Page 1 PHY205s15 Exam 2: Name: ___ 1. A heat engine with an

PHY205s15 Exam 2:
Name: __________________________
___ 1. A heat engine with an output of 300 W has an efficiency of 25% and works at 10 cycles/s. How much
heat is absorbed (Qh) and how much rejected (Qc) in each cycle?
A) Qh = 150 J, Qc = 120 J
D) Qh = 1200 J, Qc = 900 J
B) Qh = 1500 J, Qc = 1200 J
E) Qh = 120 J, Qc = 90 J
C) Qh = 40 J, Qc = 10 J
___ 2. What is the maximum possible efficiency of a steam engine operating between a high and low
temperature of 550°C and 180°C?
A) 82% B) 67% C) 55% D) 33% E) 45%
___ 3. Entropy is related to probability. An isolated system moves toward
A) a highly ordered state of low probability and high entropy.
B) a highly ordered state of high probability and high entropy.
C) a state of low order, high probability, and low entropy.
D) a state of low order, low probability, and high entropy.
E) a state of low order, high probability, and high entropy.
___ 4. Two moles of a gas at T = 350 K expand quasistatically and isothermally from an initial volume of 20
L to a final volume of 60 L. The change in entropy of the gas during this expansion is (R = 8.314
J/mol·K)
A) –17.4 J/K B) 20.4 J/K C) 18.3 J/K D) –24.6 J/K E) 27.8 J/K
___ 5. A heat engine absorbs 64 kcal of heat from a hot reservoir and exhausts 42 kcal to a cold reservoir each
cycle. Its efficiency is
A) 30% B) 40% C) 38% D) 34% E) 42%
___ 6. If α is the coefficient of linear expansion of a material at 0ºC, the volume thermal-expansion
coefficient of this material at 0ºC is
A) ~3α B) ~α C) ~α3 D) ~α1/3 E) None of these is correct.
___ 7. A glass rod that is 1.0 m long at 0ºC increases its length by 0.68 mm when heated to 80ºC. The
coefficient of linear expansion of this glass is approximately
A) 6.8 × 10–4 /Cº B) 8.5 × 10–6/Cº C) 0.013/Cº D) 1.0/Cº E) 80/Cº
___ 8. Two types of wall separate a refrigerated room from the rest of a building. Wall 1 has half the thermal
conductivity of wall 2. Wall 2 is twice as thick as wall 1. The two walls have the same area. The rate
of heat flow through wall 2 compared with the rate of heat flow through wall 1 is
A) four times greater.
D) the same.
B) twice as great.
E) one-quarter as great.
C) half as great.
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___ 9. If the absolute temperature of the filament of a lamp were doubled, the energy radiated per second by
the filament would
A) remain the same.
D) increase by a factor of 16.
B) increase by a factor of 4.
E) increase by a factor of 8.
C) increase by a factor of 2.
___ 10. The temperature gradient in a wall is 80º C/cm, and the thermal conductivity of this wall is 4.0 × 10–3
cal/s · cm · Cº. The amount of heat conducted per minute per square centimeter through this wall is
approximately
A) 27 cal B) 19 cal C) 35 cal D) 42 cal E) 0.42 kcal
___ 11. A string of mass 2.4 × 10–3 kg and length 0.60 m vibrates transversely in such a way that its
fundamental frequency is 100 Hz. The tension on this string must be approximately
A) 0.16 N B) 0.32 N C) 58 N D) 26 N E) 13 N
___ 12. When the frequency of a source is doubled, the sound produced
A) travels at half its former speed.
D) has twice its former wavelength.
B) travels at twice its former speed.
E)
has half its former wavelength.
C) is greatly improved in quality.
___ 13. A sinusoidal wave train is moving along a string. The equation giving the displacement y of a point at
coordinate x has the form
y(x, t) = 0.15 sin[10π(t – x/60)]
where the units are SI. The wavelength is
A) 8.0 cm B) 15 cm C) 6.0 m D) 60 m
E) 12 m
___ 14. A noisy workplace has a noise level of 90 dB. You want to reduce this to a more comfortable 75 dB.
By what factor must the power of the noise source be reduced? (That is, what is the ratio of the new
power to the old?)
A) 0.83 B) 0.67 C) 0.032 D) 0.32 E) 0.010
___ 15. A train traveling at 90 km/h is blowing its whistle at 440 Hz as it crosses a level crossing. You are
waiting at the crossing and hear the pitch of the whistle change as the train passes you. The sound you
hear changes from a frequency of _____ to a frequency of _____. (Take the speed of sound to be 340
m/s.)
A) 475 Hz; 410 Hz
D) 472 Hz; 408 Hz
B) 410 Hz; 475 Hz
E) 598 Hz; 348 Hz
C) 408 Hz; 472 Hz
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Answer Key
1.
2.
3.
4.
E
E
E
C
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
B
C
E
E
C
A
W=300J/s·10cycle/s =30J 30J/Qh=.25, Qh=120J, Qc=(120-30)J =90J
1-(273+180)/(273+550) = .4496
a state of low order, high probability, and high entropy.
dQ=0+pdV=nRTdV/V, ∫dQ/T=nR∫dV/V=nRln(V2/V1)=2mole(8.314J/mole·K)ln(60/20)
=18.27J/K
D 1-(42/64) =.344
A 3α
B α=.68×10-3m/1m·80Co=8.5×10-6 /Co
D Q1/t =(½·κ2)(ΔT/L1)A Q2/t =(κ2)(ΔT/2L1)A= Q1/t
D T4→24 times=16 times
Q/tA=(4×10-3cal/s·cm·oC)(80oC/cm)(60s/min)=19.2cal/cm2·min
v=λf=√(F/µ), F=µ(λf)2=(2.4×10-3kg/.6m)(1.2m·100Hz)2=57.6N
has half its former wavelength
2π/λ =10π/60 λ=12m
90dB=10ln(I1/I0), 75dB=10ln(I2/I0), (90-75)dB=10ln(I1/I2), I1/I2=101.5 =31.6, I2/I1=.0316
440Hz/(1±[(90km/h)(103m/km)(1h/3600s)]/340m/s)=409.9Hz, 474.9Hz
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