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Category Archives: 06 Pressure
Polar bear understands the concept of preasure
How to mark 2 points (at a distance apart) at the same level? (water finds its own level)
The video below demonstrates that water will find its own level.
An olevel question requires this concept:
An apartment block receives water from a nearby reservoir. A pump is necessary to lift the water into a storage tank at the top of the building.
The gravitational field strength is 10 N / kg. How much energy does the pump supply to lift each kilogram of water into the tank?
Solutions: 100J
Water will always find its own level. Hence with or without the pump, the water level in the pipe in the building will be of the same level as the reservoir. Hence, the pump is only needed to pump water up a height of 10m only (instead of the 50m).
Energy, E = mgh = 1 kg x 10 N/kg x 10m = 100 J
Hand Boiler
Hand boiler toy is a demonstration of thermal transfer and how the 3 states of matter interact.
The coloured liquid inside is usually ethyl alcohol, which is a volatile liquid (i.e. a liquid with low boiling point). Similar to the alcohol swab you applied on hand during injection. Such liquid evaporates easily in room temperature or when heat is applied to it.
Thermal energy flows from a region of higher temperature to a region of lower temperature. When we hold the boiler in our hand, the thermal energy from our hand (body temp about 37oC) is transferred to the lower bulb of the boiler. The ethyl alcohol evaporates and also due to heated air above the liquid, the air expands and the pressure increases. The air pushes the liquid down, hence pushing the liquid up the shaft into the top bulb.
The bubbling of the liquid in the top bulb is not due to ‘boiling’ of the liquid. Rather, the gas in the lower bulb continues to expand and it goes to the top bulb, hence giving the illusion that the liquid is ‘boiling’.
Understanding Hydraulic System Overview
Understanding Hydraulic System 01
Understanding Hydraulic System + Lever 02
Barometer with a layer of water – Find height of mercury H
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.
Man Jumps Vertically Upwards, Pressure On Ground Is Greater During The Jump
This concept is similar to a 2016 O-Level Pure Physics Question P2 Q2, on why the pressure acting on the ground is greater during the jump, compared to when he is standing stationary on the ground.
During the jump, his leg will exert an upward force. This upward force (equivalent to normal force or force on the man by the ground) is greater than the weight of the man. Hence there is a net (resultant force) upwards, causing him to accelerate upwards.
That force on the man by the ground is equal and opposite to the force on the ground by the man. This is an action-reaction pair. Since the force exerted on the ground by the man is greater (greater than weight), the pressure exerted on the floor is greater.
(NOTE: Normal force and Weight is not an action-reaction pair)
Pressure – Height of ethanol in the column
Answer: Option D
Watch “Michelin and General Motors Unveil Airless, Puncture-proof Tires” on YouTube
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
Another P1V1 = P2V2 question
Solution: Option C
When to use the concept PV = constant and P1V1 = P2V2 to solve?
When temperature is constant (for o-level), when a fixed mass of gas (fixed number of air molecules) is compressed in a closed system (e.g. piston), the volume V decreases and pressure P increases, and vice versa.
But when you multiply pressure and volume, PV, it is always a constant.
PV = constant
Hence we can always equate the PV of the first scenario = to the PV of the second scenario, provided there is no addition or removal of air molecules from the system.
Hence, you have P1V1 = P2V2
The followings are 4 different questions which require this concept to solve. Do revise them.
Solutions: Option D (refer to the worked solutions below)
Solutions: Option D
Solutions: A
4)
Solutions:
5)
Solution: Option C
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.
Magnus Effect
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
Examples of a hydraulic system
Pressure – Manometer
Solutions: Option A
3E1 2011 Project Gp6: Pressure
A very detailed video! Great job!
Done by:
Milton, Xin Xuan, Kai Jun, Danry, Joan
3E1 2011 Project Gp5: Various Physics Experiments
Another great video by:
Terri, Coryn, Yahui, Desiree, Sherilyn and Ziao Cinch
You gals are funny haha…enjoyed the video.
Boiling water at 60 °C in lab!
Water boils at 100°C. In the view below, the water is initially at around 60°C.
How is this possible?
Explanation:
At sea level, where the pressure is atmospheric pressure (approx 105 000 Pa), the boiling point of water is 100°C.
In the video as air is sucked from the sealed container, the pressure in the container decreases. This lowers the boiling point of the water to around 60°C, hence water will boil and bubbles are formed.
It is similar to boiling water at high altitude, like on top of Mount Everest (about 8 km high). Water will boil around 70°C to to lower atmospheric pressure at high altitude.
Physics of Construction Lifting Truck
1) Why are the 4 extended legs necessary?
This is to increase the area of base of the truck to increase the stability. From our theory, as long as the weight acting vertically downward from the centre of gravity is within the area of base, there will be a restoring moment to bring the truck back to its original position, hence increases its stability.
2) In what situation will the truck topple? (note the the centre of gravity (CG) of the truck is in general very low due to mass concentration at the base)
Basically, there are 2 situdation:
– if the load that is lifted is too heavy, the overall new CG of the truck and load might shift outside the area of base, hence causing the truck to topple. i.e. the clockwise moment created by the load is created than the anticlockwise moment created the weight of truck.
