exweek3
TRANSCRIPT
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8/12/2019 exweek3
1/4
T h e r m o d y n a m i c s o f M a t e r i a l s 3 . 0 0 F a l l 2 0 0 1
E x a m p l e P r o b l e m s f o r W e e k 3
E x a m p l e P r o b l e m 3 . 1
O n e m o l e o f a n i d e a l g a s , i n a n i n i t i a l s t a t e P = 1 0 a t m , V = 5 L , i s t a k e n , r e v e r s i b l y , i n
a c l o c k w i s e d i r e c t i o n , a r o u n d a c i r c u l a r p a t h g i v e n b y ( V ; 1 0 )
2
+ ( P ; 1 0 )
2
= 2 5 . C a l c u l a t e
t h e a m o u n t o f w o r k d o n e b y t h e g a s a s a r e s u l t o f t h i s p r o c e s s , t h e c h a n g e i n i n t e r n a l e n e r g y
o f t h e g a s , a n d t h e m a x i m u m a n d m i n i m u m t e m p e r a t u r e s a t t a i n e d b y t h e g a s d u r i n g t h e
c y c l e . H o w d o t h e s e a n s w e r s c h a n g e i f t h e n u m b e r o f m o l e s o f g a s d o u b l e f o r t h e s a m e P - V
c y c l e ?
S o l u t i o n 3 . 1
T h e i n t e r n a l e n e r g y i s a s t a t e f u n c t i o n a n d o n l y d e p e n d s o n t h e i n i t i a l a n d n a l s t a t e s
o f t h e s y s t e m . T h i s i s a c l o s e d i n t e g r a l s o t h e c h a n g e i n i n t e r n a l e n e r g y i s z e r o .
H
d U = 0 .
T h e r e a r e t w o w a y s t o c a l c u l a t e t h e w o r k d o n e b y t h e s y s t e m d u r i n g t h e c y c l e . T h e
r s t i s t o a n a l y t i c a l l y i n t e g r a t e
R
P d V , t h e s e c o n d i s t o a s s e s s t h e a r e a u n d e r t h e c u r v e
g r a p h i c a l l y . ( P ; 1 0 )
2
+ ( V ; 1 0 )
2
= 2 5 i s t h e e q u a t i o n o f a c i r c l e i n t h e P - V p l a n e w i t h
c e n t e r a t ( 1 0 1 0 ) a n d r a d i u s 5 . T h e i n i t i a l s t a t e i s a t t h e l o w e s t a l l o w e d v o l u m e f o r t h e
s y s t e m , ( V
0
P
0
) .
P
V10
10=P_o=P_1
V_o V_1
F i g u r e 1 : P - V d i a g r a m f o r t h e w o r k c y c l e .
F i r s t w e m u s t n d P = P ( V ) i n o r d e r t o p e r f o r m t h e i n t e g r a t i o n . P = 1 0
q
2 5 ; ( V ; 1 0 )
2
w h e r e t h e p l u s s i g n c o r r e s p o n d s t o t h e u p p e r h a l f o f t h e c i r c l e ( P
+
( V ) ) a n d t h e m i n u s s i g n
c o r r e s p o n d s t o t h e l o w e r h a l f ( P
;
( V ) ) . N o w l e t V
0
= V ; 1 0 i n t h e f o l l o w i n g .
W
s
=
Z
P d V
W
s
=
Z
V
1
= 1 5
V
0
= 5
P
+
( V ) d V +
Z
V
0
= 5
V
1
= 1 5
P
;
( V ) d V
W
s
=
Z
V
0
1
= 5
V
0
0
= ; 5
1 0 +
p
2 5 ; V
0 2
d V
0
+
Z
V
0
0
= ; 5
V
0
1
= 5
1 0 ;
p
2 5 ; V
0 2
d V
0
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8/12/2019 exweek3
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W
s
=
2 5
2
( + 8 ) +
2 5
2
( ; 8 ) = 2 5 L a t m
B y t h i s m e t h o d t h e w o r k d o n e b y t h e s y s t e m i s 2 5 1 0 1 : 3 J = 7 9 5 6 J . T h e s e c o n d m e t h o d
i n v o l v e s e x a m i n i n g t h e a r e a u n d e r t h e c u r v e , a c i r c l e w i t h r a d i u s 5 . S o t h e a r e a i s 2 5 L a t m .
T h i s a g a i n i s t h e w o r k d o n e b y t h e s y s t e m e q u a l t o 7 9 5 6 J .
T h e m a x i m u m a n d m i n i m u m t e m p e r a t u r e s c a n b e f o u n d f r o m t h e i d e a l g a s l a w f o r o n e
m o l e o f g a s , P V = R T . T
m a x
a t t a i n s w h e n ( P V ) i s a m a x i m u m a n d T
m i n
a t t a i n s w h e n
( P V ) i s a m i n i m u m . L e t P
0
= P ; 1 0 a n d V
0
= V ; 1 0 s o P V i s a n e x t r e m a l w h e n P
0
V
0
i s e x t r e m a l .
