# Pressure problem

1. Oct 12, 2011

### Misr

Hello,world
[PLAIN]http://img257.imageshack.us/img257/5245/unled2vw.jpg [Broken]

I don't understand where those equations have come from:
A. Pressure of the gas inside the tube=Pa
B.Pressure of the mercury+pressure of the gas inside the tube=Pa
C.Pa+Pressure of the mercury=pressure of the gas inside the tube

Last edited by a moderator: May 5, 2017
2. Oct 12, 2011

### Low-Q

If Pa = surrouding air pressure

A=Pa
B>Pa (Weight of mercury compress volume B)
C<Pa (Weight of mercury decompress volume C)

Last edited by a moderator: May 5, 2017
3. Oct 12, 2011

### Misr

What makes A=Pa in the first case?

4. Oct 12, 2011

### Low-Q

The gravitational force of a mass points vertically, and will therfor not apply pressure or sub-pressure to the volum in case A. The mercury will therefor position horizontally where there is an equilibrium between outside pressure and volume(A) pressure.

In the other cases the weight of mercury will have an impact on the pressure in volume B and C.

Vidar

5. Oct 12, 2011

### Misr

I see.
In A,If we try to move the mercury to the left-increasing the gas pressure-,it will return again to its position because fluids transfer from high pressure to low pressure.Is that right?

No.B is okay
but I can't imagine case C,could you explain more?

6. Oct 13, 2011

### Low-Q

1. Yes

2. If we assume that the mercury acts like a piston with a given weight, that weight will be forced downwards. That will expand the volume i C. Since no air is flowing into volume C in this operation, the air density and pressure will decrease.

However, the mercury will in all three cases find its equilibrium except the pressure inside volume B and C will change due to the weight of mercury. If you open B and C, allowing air to escape out of the bottom (B) or enter at the top (C), the mercury will fall down to the ground.

Vidar

7. Oct 13, 2011

### Misr

Well,I have thought of what I said again and I find that it is not true.
on moving the mercury to the left,the mercury would not return to its orginal position
because The mercury doesnot compress air in A even if we move it to the left because gravity doesn't act horizontally
so what do you think?
I still can't imagine how the volume is decreased .Is it beacause both of mercury and the trapped gas are pushing upon the atmospheric pressure?

8. Oct 13, 2011

### sophiecentaur

In all three cases, the pressure at the interface between the mercury and the outside is Atmospheric pressure once equilibrium has been reached.

A. Pressure is just Atmospheric
B. Pressure is Atmospheric + weight of mercury / cross sectional area of tube
C. Pressure is Atmosphjeric - weight of mercury / cross sectional area of tube

('weight of mercury / cross sectional area of tube' is just another way of saying 'the hydrostatic pressure due to the column of mercury')

9. Oct 13, 2011

### Misr

I can't imagine the third case

could you comment on post 7?

10. Oct 13, 2011

### sophiecentaur

The pressure on the underneath surface of mercury is equal to the pressure inside plus the weight of the mercury / csa.

Which bit did you want a comment on?

11. Oct 13, 2011

### Low-Q

In case A YOU have to use energy to move the mercury. If the air pressure inside volume A is getting higher because you compress the air by hand, the mercury will go back to its origin when you take away your hand. The pressure is controlling the position in case A. In case B and C BOTH pressure AND gravity is controlling position of the mercury.

Vidar

12. Oct 15, 2011

### Misr

but Mr Vidar already convinced me

What do you mean when we say that the mercury "decompresses" the air inside the tube?

13. Oct 16, 2011

### Low-Q

The merqury have a mass, right? That mass will move towards the ground. However, the air inside volume C, which is a compressable and decompressable gas which allow the mercury to move slightly downwards. This will ofcourse increase the volume in C, but since no extra gas has entered volum C the gas must respond by getting less densed, which again means lower pressure.

I was once told at school: "If you don't understand it, just accept it, and learn it."

Vidar

14. Oct 16, 2011

### sophiecentaur

Hard to accept being told that, I agree BUTTTT-
That advice is often very applicable. I could modify it and say that, unless you have a proveable alternative then what you have been told could well be correct enough. You can always verify, by reading around, that what you've been told by your teacher is what's accepted.
Whilst you can almost guarantee that there are better, more sophisticated or up-to-date, versions available, there is no reason why you shouldn't try to understand the 'elementary' views. Only when you fully have sussed these out, will you be in a position to appreciate the more advanced versions.

It may be galling to accept that the teacher 'know best' but he/she or the text book just may do, in this respect. You need to build on, rather than reject what you are told in School.

15. Oct 16, 2011

### Misr

may be you are right.but I tried to apply this several times and every time i fail..some times you can really get some great results rather than just accept it.This happened with me two years ago on using this website I could understand some great ideas,that's why i was the only person who got the full mark in physics that year,because all the students "just accpect it and learn it"
I feel very curious and I failed to kill this curiosity by just accepting what I'm told

Anyways i guess i have a better view of case C now
Thanks very much

16. Oct 16, 2011

### Misr

may be you are right.but I tried to apply this several times and every time i fail..some times you can really get some great results rather than just accept it.This happened with me two years ago on using this website I could understand some great ideas,that's why i was the only person who got the full mark in physics that year,because all the students "just accpect it and learn it"
I feel very curious and I failed to kill this curiosity by just accepting what I'm told

Anyways i guess i have a better view of case C now
Thanks very much

17. Oct 17, 2011

### Low-Q

I have not allways accepted what I've been told at school. Learning by accepting has never worked for me... That's why I still try to violate laws of thermodynamics

Good to read that you understand point C. Hope you find it useful :-)

Vidar