger is pair are the roots of equation x2+5x-3=0
Wayne and Dennis like to ride the bike path from Riverside Park to the beach. Dennis’s speed is seven miles per hour faster than Wayne’s speed, so it takes Wayne 2 hours to ride to the beach while it takes Dennis 1.5 hours for the ride. Find the speed of both bikers.
from theory: distance [miles] = speed [mph] × time [hours]
info #1
speed_Dennis × 1.5 = speed_Wayne × 2
=> speed_Wayne = 0.75 × speed_Dennis (i)
info #2
speed_Dennis = speed_Wayne + 7 [mph] (ii)
use (i) in (ii) => [...]
speed_Dennis = 28 mph
speed_Wayne = 21 mph
George
Let W be Wayne's speed in miles per hour and D be Dennis's speed in miles per hour. We know that W + 7 = D and W * 2 = D * 1.5.
Substituting the first equation into the second:
W * 2 = (W + 7) * 1.5
W * 2 = W * 1.5 + 7 * 1.5
0.5 * W = 7 * 1.5
W = 7 * 3 or 21
W is 21
D = W + 7
D = 21 + 7
D = 28
Salma
Devon is 32 32 years older than his son, Milan. The sum of both their ages is 54 54. Using the variables d d and m m to represent the ages of Devon and Milan, respectively, write a system of equations to describe this situation. Enter the equations below, separated by a comma.
find product (-6m+6) ( 3m²+4m-3)
what is the solution
bill
how did you arrive at this answer?
bill
i really want to learn
Amira
I only got 42 the rest i don't know how to solve it. Please i need help from anyone to help me improve my solving mathematics please
Amira
Hw did u arrive to this answer.
Aphelele
-6m(3mA²+4m-3)+6(3mA²+4m-3)
=-18m²A²-24m²+18m+18mA²+24m-18
Rearrange like items
-18m²A²-24m²+42m+18A²-18
Salma
complete the table of valuesfor each given equatio then graph.
1.x+2y=3
given that (7x-5):(2+4x)=8:7find the value of x
Nandala
please why isn't that the 0is in ten thousand place
please why is it that the 0is in the place of ten thousand
Grace
Send the example to me here and let me see
Stephen
A meditation garden is in the shape of a right triangle, with one leg 7 feet. The length of the hypotenuse is one more than the length of one of the other legs. Find the lengths of the hypotenuse and the other leg
state in which quadrant or on which axis each of the following angles given measure. in standard position would lie 89°
I am eliacin, I need your help in maths
Rood
hmm can we speak here?
Amoon
however, may I ask you some questions about Algarba?
Amoon
what the last part of the problem mean?
Roger
The Jones family took a 15 mile canoe ride down the Indian River in three hours. After lunch, the return trip back up the river took five hours. Find the rate, in mph, of the canoe in still water and the rate of the current.
Shakir works at a computer store. His weekly pay will be either a fixed amount, $925, or $500 plus 12% of his total sales. How much should his total sales be for his variable pay option to exceed the fixed amount of $925.
I'm guessing, but it's somewhere around $4335.00 I think
Lewis
12% of sales will need to exceed 925 - 500, or 425 to exceed fixed amount option. What amount of sales does that equal? 425 ÷ (12÷100) = 3541.67. So the answer is sales greater than 3541.67.
Check:
Sales = 3542
Commission 12%=425.04
Pay = 500 + 425.04 = 925.04.
925.04 > 925.00
Munster
difference between rational and irrational numbers
When traveling to Great Britain, Bethany exchanged $602 US dollars into £515 British pounds. How many pounds did she receive for each US dollar?
how to reduced echelon form
Jazmine trained for 3 hours on Saturday. She ran 8 miles and then biked 24 miles. Her biking speed is 4 mph faster than her running speed. What is her running speed?
d=r×t
the equation would be 8/r+24/r+4=3
worked out
Sheirtina
Got questions? Join the online conversation and get instant answers!
Source:
OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
Google Play and the Google Play logo are trademarks of Google Inc.