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Understand pressure through cooker

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Overview         (For age - group : Above 16 )

This animated science topic "Pressure" is related to force and area that seem a macroscopic quantity, but pressure in microscopic physical aspect is collision between molecules. pressure exists everywhere, at times we dont even feel it, like the atmospheric pressure on our bodies. To understand pressure we have to look minutely at things --- at molecular level

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Category : Science
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Total animation length
: 45 minutes
Animation type : Advanced

animation contains details about

  • Pressure in every day life- experiencing pressure
  • What is pressure?
  • How atmosphere generates pressure?
  • Experiment to show air generates pressure
  • Why atmospheric pressure decreases with increase in altitude?
  • Understanding concept of pressure - two examples

    more about pressure in explanatory notes

    Atmospheric pressure and climate
    Measurement of atmospheric pressure

as pressure increases the depth due to weight will increase
Pressure as force /area

Application of pressure as shown on a can filled with air

Atmosphere exerts a pressure on us
Application of pressure
Atmosphere exerts a pressure on us

application of pressure - skating is possible due to pressure only

Molecules exert pressure  by collision

Application of pressure - skating is possible due to pressure only
Molecules exert pressure

Atmospheric pressure is due to air in atmosphere exerting pressure on us

weight and pressure

Atmospheric pressure is due to air in atmosphere exerting pressure on us
Weight and pressure
Details of the animation/ movie /software

pressure example a sharp knife cuts easier

Why is it difficult to cut a fruit using a blunt knife? Why drawing pins have flattened heads? On blowing why does a rubber balloon swells? The answers to all these questions are related to the effect of a force applied on a surface. This effect is termed as pressure.

A sharp nail can penetrate a wood but a flat surface cannot

Pressure is the force acting on an unit area of the surface in a direction perpendicular to it.
Pressure =

Perpendicular contact force (Newton N)
surface area of contact (square meter or m2)

The S.I unit of pressure is Pascal( N/m2 )

When the force of one Newton acts on an area of one square meter, the pressure on that surface is said to be one Pascal.

Pressure due to molecules


1.Pressure is directly proportional to force
on same contact area.
2. Pressure is inversely proportional to surface area

i.e. When same force is applied on different areas, greater is the pressure for smaller contact area while smaller is the pressure for larger contact area.
Similarly when different forces are applied on the same surface area, greater the magnitude of force greater is the pressure.

Though force is a vector quantity pressure is a scalar quantity since it is independent of direction.

atmospheric pressure is the molecular pressure

All things on earth are constantly hit by large number of air molecules present on the surface of earth (the envelope of air extends up to 320 miles).
These molecules though invisible have weight and cumulatively exert pressure on all things on earth. The pressure exerted by these air molecules is called as Atmospheric pressure. The atmospheric pressure at a place is the weight of a column of air over a unit area of the earth's surface at that place.

Atmospheric pressure decreases with increase in altitude. This is because the number of air molecules i.e. air density decreases at higher altitudes.

At higher altitudes due to weak gravitational force, the cumulative force exerted by the air molecules on an object decreases and hence atmospheric pressure decreases.

A balloon rising up bursts  as it expands

When a rubber balloon is blown into, it swells in all directions and takes a rounded shape. This happens because as one blows into a balloon, air molecules are injected into it. These molecules while moving randomly around the space collide with the skin of the balloon and thus exert pressure in all directions. This pressure acts perpendicular to the surface and helps the balloon to swell and take a round shape.

