Final, Physics 9HB, Tuesday, March. 16, 2010 Name:
Directions:
Test is open book, open note.
Please write your name on every
page. If you think there is something missing from the test which you need,
please ask Jeremy.
Use the backside of the paper as needed.
1) Short answer, 40 pts, 5 pts each.
Explain your reasoning in the space
provided or on the back of the page.
A)A refrigerator uses 100W of electricity and discards 600W into the
kitchen.
Its coefficient of performance is
a) .17 b) .2 c) 5 d) 6 e) Other. f) Impossible to answerviolates energy
conserv.
600W is discarded, with 100W of work done.
So Q
H
= 600 W, and thus
Q
c
= 500W = Q
H
– W – hence COP = 500/100 = 5
B) A certain gas of atomic fermions has spin 3/2 (four spin orientations).
Compared to a gas of spin
½
fermions of the same density and mass, what
is its Fermi temperature?
All else being equal, the Fermi momentum will
go as p
F
= (h/2)(6
ν
/[
π
#spin
components])
1/3
so that for the same mass and density
T
F
(S=3/2)/T
F
(S=1/2) = (2/4)
2/3
=2
2/3
=0.63
since T
F
~ p
F
2
C) Scientists in Iceland discover geothermal vents that produce abundant
pressurized steam with a temperature of 300
0
C.
Engineers construct a heat
engine that uses this steam as a hot reservoir and a nearby glacier as a
cold reservoir.
The maximum possible efficiency of this engine is
a) 300% b) 100% c) 52% d) 45% e) 22% f) other
Ice is at 0
0
C = 273K, so max efficiency is (1273/573) = 52%
D) 100 g of water is slowly heated from 5
0
to 25
0
C.
What is its change in
entropy? (For water, c = 4186 J/(kgK))
Δ
S = mc
∫
dT/T = mc ln(T
2
/T
1
) where T
1
,T
2
Hence
S = 0.1 kg x 4186 J/(kgK) x ln(298/278) = 29 J/K
T
1
T
2
This preview has intentionally blurred sections. Sign up to view the full version.
View Full DocumentE) A sample of He gas is held at constant temperature while it I s slowly
compressed in a cylinder until its volume has decreased by a factor of 4.
During this time the gas’s entropy
a) increases b) decreases c) does not change d) does something else
(explain).
Decreases.
This is the end of the preview.
Sign up
to
access the rest of the document.
 Winter '10
 DanielCox
 Physics, Energy, Kinetic Energy, Statistical Mechanics, The Land

Click to edit the document details