P
0
V
0
= f ( P
0
( V
0
) V
0
) =
p
2 5 ; V
0 2
V
0
d f
d V
0
= 0 =
p
2 5 ; V
0 2
V
0
1
2
; 2 V
0
p
2 5 ; V
0 2
0 =
"
2 5 ; V
0 2
; V
0 2
p
2 5
;V
0 2
#
T h e e x t r e m a a r e f o r V
0
=
q
2 5
2
. E x a m i n a t i o n o r c h e c k i n g t h e s i g n o f t h e s e c o n d d e r i v a t i v e ,
d
2
F
d V
2
> 0 f o r a m i n i m u m a n d
d
2
F
d V
2
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8/12/2019 exweek3
3/4
C a l c u l a t e t h e v a l u e o f P w h i c h m a k e s t h e w o r k d o n e o n t h e g a s i n t h e r s t c y c l e e q u a l t o
t h e w o r k d o n e b y t h e g a s i n t h e s e c o n d c y c l e .
S o l u t i o n 3 . 2
F o r o n e m o l e o f a n i d e a l g a s P V = R T a n d W =
R
P d V . W i s t h e w o r k d o n e b y t h e
g a s . T h e r e a r e 3 k i n d s o f p r o c e s s e s h e r e a n d t h e w o r k m u s t b e c a l c u l a t e d a c c o r d i n g l y .
1 . F o r a n i s o t h e r m a l p r o c e s s ( T i s c o n s t a n t ) : P = R T = V s o W = R T l n
V
1
V
0
= R T l n
P
0
P
1
.
2 . F o r a n i s o b a r i c p r o c e s s ( P i s c o n s t a n t ) : W = P ( V
1
; V
0
) = R ( T
1
; T
0
)
3 . F o r a c o n s t a n t v o l u m e p r o c e s s ( o n e o f t h e i s o c h o r i c p r o c e s s e s ) W = 0 b u t t h e p r e s s u r e
a n d t e m p e r a t u r e w i l l c h a n g e .
L e t ' s b e g i n b y s u m m a r i z i n g t h e t r a n s f o r m a t i o n s o f t h e g a s b y r e c o r d i n g t h e T , P , V b e f o r e
a n d a f t e r e a c h p r o c e s s i n l i t e r s , a t m o s p h e r e s a n d K e l v i n a n d a l s o t h e f o r m o f t h e w o r k d o n e
b y t h e s y s t e m .
T
0
P
0
V
0
T
1
P
1
V
1
W
i
2 9 8 1 . 2 9 8 0 : 5 . W
1
= R T l n
P
0
P
1
2 9 8 0 : 5 . 3 7 3 0 : 5 . W
2
= R ( T
1
; T
0
)
3 7 3 0 : 5 . 3 7 3 1 . W
3
= R T l n
P
0
P
1
3 7 3 1 . 2 9 8 1 . W
4
= R ( T
1
; T
0
)
2 9 8 1 . 3 7 3 1 V
u
W
0
1
= R ( T
1
; T
0
)
3 7 3 1 V
u
T
3
P
u
V
u
W
0
2
= 0
T
3
P
u
V
u
T
4
P
u
2 4 : 5 W
0
3
= R ( T
1
;T
0
)
. P
u
2 4 : 5 . 1 2 4 : 5 W
0
4
= 0
F o r t h e r s t r e v e r s i b l e p r o c e s s t h e w o r k d o n e b y t h e s y s t e m i s W =
P
4
i = 1
W
i
.
W =
4
X
i = 1
W
i
= 2 9 8 R l n 2 + R ( 3 7 3 ; 2 9 8 ) + 3 7 3 R l n 0 : 5 + R ( 2 9 8 ; 3 7 3 )
W = ( 2 9 8 ; 3 7 3 ) R l n 2 = ; 7 5 R l n 2
F o r t h e s e c o n d c y c l i c p r o c e s s t h e w o r k d o n e b y t h e s y s t e m i s W
0
=
P
4
i = 1
W
0
i
.
W
0
=
4
X
i = 1
W
0
i
= R ( 3 7 3 ; 2 9 8 ) + 0 + R ( T
4
; T
3
) + 0
W
0
= 7 5 R + R ( T
4
; T
3
)
S o w e m u s t n d t h e u n k n o w n t e m p e r a t u r e s u s i n g T = P V = R a n d V = R T = P .
T
3
=
P
u
R
V
u
=
P
u
R
3 7 3 R
1
= 3 7 3 P
u
T
4
=
P
u
2 4 : 5
R
N o w W
0
= 7 5 R + R (
P
u
2 4 5
R
; 3 7 3 P
u
) = 7 5 R + R P
u
(
2 4 5
R
; 3 7 3 ) .
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8/12/2019 exweek3
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T h e r e s t r i c t i o n t h a t t h e w o r k d o n e o n t h e g a s i n t h e r s t c y c l e e q u a l s t h e w o r k d o n e
b y t h e g a s i n t h e s e c o n d c y c l e i s e q u i v a l e n t t o ; W = W
0
.
W + W
0
= 0 = ; 7 5 R l n 2 + 7 5 R + R P
u
(
2 4 : 5
R
; 3 7 3 )
P
u
=
7 5 ( l n 2 ; 1 )
2 4 5
R
; 3 7 3
I n t h e a b o v e R = 8 : 3 1 4
J
m o l e K
1 a t m
1 0 1 3 1 0
5
P a
1 0
3
L
m
3
= 0 : 0 8 2 0 7
L a t m
m o l e K
. S o P
u
= 0 : 3 a t m .