Identify process in a heat engine

In summary, the student is having trouble with a heat engine problem and needs some help. The three parts of the problem are a pressure graph, a volume graph, and a temperature graph. The pressure graph shows a straight line, while the volume and temperature graphs show an increase in volume and temperature. The student is not able to solve the problem and needs help from others. The student finds that the process is not adiabatic and that temperature changes. The student uses other equations to find W and Q. The student is not sure if the process is isobaric/choric or if it is a general transformation for a perfect gas.
  • #1
physics.stu
5
0

Homework Statement


I'm taking a practice test and I need some help with a heat engine problem.
There are three parts, the one that's giving me trouble is a-->b. The Volume v. Pressure graph shows a straight line. Pa= 5 atm Va= 5 L Ta=300K and Tb=800K are given.
So pressure is decreaseing, while volume and temperature are increasing. What type of process is this? How could I find Pb or Vb?

Homework Equations



For an adiabatic process:
PaVa^(5/3)=PbVb^(5/3)=constant
TaVa^(2/3)=TbVb^(2/3)=constant
W=-dU(internal energy)=-(3/2)RTln(Vb/Va)

The Attempt at a Solution


I initially thought this was adiabatic expansion but that cannot be true because of the temperature increase (as far as my limited knowledge can tell). All my calculations yield nonsense answers so I really do not think it could be adiabatic; but I may be wrong of course. Other than that I have no idea
 
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  • #2
physics.stu said:
I initially thought this was adiabatic expansion but that cannot be true because of the temperature increase (as far as my limited knowledge can tell). All my calculations yield nonsense answers so I really do not think it could be adiabatic; but I may be wrong of course. Other than that I have no idea

In an adiabatic process, temperature DO change: the adiabatic process is the one in which there is no heat exchange with the outside, but temperature will change.

This said, you're right, it can't be an adiabatic process anyway, because i) if pressure decreases and volume increases, then temperature would decrease in an adiabatic process, and also ii) it is not represented with a straight line, but with a curve...

A straight line with both pressure and volume increasing sounds as no elementary transformation to me (it is neither an isochoric transf. nor an isobaric one). Also isothermal would have constant temperature so nothing to do. The only chance I see is to use a composite transformation. You could try seeing if you can connect the starting and arriving point with an isothermal and an adiabatic transformation, but the simplest way should be to use an isochoric and an isobaric.

In practice you can try to reach the final pressure with constant volume and then leave the same pressure and vary volume to reach also final volume. This is the only way I can think of anyway.
 
  • #3
Can you use the information of b-->c to figure out the conditions at b?
 
  • #4
I found that Pa= 10Pc so I was able to find Vc and b-->c is isovolumetric so Vc=Vb. The only question remaining is what type of process could this be?!
Thank you all for your replies. It's not isobaric/choric because both pressure and volume are changing. I'm glad you all agree that it can't be adiabatic. Maybe a-->b isn't a process that has any sort of special characteristics other than the ideal gas law. I should be able to find W and Q from other equations.
 
  • #5
I am not aware of any special name for a transformation which is not isobaric/choric and is straight at the same time... then it has to be a general transformation for a perfect gas, and that's all
 
  • #6
I think you're right Tia. I used W=PdV and dU=Q-W to find W and Q. Thanks for your help!
 

Related to Identify process in a heat engine

1. What is a heat engine?

A heat engine is a device that converts heat energy into mechanical work. It follows the principles of thermodynamics and operates by extracting heat from a high-temperature source, using it to perform work, and then releasing the remaining energy to a lower-temperature sink.

2. How does a heat engine work?

A heat engine works by utilizing the temperature difference between a hot source and a cold sink to drive a cycle of processes that convert heat into work. This can be achieved through various mechanisms such as combustion, steam pressure, or gas expansion.

3. What is the purpose of identifying the process in a heat engine?

Identifying the process in a heat engine helps to understand how the engine is converting heat into work. It allows for the analysis of the efficiency and performance of the engine and helps in identifying any potential improvements that can be made.

4. How do you identify the process in a heat engine?

The process in a heat engine can be identified by analyzing the thermodynamic cycle it follows. This can be done by measuring the changes in temperature, pressure, and volume of the working fluid as it moves through the engine. The type of working fluid and the mechanism used to convert heat into work can also provide clues about the process.

5. What are the different types of processes in a heat engine?

There are four main types of processes in a heat engine: isothermal, adiabatic, isochoric, and isobaric. An isothermal process occurs at a constant temperature, an adiabatic process occurs without heat exchange, an isochoric process occurs at a constant volume, and an isobaric process occurs at a constant pressure.

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