# Homework Help: Into Thermodynamics Review

1. Dec 13, 2004

### PhysicsWorst

Hey watsup guys,
I'm up to my last test (YESSS!) this wednesday and while reviewing the practice tests I took some stabs at a few but would like to ask for a little help with them. I'll post the questions with my answer choices(bold) and appreciate any corrections, advice, explanations, etc. you all can provide. THANKSSSS a bunch

1.)The efficiency of a real engine can be improved by __________ the input temperature and ____________ the exhaust temperature.
a- increasing.... increasing
b- increasing..........decreasing
c- decreasing........increasing
d- decreasing.......decreasing

2.)A heat engine takes in energy at a rate of 1600 W at 1000K and exhausts heat at a rate of 1200 W at 400 K. What is the maximum theoretical efficiency of this engine?
a- 40%
b- 50%
c- 60%
d- 70%

3.)An ideal heat engine has a theoretical efficiency of 60% and an exhaust temperature of 27C. What is its input temperature?
a- 227 C
b- 477 C
c- 500 C
d- 750 C
(for this one i'm guessing that the C in the answer options should b K's and I converted 27C to 330K)

4.)How much work per second (power) is required by a refrigerator that takes 800 J of thermal energy from a cold region each second and exhausts 1200 J each second to a hot region?
a- 400 W
b- 800 W
c- 1200 W
d- 2000 W
(not sure how to plug in numbers for this one)

5.)A cold piece of metal is dropped into an insulated container of hot water. After the system has reached an equilibrium temperature, the...
a- entropy of the metal has decreased
b- entropy of the water has decreased
c- net change in entropy of the system is zero
d- entropy of the system has decreased

6.)What restrictions does the second law of thermodynamics place on building of perpetual motion machine?
a- It does not place any restriction on this possibility
b- Energy must be conserved
c- Thermal energy cannot be completely converted into mechanical energy
d- The machine must have a very long cyclical period

7.)It is impossible to run an ocean liner by taking in seawater at the bow of the ship , extracting internal energy from the water, and dropping ince cubes off the stern because this process violates the __________law of thermodynamics
a- zeroth
b- first
c- second
d- third

8.)As a baby grows into an adult, the entropy of the universe...
a- increases[
b- stays the same
c- decreases
c- impossible to say without knowing this particular baby.

Last edited: Dec 13, 2004
2. Dec 13, 2004

### Bystander

Review? OK ---

Okay, technically, "a" and "d" are also possible --- magnitudes of increases and decreases can be picked that'll do the trick --- lousy question writing.

Sounds 'bout right.
27 + 273 = 330? You did mean 300? Otherwise, okay.
"Worng." Conservation of energy, no matter the form. Think about it.

"Thank you for playing. You have guessed incorrectly, please guess again." Think about the other 2nd law statements you've seen.

Good enough.
Yer there.

Wrong again --- reread the other 2nd law statements.

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