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Tag Archives: pressure
Polar bear understands the concept of preasure
Balloon at reduced pressure
A partially inflated balloon is placed inside a sealed container as air is pumped out of the by syringe. As the pressure of the container is reduced, the volume of the balloon increases. The pressure inside the balloon decreases.
Note that the pressure inside the balloon is not equal to the pressure of the container. The number of air molecules in the balloon is fixed. As the pressure of the container decreases, this creates a pressure difference inside and outside the balloon. This causes the balloon to expand and its volume to increase. The number of air molecules per unit volume inside the balloon decreases, hence pressure inside the balloon decreases.
But as the balloon is elastic, the wall of the balloon is stretched. Hence the pressure of the balloon will be greater than the pressure of the container. Refer to the video below.
Boiling water at reduced pressure – Water boils at 75 oC
At sea level (where most of us are), the standard atmospheric pressure is about 101325 Pa. The boiling point of water is at 100 oC which we are familiar with.
But as you climbed up e.g. Mount Everest at 8,848 m, the pressure is low and the boiling point of the water is about 71oC. So that’s the hottest cofe you can have on top of the cold mountain!
Hence as the pressure decreases, the boiling point of the water decreases. As with lower pressure, the water molecules requires lesser energy to break the intermolecular forces to escape into the atmosphere, hence boiling point is lower.
This video shows the same effect. Using the syringe, the air is pumped out of the container to reduce the pressure. The water at 75 oC , (below the usual boiling point of 100 oC) will start to boil and you can observe the bubbles forming!
Marshmallow Hulk in Vacuum Jar
When the pump is switched on and the air in the jar is gradually removed, the pressure in the jar decreases. There will be fewer air molecules per unit volume in the far. Hence rate of collision of the air molecules with one another and with the wall and hulk will be reduced. As pressure P = F/A, the force acting per unit area decreases, the pressure decreases.
In the marshmallow, there are pockets of air at normal atmospheric pressure initially. As the pressure in the jar decreases, the pockets of air in the marshmallow expands due to this pressure difference. Hence the hulk expands and its volume increases.
Liquid Pressure Summary Part 2
It’s Not an Impossible Fish Tank, It’s Just Physics | WIRED
Fluid pressure in a capillary tube
A uniform capillary tube closed at one end, contained air trapped by a thread of mercury 85 mm long. When the tube was held horizontal, the length of the air column was 50 mm. When it was held vertically with the closed end downwards, the length was 45 mm. Find the atmospheric pressure in Pa. (Density of mercury = 14 x 103 kg/m3 )
Solutions:
Manometer – mercury levels difference changes with different density of liquid used
The pressure of a gas is measured using a manometer as shown in the diagram.
The mercury in the manometer is replaced with a liquid which is less dense. How does the value of h change?
A It becomes zero.
B It decreases, but not to zero.
C It stays the same.
D It increases.
Solutions: Option D
The pressure to be measured remains constant. Since P = pgh, where p is the density of the liquid used in the manometer. If a liquid of lower density is used, height h of the liquid (level difference) will be greater. The gravitational field strength g remains constant.
2013 Nov Sci Phy P2 Q9 – Pressure and Moment
Solutions:
(a) (i) A bigger force than F can be obtained due to the lever system and hydraulic system.
Lever system: Applying principle of moments, the anticlockwise moment by the F is equal to the clockwise moment by the force on piston A (note that the handle is pushing the piston down, but the piston A is pushing on the handle upwards – action = reaction). As the perpendicular distance from F to the pivot is greater than the perpendicular distance of the force by piston to the pivot, the force on the piston A is greater than F at handle.
Hydraulic system: As the pressure transmitted in the liquid is the same, pressure at piston A = pressure at piston B. As P = F/A and area of piston A is smaller than area of piston B, a larger force is obtained in piston B.
Hence these two systems allow the force on piston B to be greater than F at the handle.
(a) (ii) Both liquid and gas molecules are in a continuously random motion. But in liquid, the molecules are closely packed together and able to slide around one another. There is very little empty space between the molecules hence liquid is not compressible. Gas molecules are far apart from one another, hence gas can be easily compressed.
(b) P = F/A = 12000 / 0.060 = 200 000 Pa
(c) Velocity is the vector quantity while speed is a scalar. As the car goes round the bend, the direction of the car changes. Hence velocity is changing even though speed is constant.
Likewise, as the velocity is changing, the car is considered to have an acceleration (not in the sense of increasing speed though).
Relationship between Pressure and Length of a Cube
Option: A
Liquid pressure – U-tube
A U-tube with 30 cm of water is set up vertically and 12 cm of olive oil is poured carefully into the left-hand limb of the tube as shown below. It is observed that water will be pushed up in the other limb. The density of water and olive oil are 1000 kg m-3 and 920 kg m-3 respectively.
Given A is at same level as the boundary between olive oil and water.a) What is the length of water column above point A?b) A further 6 cm of olive oil is added into the left-hand limb. How much further will the water level rise in the right limb?
Solutions: View video tutorial for part (b)a)
Poil = Pwater above A
ρgh = ρgh920 x 10 x 12 = 1000 x 10 x h
h = 11.04 cm
b) Pressure of water = pressure of olive oil
1000 x 10 x (2x + 11.04)/100 = 920 x 10 x 18/100
x = 2.76 cm
N2008P1Q15 – Kinetic Model of Matter
Gas inside a cylinder is heated slowly to a higher temperature. The pressure inside the cylinder remains constant as the piston moves outwards.
How do the speed of the gas molecules and their rate of collision with the piston compare with their initial values at the lower temperature?
Solutions:
In short: Temperature increases, Kinetic Energy increases, Rate of Collision decreases, Average Force on wall increases, Pressure constant.
As temperature increases, the speed of molecules increases, the kinetic energy of the air molecules increases.
As piston is free to move, it will move to the right such that the pressure remains constant (equal to atmospheric pressure outside). As the piston moves to the right, the volume inside the piston increases.
Surface area in which the air molecules collide increases.
The rate of collision decreases as the number of molecules remains constant. With higher KE of molecules, the molecules will collide the wall with greater force. Though rate of collision decreases, with each collision having greater impact force, the average force acting on the wall of piston increases.
Since P = F / A, with greater force F, over a bigger area A, the pressure P remains constant. (Compared with previously, smaller F over smaller A, but P constant)
Misconception: Many think that the rate of collision remains the same, which is wrong. Apparently the effect of volume increases is more significant, hence rate of collision decreases, even though they collide with greater impact force. Hence overall force on wall still increases. If the speed of the molecules increases but the pressure remains constant, then the molecules must collide less frequently. If the rate of collision stayed the same, the pressure would increase. If the rate of collision increased, the pressure would increase even more.
Evan's Space 