Thermodynamics-why is this constant pressure?

Click For Summary
SUMMARY

The discussion centers on the behavior of a piston-cylinder device containing 0.85 kg of refrigerant R-134a, initially at -10°C, and heated to 15°C. The final pressure remains equal to the initial pressure of 88 kPa, confirming the process is isobaric. This occurs because the piston is free to move, allowing the system to reach equilibrium where the internal gas pressure equals the external atmospheric pressure. The movement of the piston facilitates energy transfer, balancing the forces exerted by the gas and the atmosphere.

PREREQUISITES
  • Understanding of thermodynamic processes, specifically isobaric processes.
  • Familiarity with the properties of refrigerants, particularly R-134a.
  • Knowledge of piston-cylinder device mechanics and pressure equilibrium.
  • Basic principles of heat transfer and energy conservation in thermodynamics.
NEXT STEPS
  • Study the principles of isobaric processes in thermodynamics.
  • Learn about the properties and applications of refrigerants like R-134a.
  • Explore the mechanics of piston-cylinder devices and their role in thermodynamic systems.
  • Investigate heat transfer methods and their impact on gas behavior in closed systems.
USEFUL FOR

Students and professionals in mechanical engineering, thermodynamics, and HVAC systems who seek to understand the principles governing pressure and temperature changes in refrigerant systems.

pyroknife
Messages
611
Reaction score
4
A piston-cylinder device contains 0.85kg of refrigerant at -10 deg C. The piston that is free to move has a mass of 12kg and diameter of 25cm. the local atmospheric pressure is 88kPa. Now, heat is transferred to the refrigerant-134a until the temperature is 15 deg C. Determine the final pressure.

The solution in the textbook says that the final pressure is equal to the initial pressure. I don't understand how this is an isobaric process.
 
Physics news on Phys.org
pyroknife said:
A piston-cylinder device contains 0.85kg of refrigerant at -10 deg C. The piston that is free to move has a mass of 12kg and diameter of 25cm. the local atmospheric pressure is 88kPa. Now, heat is transferred to the refrigerant-134a until the temperature is 15 deg C. Determine the final pressure.

The solution in the textbook says that the final pressure is equal to the initial pressure. I don't understand how this is an isobaric process.
The piston is free to move.
 
I don't understand why that means it's constant pressure. Are all pistons that are free to move considered constant pressure? If the piston is not free to move that means it's constant volume right?
 
The piston is free to move and the atmospheric pressure on the outside is constant. If energy is transferred into the system as heat, the energy only has one place to go, into the kinetic energy of the gas, making it move faster. The more energetic gas particles push harder on the piston than air outside, so it moves. As the piston moves away particles that bounce off it lose momentum so they slow down, losing energy. The piston moves until it reaches the equilibrium point where the gas inside pushes just as hard as the gas outside.

Some of the heat energy has gone into moving the piston, some of the energy has gone into increasing the temperature of the gas. Work has been done to move the piston.

Any amount of gas will push on the walls of the container its in. If a piston is free to move, it requires a equal and opposite force to keep it where it is. Therefore, yes a piston that is free to move will move unless the pressure is equal on each side. Yes a piston that is not free to move will preserve the volume of the container.
 

Similar threads

Adiabatic Expansion of Pressurized Air in a Piston-Cylinder Setup
  • · Replies 8 ·
Replies
8
Views
2K
Mixing two ideal gases with different V, T at constant pressure
  • · Replies 16 ·
Replies
16
Views
4K
Proof question related to the Ideal Gas Law
  • · Replies 8 ·
Replies
8
Views
2K
High School Gas cylinder at constant pressure being exposed to atmosphere, how?
  • · Replies 8 ·
Replies
8
Views
2K
Thermodynamics Enthelpy Problem
  • · Replies 10 ·
Replies
10
Views
2K
Thermally insulated container with alternating series of walls
  • · Replies 49 ·
2
Replies
49
Views
3K
Thermodynamics- Piston problem, pressure, internal energy
  • · Replies 5 ·
Replies
5
Views
2K
Why is PV work in thermodynamics so difficult to understand?
  • · Replies 3 ·
Replies
3
Views
3K
Why is temperature constant after gas has expanded?
  • · Replies 33 ·
2
Replies
33
Views
2K
Pressure and force in a cylinder
  • · Replies 11 ·
Replies
11
Views
4K