Thermodynamics: Internal Energy

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Homework Help Overview

The discussion revolves around thermodynamics, specifically focusing on the internal energy of gases in various scenarios involving work and heat transfer. The original poster presents multiple problems related to ideal gases, work done on or by the gas, and heat exchange, seeking clarification on the appropriate equations and concepts to apply.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the application of the first law of thermodynamics, questioning the relationships between internal energy, work, and heat. There are inquiries about unit consistency and the implications of constant temperature during processes. Some participants express uncertainty about the correctness of the original poster's attempts and suggest reconsidering assumptions regarding pressure and heat flow.

Discussion Status

Participants are actively engaging with the problems, offering insights into the relationships between variables and questioning the setup of the equations. There is no explicit consensus, but some guidance has been provided regarding the application of thermodynamic principles and the need for consistent units.

Contextual Notes

Some participants note potential discrepancies in the problem statements, such as the relationship between heat input and work done, which may affect the interpretation of the scenarios. The original poster has expressed difficulty in obtaining feedback from their instructor, which may contribute to the uncertainty in their understanding.

naspri4
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Homework Statement



I need some help on these problems, i got most of the other problems right, but these ones I am not sure what to do with. I don't even know if those are the right equations for it. Someone please help.


1. A cylinder-and-piston containing an ideal gas is placed in contact a thermal reservoir. The volume of the gas is very slowly changed from 50 liters to 12 liters as 50 J of work is done on it by an external agency. Determine the change in the internal energy of the gas.
The answer is in Joules

1.2. Determine the change in the amount of heat flowing into or out of the system. (For reference, a negative value of heat means it is flowing out of the system.)
the answer is in Joules

2. A gas in a thin plastic bag at atmospheric presure receives 10480 J of heat and, in the process, puffs up the bag, increasing its volume by 0.90 m3. By how much is the internal energy of the gas altered?
the answer is in kJ

3. Heat is added to 4.00 m3 of helium gas in an expandable chamber that thereupon increases its volume by 2.00 m3. If in the process 2.22 kJ of work are done by the gas, what was its original pressure?
the answer is in kPa


Homework Equations



change in energy=W
change in energy=W+Q
change in energy=W+Q+Em

The Attempt at a Solution



1. (50-12)+50
1.2. 88-50
2.(10480)(.9)
3.?
 
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Hi naspri4, welcome to PF. One serious problem I see is that you're not making sure your units match. For example, your first answer combines Joules with liters, which can never be right.

I agree with using the equation \Delta U = W + Q for change in energy. The problem is that both \Delta U and Q or W are sometimes unknown. There's also another equation you'll need that connects internal energy to temperature for an ideal gas, and that should be enough to solve these problems.
 
naspri4 said:

Homework Statement



I need some help on these problems, i got most of the other problems right, but these ones I am not sure what to do with. I don't even know if those are the right equations for it. Someone please help.1. A cylinder-and-piston containing an ideal gas is placed in contact a thermal reservoir. The volume of the gas is very slowly changed from 50 liters to 12 liters as 50 J of work is done on it by an external agency. Determine the change in the internal energy of the gas.
The answer is in Joules
If the gas is in contact with a thermal reservoir and it is compressed slowly, what can you say about the temperature of the gas during the process?

1.2. Determine the change in the amount of heat flowing into or out of the system. (For reference, a negative value of heat means it is flowing out of the system.)
the answer is in Joules
Use the answer from the first to determine the heat flow (is it out of or into the gas?).

2. A gas in a thin plastic bag at atmospheric presure receives 10480 J of heat and, in the process, puffs up the bag, increasing its volume by 0.90 m3. By how much is the internal energy of the gas altered?
the answer is in kJ
I think the question must be wrong because the work done exceeds the heat input by about 9 times.

3. Heat is added to 4.00 m3 of helium gas in an expandable chamber that thereupon increases its volume by 2.00 m3. If in the process 2.22 kJ of work are done by the gas, what was its original pressure?
the answer is in kPa
I think you have to assume that the pressure is constant here.

AM
 
Ok, so for the first question, the temperature would be constant meaning there wouldn't be any change. So how would that help me solve the problem? Did I have my equation set up right at the bottom of the first thread. I tried emailing my teacher via webassign but he has not responded for some unknown reason.

The second one was correct being zero, i don't know how but it was.

What about the third question? Any tips on how to solve it?
 
naspri4 said:
Ok, so for the first question, the temperature would be constant meaning there wouldn't be any change. So how would that help me solve the problem? Did I have my equation set up right at the bottom of the first thread. I tried emailing my teacher via webassign but he has not responded for some unknown reason.
Apply the first law. If there is no change in U what is the relationship between Q and W?

The second one was correct being zero, i don't know how but it was.
The work done is 101KPa x .9 m^3 =91KJ. If there is a heat flow of only 10.480 KJ this can't happen.

What about the third question? Any tips on how to solve it?
If pressure is constant, what is the relationship between Work and total change in volume?

AM
 
Last edited:

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