GR0877 #35

issacnewton
Posts: 11
Joined: Sat May 14, 2011 12:41 am

GR0877 #35

Postby issacnewton » Tue Nov 08, 2011 11:49 pm

A heat pump is to extract from an outdoor environment at 7oC and heat the
environment indoors to 27 celcius. for each 15000 J of heat delivered indoors, the smallest
amount of work that must be supplied to the heat pump is approximately

a)500 J
b)1000 J
c)1100 J
d)2000 J
e) 2200 J

this is how I did my work.

T_c = (7+273) K

T_h=(27+273) K

the highest possible efficiency is that of carnot engine

\eta_{max}=1-\frac{T_c}{T_h}=\frac{1}{15}

let the \eta be the efficiency of this pump . then

\eta \leqslant \frac{1}{15}

\because \; \eta=\frac{W_{eng}}{|Q_h|}

\therefore \; \frac{W_{eng}}{|Q_h|} \leqslant \frac{1}{15}

Q_h=15000 J\;\Rightarrow \; W_{eng}\leqslant \frac{15000}{15}

W_{eng} \leqslant 1000

which suggests that W has some maximum value but the problem is talking about the
minimum value of W. is something wrong ?

bfollinprm
Posts: 1197
Joined: Sat Nov 07, 2009 11:44 am

Re: GR0877 #35

Postby bfollinprm » Wed Nov 09, 2011 12:17 am

it's not likely that there's a maximum energy a system can need in moving a certain amount of heat (if you give me an engine, I can always hand you back a less efficient one, assuming I have access to a hammer). Check your definitions again.

From Wikipedia:


Wis the work done by the system (energy exiting the system as work), Q_H is the heat put into the system (heat energy entering the system)

That's opposite of what you've defined.

issacnewton
Posts: 11
Joined: Sat May 14, 2011 12:41 am

Re: GR0877 #35

Postby issacnewton » Wed Nov 09, 2011 12:59 am

thanks for reply.

i am using the book Serway and Jewett. There for the heat pump, instead of efficiency , he talks about
coefficient of performance(COP) defined as

COP=\frac{|Q_h|}{W}

and he says that , because Q_h is generally greater than W , typical values of COP are greater than unity.
So how do I relate the efficiency to this ?

Edit: I checked the answer key and it gives 1000 J as the correct answer. Though in my first post , I could find the
upper bound for the W as 1000 J, the answer IS 1000 J. So is it possible that the problem wanted to ask "greatest amount of
work" instead of "smallest amount of work" ?

bfollinprm
Posts: 1197
Joined: Sat Nov 07, 2009 11:44 am

Re: GR0877 #35

Postby bfollinprm » Wed Nov 09, 2011 1:57 am

You're hung up on the W being "work" and the Q being "heat". The 15000J in the problem is the total "work" done by the heat pump--it identifies the amount of energy it has to add to the interior environment. The amount of energy you need to run the heat pump is the "heat" (plus work) generated. Thus, you're simply identifying the 15000 with the wrong element of the equation.

The best way to check these kind of things is to go back to the physics. Does it make sense to have a maximum total work done? No! That'd imply a heat pump with a minimum total efficiency, but I can create a heat pump with 0 (or even negative) total efficiency, simply by pointing the heat pump in the wrong direction. Therefore, you MUST have the terms backwards, or an inequality sign pointing in the wrong direction. Either way, you can be confident what you actually calculated is the minimum total work, not the maximum.

issacnewton
Posts: 11
Joined: Sat May 14, 2011 12:41 am

Re: GR0877 #35

Postby issacnewton » Wed Nov 09, 2011 2:55 am

thanks......... makes sense




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