Why use c_p and not c_v as specific heat - Thermodynamics

  • #1
yjl
8
0
Hey all,

I am working on a problem that goes like this:

The cargo space of a refrigerated truck whose inner
dimensions are 12 m 3 2.3 m 3 3.5 m is to be precooled
from 25°C to an average temperature of 5°C. The construc-
tion of the truck is such that a transmission heat gain occurs
at a rate of 120 W/°C. If the ambient temperature is 25°C,
determine how long it will take for a system with a refrigera-
tion capacity of 11 kW to precool this truck.


The solutions manual calculates the mass of the air inside the truck and afterwards does
[tex]Q=mc_p\Delta T[/tex]
to calculate the amount of heat removed.

My question is: why do we use [tex]c_p[/tex] here and not [tex]c_v[/tex]
It's a closed system, such that
[tex]Q=m(u_2-u_1)=mc_v\Delta T[/tex]
right?
 
Last edited:

Answers and Replies

  • #2
BvU
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It's a closed system
How does the solution manual
The solutions manual calculates the mass of the air inside the truck
 
  • #3
rude man
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Insights Author
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A truck is not really a closed system. It's much more liely to be vented somewhere to atmospheric pressure.
 

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