Power Consumption and Entropy Generation

AI Thread Summary
The discussion revolves around a homework problem related to power consumption and entropy generation in a mechanical engineering context. The user calculates internal energy values based on temperature but expresses uncertainty about the correctness of their power calculation and how to approach the entropy generation aspect. Key equations for power, work, internal energy, and entropy are provided, emphasizing the importance of understanding specific internal energy and its units. The response highlights the need to consider the change in time and the correct units for specific internal energy to arrive at accurate results. Overall, the conversation focuses on clarifying the application of thermodynamic principles in the problem.
gloppypop
Messages
5
Reaction score
0

Homework Statement


[/B]
2-3-15

123.jpg


Homework Equations


[/B]
P = power. W = work. U = internal energy. S = entropy. t = time. Q = heat. T = temperature. F = force. d = distance.

P = ΔW/Δt= ΔU/Δt

ΔS = ΔQ/T

dm/dt = ρ⋅dV/dt

W = F ⋅ d

ΔU = Q - W

Where m is mass, V is volume, and ρ is the density of the fluid. Water ρ = 1000 kg/m3

The Attempt at a Solution


[/B]
Since Uint = 4.2T
Uin = (4.2)(300K) = 1260J
Uout = (4.2)(370K) = 1554J

P = ΔU/Δt = (1554J - 1260J) = 294 J/sec

I don't know if this is correct. And I'm not sure how to approach (b), regarding entropy generation.
 
Last edited:
Physics news on Phys.org
https://www.physicsforums.com/attachments/95666
 
I'm really lost. Any hints? Maybe this should be in engineering. It's for my Mechanical Engineering 205 class...
 
gloppypop said:
I'm really lost. Any hints? Maybe this should be in engineering. It's for my Mechanical Engineering 205 class...
The problem statement did tell you that the specific internal energy and specific entropy were u = 4.2 T and s = 4.2 \ln T respectively. Let's just concentrate on the specific internal energy for now.

The "specific" part of specific internal energy tells you that it's quantifying the energy per amount of stuff. This amount of stuff is usually measured in units of kilograms (kg) in problems like these, but don't be surprised in future coursework if it's measured in moles, pounds, tons, etc.

So here, u isn't in units of energy. But rather it's in units of energy per unit mass. I'm guessing, probably J/kg.

Also, you haven't figured the change in time into your equations either. You've suddenly converted units of J to J/sec without any rationale.

Figure out both of those, and I suspect you'll get the right answer. :smile: [Hint: if you're clever, you might be able to figure them both out in a single step.]
 
TL;DR Summary: I came across this question from a Sri Lankan A-level textbook. Question - An ice cube with a length of 10 cm is immersed in water at 0 °C. An observer observes the ice cube from the water, and it seems to be 7.75 cm long. If the refractive index of water is 4/3, find the height of the ice cube immersed in the water. I could not understand how the apparent height of the ice cube in the water depends on the height of the ice cube immersed in the water. Does anyone have an...
Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .

Similar threads

Back
Top