Perplexing Thermodynamics problem.Help

In summary, the conversation revolved around a problem involving an air pistol and its expansion in an isothermal process. The question asked for the final volume and mass of air, as well as the work done by the air and on the atmosphere, and the work done to the bullet and the bullet's exit velocity. The main challenge was in calculating the work done to the bullet and its exit velocity, as the mass of the bullet was not given. A possible solution was suggested involving the integration of W = integral P dV and considering isentropic expansion.
  • #1
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This problem has been puzzling me and several classmates for days now. The answer is due monday. I need some help.

"An air pistol contains compressed air in a small cylinder. Assume that the volume is 1 cm^3, pressure is 1 Mpa and the temperature is 27 C when released, the air expands in an isothermal process. If the air pressure is .1 Mpa in the cylinder as the bullet leaves the gun, find A) the final volume and the mass of air B) the work done by the air and the work done on the atmosphere C) the work to the bullet and the bullet exit velocity.

I have no problem with part A, but part B and C pose the problem. Since the mass of the bullet is not given, how can you calculate the work done to the bullet and the bullet exit velocity? I don't even know where to begin.

Help!
 
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  • #2
Just integrate W = integral P dV for V1 to V2 (which you know both) isohermal expansin PoVo is constant (isentropic expansion is probably more realistic.

Jaap
 
  • #3


Thermodynamics problems can definitely be challenging, especially when it comes to calculating work and velocities. It sounds like you have a good understanding of part A, and that's a great start. For part B and C, there are a few different approaches you can take to solve the problem.

One way to approach it would be to use the ideal gas law to calculate the final volume of air and then use the work-energy principle to find the work done by the air and the work done on the atmosphere. From there, you can use the conservation of momentum to find the velocity of the bullet. Another approach could be to use the isentropic process equations to find the work done by the air and the work done on the atmosphere, and then use the impulse-momentum principle to find the velocity of the bullet.

If you're still feeling stuck, I would recommend reaching out to your professor or a tutor for further guidance. They may be able to provide you with more specific steps or examples to help you solve the problem. Additionally, reviewing any relevant class notes or textbook material could also be helpful. Don't be afraid to ask for help, as it can often lead to a better understanding of the problem and the concepts involved. Best of luck and I hope you're able to find a solution before the deadline!
 

1. What is a "perplexing thermodynamics problem"?

A perplexing thermodynamics problem is a question or scenario related to the study of energy and its transformations, which may be complex or puzzling in nature.

2. Why are thermodynamics problems often considered difficult?

Thermodynamics problems can be challenging because they involve a combination of complex mathematical equations and abstract concepts related to energy, heat, and work.

3. How can I approach solving a perplexing thermodynamics problem?

Start by breaking down the problem into smaller, more manageable parts. Then, use fundamental thermodynamics principles and equations to analyze each part and piece them together to arrive at a solution.

4. What are some common mistakes to avoid when solving thermodynamics problems?

Some common mistakes include not carefully considering all given information, mixing up units, and not checking the units of the final answer. It is also important to pay attention to the signs (+/-) in equations and to be consistent in using them.

5. Are there any useful resources for practicing thermodynamics problems?

Yes, there are many textbooks, online resources, and practice problems available for students to improve their understanding of thermodynamics and practice problem-solving techniques. Working with a study group or seeking help from a tutor can also be beneficial.

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