Please explain for the following question: 4. A household refrigerator (see diag
ID: 497267 • Letter: P
Question
Please explain for the following question:
4. A household refrigerator (see diagram) is essentially a device that uses electrical work to transfer heat from a cold system (the contents of the refrigerator to a warmer system (the kitchen). (a) Explain why, on the basis of the second law of thermodynamics universe 20) some minimum amount of electrical work is necessary for the refrigeration process to occur, and (b) compute the minimum amount of work necessary to transfer 1 kJ ofheat from the refrigerator at 5 oC to the kitchen at 25 oC. hot kitchen dai compressor dw electric grid fridge refrigerant ColdExplanation / Answer
(a) if you see heat is taken from low temperature and released at higher temperature in overall system which is not possible according to 2nd law of thermodynamics.
if you see delta S = delta q/T so when heat is taken up from low temperature the same heat looses more entropy as q is same taken and rejected but the temperature at which it is released is different and hence delta s universe will become negative which is not possible.it needs and external work to obtain this effect and makes the delta s universe +ve by adding electrical work.
b) efficiency of refrigerator are defined in terms of COP(coefficient of performance)
desired effect/input work
Qcold/Work input = Qcold/(Qhot - Qcold) = Tc/(Th-Tc)
Qcold heat rejected by cold body
Qhot heat given to hot body
Tc = cold body temperature
Th = hot fluid temperature
Qcold = 1KJ
Tc = 273.15+5 = 278.15K
Th = 273.15 + 25 = 298.15K
278.15K/(298.15K-278.15K) = 1KJ/work
work = 71.90 J