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Q & A
Q1: why there is pressure difference beween accume and air rohan raj 3/5/2014
Q2: what is the relation between pressure and teprature? anish 16/2/2010
Q3: what is force? abigail 5/11/2009
Q4: what is the relation between pressure, temp, viscosity, discharge and velocity for fluids? im really confused please help! ratheesh 11/9/2009
Q5: why moon moves around earth but it does not moves around the sun amar 19/2/2009
Q6: why velocity increases / decrease with drop/rise in pressure? you didn't answer first one in 48 hours. that's why i am again asking yogesh mehndiratta 22/12/2008
Q7: when area decreases pressure decreases & velocity increases. why velocity increases & vice versa decreases. yogesh mehndiratta 20/12/2008
Q8: why dose there need to be changes in some cooking recipes for cooking and baking at high elevations kristinn 25/9/2008
Q9: How to convert the pressure drop say 100 mm of Hg to flow rate? skarp 25/8/2008
Q10: Kindly delete the below question.I find out the answer.For constant DP ,whatever may be the pressure variation,Mass flow rate constant skarp 28/4/2008
Q11: In a pipe, one side is 3.2bar and other side is 2 bar, differential pressure is 3.2-2=1.2bar.The massflow rate is 400lpm.If the pressure on two side of the pi varies but Differential pressure is same (2.2-1=1.2bar) whether mass flow rate will be same skarp 22/4/2008
Q12: Why can't we feel pressure due to the atmoshere? Karen 13/3/2008
Ans: We don't feel the pressure of the atmosphere because the pressure within our bodies counterbalances the pressure of the gas in the atmosphere. This is because the air spaces inside our bodies are at much the same pressure as the air outside. The puncture of a space suit in the vacuum of outer space immediately leads to the rupture of the body because there is nothing outside to balance the body's inner pressure.
Q13: i know alot now about pressure but still one confusion is in my mind. whether the pressure velocity are directly propotional to each other or inversely propotional to each other? kindly answer. TalhaAwan 25/12/2007
Ans: They are inversely proportional, as per "Bernoulli effect", an increase in the flow velocity causes a lowering of the fluid pressure and vice versa.
Q14: relation ship between temperature and vaccume means negetive pressure mukesh 2/12/2007
Ans: Actually no, there is nothing like a negative pressure, a vacuum pressure will keep on reducing to values 10(-4) or 10(-6) torr but it will not go negative. The relationship of proportionality exists between temperature and pressure in vacuum, as one increases so does the other. So as such negative pressure as a concept does not exist.
Q15: Do you have trial version? ATHK 15/11/2007
Ans: No we do not have any trial version as yet, actually you can see a preview of the animtion, which in most cases gives a fair idea of what the animation contains
Q16: What is the relation between flash and fire point of bitumen and atmospheric pressure? ammar hassis 5/10/2007
Ans: Vapor production at a given temperature is opposed by atmospheric pressure. A flammable liquid can form an ignitable mixture only if sufficient concentration of vapor is present in air. If the concentration increases, then vapor pressure increases. Boiling or vapor production occurs at a lower temperature if atmospheric pressure is reduced. Therefore, flash/ fire point decreases as atmospheric pressure decreases. Flash point decreases linearly by 1deg F for every 800 ft increase in altitude.
Q17: A closed container consists of air molecules.The temperature of the container is increased.viscosity will also gets increased.what happens to pressure it will increase or decrease ? skarp 26/9/2007
Ans: Actually the viscosity decrease with temperature increase but for a gas it actually it does not matter at molecular level. Pressure is determined by collisions of molecules with the walls. The molecules of air will start moving faster as increase in temperature is proportional to KE, the collisions between the walls of the container and molecules and among molecules will be more vigorous due to higher KE, this will lead to higher pressure. Thus upon heating the pressure will increase.
Q18: When heat is applied to the hollow cylinder ,the outer portion expanded and what will happen to inner portion of cylinder ? skarp 18/9/2007
Ans: Take a solid cylinder; heat it, you will see it expands (not contracts) on heating. Now cut an inner cylinder from it to get a hollow cylinder, place the cut cylinder inside it again and heat them both, both will expand and at the same rate as if they were one solid cylinder. The outer cylinder did not expand inwards. Why? Because heating increases distance between atoms, contraction will bring atoms closer, that will violate the law. For a disc or cylinder on heating hole will become bigger.
Q19: what is the reynolds no for the pipeflow maruti 10/9/2007
Ans: The Reynolds (Re ) is a non-dimensional parameter used to estimate the relation between inertia and viscous forces. It is used to find if a fluid flow is laminar, transition (where the flow is neither fully laminar nor fully turbulent) or turbulent. It is a function of fluid speed is V; the orifice diameter is d, density (rho) or absolute dynamic viscosity (mu). If Re>2320, turbulent flow will occur, and if viscous forces are big enough in compare to inertia, Re<2320, laminar flow will occur.
Q20: what will happen if vacuum is applied to pressure cooker parag vandkar 10/9/2007