– If the angle of tilt is too much which increases the perpendicular distance from load to pivot. Likewise, the overall CG might be outside the area of base and there is a net clockwise moment.
3) What is the purpose of the metal plate underneath the legs?
The purpose is to increase the area of base. The whole weight of the truck is spread over the 4 legs. Since P = F/A, with a bigger base area, the pressure acting on the ground will be reduced. This is to minimise any damages done to the ground.
N2004 P1 Q15 – Pressure P1V1 = P2V2
Water if depth 10 m exerts a pressure equal to atmospheric pressure. An air bubble rises to the surface of a lake which is 20 m deep. When the bubble reaches the surface, its volume is 6 cm3.
What was the volume of the air bubble at the bottom of the lake?
A) 2 cm3 B) 3 cm3 C) 12 cm3 D) 18 cm3
Solutions: Option A
Since the air bubble is enclosed, PV at A is equal to PV at B.
PAVA = PBVB
(Patm + Pwater) VA = (Patm) VB
(10 + 20) VA = 10 x 6
VA = 60 / 30
= 2 cm3
Using barometer to estimate height of mountain
At sea-level, the barometer which Pie is holding reads 760 mm of Hg. When he is at the top of Mount Pie, the same barometer reads 230 mm of Hg.
Given take the density of mercury to be 13.6 g cm-3 and the density of air, 1.23 x 10-3 g cm-3.
Estimate the height of the mountain?
Solutions: 5860 m
Pressure difference in mercury, Pmercury = pgh
= 13600 x 10 x (0.76 – 0.23)
= 72080 Pa
Pressure difference, Pair = pgh
72080 = 1.23 x 10 x h
h = 5860 m
In general, barometer can be used to estimate altitude. As height increases, height of mercury column will decreases. Such measurement is just an estimation as the density of air varies with various altitudes. Density of air is lower higher up in the mountain as the air is thinner.
In addition, It is not practical to bring around a mercury barometer as mercury is a heavy liquid and is poisonous.
An aneroid barometer (no mercury inside though) is used to measure altitude more accurately.
Pressure – Manometer and how mercury levels in both limbs move as pressure changes
The diagram shows a manometer connected to a gas cylinder of large volume. The atmospheric pressure is 76 cm Hg.
Due to the pressure of the gas cylinder, the mercury level in the left limb is at 30 cm while the mercury level in the right limb is at 0 cm.
Actually there is a short-cut. If atmospheric pressure remains constant, when the pressure of the gas cylinder decreases by 20 cm Hg, the difference in mercury levels between the left and right limbs decreases from 30 cm Hg to 10 cm Hg.
If you understand this, straight away you will be able to tell that the mercury height difference between left limb and right limb is 10 cm Hg.
You can then consider the markings on the manometer. When the drop in pressure is 20 cm Hg, note that mercury level on one limb will increase by 10 cm and the other limb will decrease by 10 cm. Shared equally!
Liquid Pressure Video
Liquid pressure, P = ρgh.
ρ is the density of the liquid (kg/m3) ,
g is the gravitational field strength (10 N/kg), and
h is the vertical height from the surface of the liquid (m)
In this video, since the last outlet is the lowest (greatest height from the liquid surface), the pressure is the greatest. Hence water ejects out with the greatest force and longest distance.
<p><a href=”https://vimeo.com/24961365″>Liquid Pressure</a> from <a href=”https://vimeo.com/user7367248″>evantoh</a> on <a href=”https://vimeo.com”>Vimeo</a>.</p>
Water finds its own level
Hydraulic System – Basic Concepts
Hydraulic System is a useful system which allows you to lift heavy load by just applying a small force.
Liquid is used (over gas) in hydraulic system as liquid is incompressible.
Enclosed liquid is able to transmit pressure to all parts of the system. In other words, pressure throughout the system is constant.
View the video tutorial for the concept of hydraulic system and Conservation of Energy.
Hydraulic System
A common question on P=F/A but students always get confused.
Take note of all units. To find the pressure in Pa, the units for F must be N and the area must be m2.
For (a) and (b)
Note that pressure acting on the floor by all the 3 legs is the SAME as the pressure acting on the floor by each of the leg.
In this case, the total weight of Pie and the stool is spread over 4 legs.
F = weight = 65 x 10 = 650 N
A = 25 cm2 = 25 ÷ 1002 = 0.0025 m2
P = F/A = 650 / (0.0025 x 4) m2 = 65 000 Pa = 65 kPa
For (c)
When there are only 3 legs, the weight (still the same) is now spread over a smaller area. Hence pressure should increase.
P = F/A = 650 / (0.0025 x 3) m2 = 86666.67 Pa = 86 700 Pa = 86.7 kPa
Women = Dangerous
Basic of P = F/A – 1 foot vs 2 feet on ground
Basic formulae for Pressure
The 2 important formulae for Pressure.
It is of great importance that you take note of the units.
For P = F/A
– If finding pressure (P) in Pascal (Pa), make sure that the area is in m3, if you use cm3, it will be wrong.
All units must be consistent.
To revise on unit conversion for density – kg/m3 to g/cm3, or vice versa, click on the post.
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
Evan's Space 