P20 089

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89.

(a) The work done in a constant-pressure process is W = pV . Therefore,

W =

25 N/m

2

1.8 m

3

3.0 m

3

=

3 0 J .

The sign conventions discussed in the textbook for Q indicate that we should write

75 J for the

energy which leaves the system in the form of heat. Therefore, the first law of thermodynamics
leads to

E

int

= Q

− W = (75 J) (3 0 J) = 45 J .

(b) Since the pressure is constant (and the number of moles is presumed constant), the ideal gas law

in ratio form (see Sample Problem 20-1) leads to

T

2

= T

1

V

2

V

1

= (300 K)

1.8 m

3

3.0 m

3

= 180 K .

It should be noted that this is consistent with the gas being monatomic (that is, if one assumes
C

V

=

3
2

R and uses Eq. 20-45, one arrives at this same value for the final temperature).


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