# pressure of a gas

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#1
Hi Guys,
I'm struggling on part B of the question below. i've calculated the pressure difference on part A to be 100000 pa but I'm not sure what the pressure of gas would be on tube C?is it the same as pressure inside? Thanks guys
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4 years ago
#2
I think you forgot to convert from cm to meters
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4 years ago
#3
The gas in C is the one which is causing the heigh difference between A and B. Coming from C, the pressure has an effect on B. In B the water is pressed and thus the water increases up to A.

It is p = d*g*h, where d stands for density of water and h for the named heigh difference between A and B.

As the density is given in meters and the heigh difference in centimeters, you have one of the to convert. After that you are able to calculate the pressure.
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#4
(Original post by Kallisto)
The gas in C is the one which is causing the heigh difference between A and B. Coming from C, the pressure has an effect on B. In B the water is pressed and thus the water increases up to A.

It is p = d*g*h, where d stands for density of water and h for the named heigh difference between A and B.

As the density is given in meters and the heigh difference in centimeters, you have one of the to convert. After that you are able to calculate the pressure.
sorry i've just seen this. Haven't recived any notification of your reply guys anyway so basically the pressure of the gas in tube C is same as the pressure difference between A and B so you take the value of the h the same for both?
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4 years ago
#5
(Original post by Alen.m)
sorry i've just seen this. Haven't recived any notification of your reply guys anyway so basically the pressure of the gas in tube C is same as the pressure difference between A and B so you take the value of the h the same for both?
The height h is just the distance from point B to point A, or to explain that in other words: the difference in water levels in the tube. So, yeah you are right. According to the pressure, the water in the tube increases in dependence to the force which is working on the area of the water. So, the greater the force on a (constant) area, the greater the difference between A and B, thus the stronger the pressure in the tube.
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4 years ago
#6
It is the pressure difference plus atmospheric pressure, which presumably is the pressure at the open end of the apparatus. So basically it's a knowledge test (plus your understanding that a higher pressure at C causes this situation).
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#7
(Original post by Kallisto)
The height h is just the distance from point B to point A, or to explain that in other words: the difference in water levels in the tube. So, yeah you are right. According to the pressure, the water in the tube increases in dependence to the force which is working on the area of the water. So, the greater the force on a (constant) area, the greater the difference between A and B, thus the stronger the pressure in the tube.
Thanks for your great explanation. About part c, anything comes in mind of why this pressure is the pressure of the gas above atmospheric pressure?
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4 years ago
#8
(Original post by Alen.m)
Thanks for your great explanation. About part c, anything comes in mind of why this pressure is the pressure of the gas above atmospheric pressure?
Consider the situation where the tube is open to the atmosphere at both ends... the fluid level is equal in both arms not because there is no pressure but because there is no difference in pressure.

if one of the arms is connected to a closed pressure vessel, the fluid will be pushed up on the open side wrt the other side if the mystery pressure is higher than atmospheric pressure. If the fluid is pushed up on the mystery pressure side it's because atmospheric pressure is higher than the mystery pressure... and if the level is exactly the same on both sides it's because the mystery pressure is exactly equal to atmospheric pressure.
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#9
(Original post by Joinedup)
Consider the situation where the tube is open to the atmosphere at both ends... the fluid level is equal in both arms not because there is no pressure but because there is no difference in pressure.

if one of the arms is connected to a closed pressure vessel, the fluid will be pushed up on the open side wrt the other side if the mystery pressure is higher than atmospheric pressure. If the fluid is pushed up on the mystery pressure side it's because atmospheric pressure is higher than the mystery pressure... and if the level is exactly the same on both sides it's because the mystery pressure is exactly equal to atmospheric pressure.
so basically if the fluid pushes up on the opposite side of tube c, it means that the gas pressure is higher than atmospheric pressure but if the fluid pushes up on the same side as tube c, it means the atmospheric pressure is more than gas pressure?shouldn't it be other way around?
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4 years ago
#10
(Original post by Alen.m)
so basically if the fluid pushes up on the opposite side of tube c, it means that the gas pressure is higher than atmospheric pressure but if the fluid pushes up on the same side as tube c, it means the atmospheric pressure is more than gas pressure?shouldn't it be other way around?
well no - it's a similar situation to having a straw in a glass of milkshake.

if the pressure on the milk surface in the glass is the same as the pressure on the milk surface in the straw, then the level of the milk surface in the straw will be the same as the level of the milk surface in the glass.

if you lower the pressure in the gas above the fluid in the straw (by sucking) so it's below atmospheric pressure, the atmospheric pressure on the milk in the glass pushes the milk into the bottom of the straw and raises the milk level inside the straw.

if you raise the pressure in the air above the fluid in the straw above atmospheric pressure the milk is pushed out of the bottom of the straw and the level of milk inside the straw is lowered below the level in the glass.
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#11
(Original post by Joinedup)
well no - it's a similar situation to having a straw in a glass of milkshake.

if the pressure on the milk surface in the glass is the same as the pressure on the milk surface in the straw, then the level of the milk surface in the straw will be the same as the level of the milk surface in the glass.

if you lower the pressure in the gas above the fluid in the straw (by sucking) so it's below atmospheric pressure, the atmospheric pressure on the milk in the glass pushes the milk into the bottom of the straw and raises the milk level inside the straw.

if you raise the pressure in the air above the fluid in the straw above atmospheric pressure the milk is pushed out of the bottom of the straw and the level of milk inside the straw is lowered below the level in the glass.
what i understand from your explanation is that if gas pressure inside is more than atmospheric pressure, the water level would rise below point B all the way until point A or above(in opposite side of tube c)but if atmospheric pressure is more than gas pressure, the water level would rise below point A all the way until point B or above????
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4 years ago
#12
(Original post by Alen.m)
what i understand from your explanation is that if gas pressure inside is more than atmospheric pressure, the water level would rise below point B all the way until point A or above(in opposite side of tube c)but if atmospheric pressure is more than gas pressure, the water level would rise below point A all the way until point B or above????

Atm represents atmospheric pressure

P1 = atmospheric pressure
P2 > atmospheric pressure
P3 < atmospheric pressure
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#13
(Original post by Joinedup)

Atm represents atmospheric pressure

P1 = atmospheric pressure
P2 > atmospheric pressure
P3 < atmospheric pressure
Perfectly shown. Thanks very much.
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