Why is the answer to this question "No change"

  • Thread starter Syndy
  • Start date
  • #1
25
0

Main Question or Discussion Point

image.jpg

http://postimage.org/ [Broken]
 
Last edited by a moderator:

Answers and Replies

  • #2
Svein
Science Advisor
Insights Author
2,025
649
Well, look at the piston. The force downwards is: (Atmospheric pressure)*(piston area) plus (sum of the masses of the piston and the weights)*g. These are not temperature dependent. If the piston is to stay where it is, the force upwards from the air below the piston (which is (air pressure)*(piston area)) must equal the force downward. Hence: The air pressure must stay constant.
 
  • #3
25
0
Ok . Thank you very much.
 
  • #4
Drakkith
Staff Emeritus
Science Advisor
20,737
4,442
I don't think that's correct. The example shown days nothing about the outside air. The pressure of the air inside the piston should absolutely change.
 
  • #5
A.T.
Science Advisor
9,969
1,731
The pressure of the air inside the piston should absolutely change.
I think they mean the pressure after a new equilibrium was found.
 
  • #6
Nugatory
Mentor
12,611
5,162
I don't think that's correct. The example shown days nothing about the outside air. The pressure of the air inside the piston should absolutely change.
Not as I read the problem... The volume will change in such as a way as to maintain a constant pressure.
 
  • #7
Drakkith
Staff Emeritus
Science Advisor
20,737
4,442
I think they mean the pressure after a new equilibrium was found.
Okay. That makes sense. I wasn't thinking about after equilibrium.
 
  • #8
34,038
9,880
Pressure in the piston will change if the outside pressure changes, but temperature changes won't change anything.
Even far away from a temperature equilibrium the piston will still be in mechanical equilibrium (unless you change the temperature with an explosion...).
 
  • #9
Drakkith
Staff Emeritus
Science Advisor
20,737
4,442
Pressure in the piston will change if the outside pressure changes, but temperature changes won't change anything.
Even far away from a temperature equilibrium the piston will still be in mechanical equilibrium (unless you change the temperature with an explosion...).
Can you elaborate on this?
 
  • #10
34,038
9,880
Force equilibrium, as Svein explained. If the (frictionless) piston does not move, pressure from below multiplied by the area is equal to pressure above multiplied by the area plus the weight of piston and the added weights. If we assume that pressure outside does not change, and we do not change the weights, pressure inside won't change.
 
  • #11
Drakkith
Staff Emeritus
Science Advisor
20,737
4,442
I think there's some confusion. I was under the impression that we weren't taking the outside air into account since the 'air' is labeled as being inside the piston.
 
  • #12
34,038
9,880
It is not given that we have a vacuum outside so air is a natural assumption - but even that does not matter because the answer "the pressure does not change" is independent of the absolute pressure.
 
  • #13
Drakkith
Staff Emeritus
Science Advisor
20,737
4,442
Oh oh oh, I get it now. I was confused by 'temperature equilibrium' vs 'mechanical equilibrium'.
 

Related Threads for: Why is the answer to this question "No change"

  • Last Post
Replies
3
Views
3K
  • Last Post
Replies
10
Views
3K
Replies
4
Views
885
  • Last Post
Replies
1
Views
1K
  • Last Post
Replies
1
Views
990
  • Last Post
Replies
1
Views
2K
Replies
3
Views
1K
Top