A 1000 W iron is left on the ironing board with its base exposed to the air which is at 20oC. If the surface of the iron is at 400oC, determine:
a.) The total rate of entropy generation during this process in steady operation
b.) How much of this entropy generation occurs within the iron ? (Sgen,int)
c.) The lost work in kW, assuming the temperature of the surroundings is 20oC
6 comments:
i really have no idea how to start this problem. i understand what is happening and what i have to solve. but i am unsure of what equation to use
simply use the general equations for Sgen. it simplifies to Sgen(dot) = sum (Q/Tsys).
Q/Tfinal gives the correct answer to a. but i am unsure of why. should Tinitial matter?
For part C, do we use the equation W(lost) = T(surr) * S(gen)?
Anon 8:55 PM
Draw a picture and be sure to include a system boundary...either shrink-wrapped or extended.
Apply the 1st Law to determine Q.
Apply the 2nd Law to determine the Sgen inside of whatever system boundary you chose to use.
Anon 9:53 PM
I am not sure what Tiitial is, so I cannot say for sure.
The KEY to this problem is that the T in the equation you gave is the T of the system at the boundary where the heat transfer actually occurs. By choosing an extended boundary, we made T at the boundary = 20 degC. If we choose a shrink-wrapped boundary, then the T at the system boundary is 400 degC.
The Sgen you calculate is the Sgen INSIDE of the system boundary you chose.
Anon 10:02 PM
See my comment on the previous two Anonymous remarks.
Post a Comment