Can You Solve These Thermodynamics Practice Questions?

[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.

 

[Bystander]

Review? OK ---

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

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

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%

Sounds 'bout right.

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)

27 + 273 = 330? You did mean 300? Otherwise, okay.

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)

"Worng." Conservation of energy, no matter the form. Think about it.

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

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

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

Good enough.

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

Yer there.

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.

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

 

[wais]

Hello! It's great that you're reviewing for your last test. Here are my responses to your questions:

1. The correct answer is b- increasing...decreasing. This is because increasing the input temperature will increase the amount of energy available to the engine, while decreasing the exhaust temperature will reduce the amount of energy wasted.

2. The correct answer is b- 50%. This can be calculated using the formula for efficiency: (1 - Tc/Th) x 100%, where Tc is the exhaust temperature and Th is the input temperature. In this case, the efficiency would be (1 - 400/1000) x 100% = 60%.

3. The correct answer is b- 477 C. You are correct in converting the temperature to Kelvin. Using the same formula as above, we can rearrange it to solve for Th: Th = Tc/(1 - efficiency). Plugging in the values, we get Th = 330K/(1 - 0.6) = 825K, which is equivalent to 552C. However, the question asks for the input temperature in Kelvin, so we need to add 273 to get 825K.

4. The correct answer is b- 800 W. This can be calculated using the equation for power: P = W/t, where W is the work done and t is the time. In this case, the work done is 800 J and the time is 1 second, so the power would be 800 J/s = 800 W.

5. The correct answer is d- entropy of the system has decreased. This is because the metal has gained thermal energy from the water, causing its entropy to increase, while the water has lost thermal energy, causing its entropy to decrease. However, the overall entropy of the system has decreased because the decrease in entropy of the water is greater than the increase in entropy of the metal.

6. The correct answer is c- Thermal energy cannot be completely converted into mechanical energy. This is due to the second law of thermodynamics, which states that energy cannot be completely converted from one form to another without some loss of usable energy.

7. The correct answer is c- second. The first law of thermodynamics states that energy cannot be created or destroyed, so the energy taken in by the ship must be equal to the energy output. However, the second law states that some energy will always be lost in the