Ans: A pressure cooker operates due to vapor pressure i.e. pressure of the molecules that have evaporated from the liquid counteracting further evaporation, if a vacuum is created inside a cooker it will have no effect as molecules will evaporate at lower temperature to fill up the vacuum, once the vacuum is filled up then vapor pressure will come into play again.
Q21: Does the pressure changes cause any influence on the gravitation?is there any relation between them? stanley raj 2/8/2007
Ans: Air Pressure or more correctly atmospheric pressure is a resultant of gravity pull on molecules of air, causing them to cluster nearer to earth, there are more molecules at the bottom than at the top of air column causing a pressure on everything below but this does not affect gravity at all, gravity's magnitude is much more than the pressure effect of atmosphere.
Q22: what is the difference between pressure and stress both have same units.consider 10N force and 5mm2 area .pls tell me the difference skarp 3/7/2007
Ans: In SI units both are expressed in Pascal (Pa) or N/m2 or m-1·kg·s-2 and they share a cause effect relationship. Pressure causes stress. Pressure is scalar while stress is a vector. For a solid body, Pressure is the force per unit area applied on a surface in a direction perpendicular to that surface while stress is the tensile and compressive stresses in solid objects, it is the internal force per area that balances and reacts to external pressure, these terms are often used interchangeably.
Q23: air pressure inside a pressure cooker Maheen 17/3/2007
Ans: Standard is 15 psi, the range can vary from 5 -15 psi (5 psi is rarely used sometimes for delicate food like fish or shrimp , 9-10 psi is sometimes used for rice dishes, or puddings)
Q24: Can u tell me , What are the assumption of Bernoulli s Theorem for compressible and incompressible fluid ? skarp 31/1/2007
Ans: For both conservation of mass and energy is assumed. For incompressible fluids, we ignore viscosity, compressibility, and rotational and thermal effects here incompressible flow means that density is constant along a streamline. This gives an equation P/rho + ½ v^2 + gh=constant. For a compressible flow there is a change in density along a fluid flow and is accounted for by adding a specific internal energy constant to P/rho (Errata - 18/1/2007, by mistake written ideal fluid to be compressible)
Q25: pressure=force/area,pressure is inversely proportional to area.so pressure should increase when area decrease. consider a tapper pipe 10cm on one side and 5cm on the other.according to bernoulli s theorem the pressure decreaseswhen area decrease? how skarp 29/1/2007
Ans: Pressure is the weight of molecules of fluid acting upon a horizontal pipe cross-section area. Taking 10 cm long pipe, the pressure at 10 cm end is caused by the larger number of molecules. Inside at 5 cm cross-section, “lesser” number of molecules are present so lesser force & pressure. Now make the pipe vertical then at narrow opening, higher pressure will result as “full” weight acts on a lesser area. In case of horizontal pipe, the whole weight was not acting on a cross section.
Q26: THANKS GOT IT....REALLY GREAT!!!!!!!!!!! skarp 23/1/2007
Ans: You are welcome, it was enjoyable for us too. Thanks
Q27: Thats great but at the end P(a)+1/2 rho v(a)^2=P(b)+1/2 rho v(b)^2 by this eqn .how do u say that pressure is inversely proportional.i am not able to understand .pls clear my doubt by that eqn skarp 23/1/2007
Ans: I think you are imagining a=k/b type relationship but remember a = -kb is also inverse proportionality, if b increases, a is more negative hence reduces. In Bernoulli’s eq. as energy is constant hence either side of the equation is constant or generalizing it P + K v^2 = constant, where K = 1/2 rho ( rho’s value remains same for the incompressible fluid), the eq. can be P + K v^2 = M. This changes into P = M - K v^2. If v increases, P will have a greater negative value or reduce. Get it?
Q28: how pressure drop with increase in velocity according to bernoulli.Any equation that pressure is inversely proportional to velocity.pressure= force/area so pressure is inversely proportional to area.In venturi area decreases so pressure should increa skarp 19/1/2007
Ans: A fluid has additional energy due to pressure of molecules besides KE and PE. In a tapering fluid tube if we take two cross sections near ends, the static pressure P in energy terms is P=F/A=(Fd)/(Ad)=Work/Vol=E/Vol, total energy at two cross sections is P(a)V+1/2m v(a)^2+mgh(a)=P(b)V+1/2 m v(b)^2+mgh(b), divide by volume to get P(a)+ ½ rho v(a)^2+rho gh(a)=P(b)+1/2 rho v(b)^2+ rho gh(b), neglect gravity we get P(a)+1/2 rho v(a)^2=P(b)+1/2 rho v(b)^2. If v(b) increases P(b) has to reduce.
Q29: what is the relation between pressure and velocity. skarp 18/1/2007
Ans: Relationship between pressure and velocity of a fluid is governed by Bernoulli’s principle, it implies that the pressure falls where the fluid flows faster and visa-versa. An ideal fluid (a non-turbulent, compressible and barotropic fluid undergoing steady motion example low speed air) with no external work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or gravitational energy. Application is in aircraft lift, Venturi tubes and carburetors.
Q30: Does the pressure cooker produce the same boiling point temperature at sea level as it does at higher elevations? Brian Washburn 17/4/2006
Ans: Yes, the same, cooker is an enclosed system and is unaffected by outside pressure whether below or above sea level. The need to cook longer at higher altitudes using pressure cooker occurs, because it is colder than at sea level hence more heat is required, also the relief valve is under lesser pressure from outside and releases earlier than at sea level causing loss of heat and pressure.

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