# Tag Info

1

An addition to Daniel's answer: It regularly happens that a certain definition isn't applicable when talking about liquids and gases. For example, almost all definitions in thermodynamics are defined for closed systems, which is rarely applicable in the real world. To overcome this, there is the notion of a control volume. For this, we basically take a ...

5

The pressure of a gas is defined as the force the gas would exert upon a surface or container. However, there is no need for a container for pressure to exist. For instance, the air you're breathing right now (unless you're in an airplane or submarine) has pressure due to the column of atmosphere above you. Stars are balls of gas (plasma, actually) that are ...

1

The explanation that the measured pressure is the force on external walls of a reservoir under pressure doesn't quite make sense to me, in that in order for a High-Pressure fuel reservoir to be under greater than atmospheric (30,000 psi) pressure, the fluid within must also be pressurized. The measured pressure is not a force but it is related to force. ...

1

I noticed today that the water warmed up in pressure cooker (1.5 ATM) dissolves sugar much faster and much more than water cooked in 1 ATM. The effect you observe is one of temperature, not pressure. Water soluble substances dissolve quicker when the temperature is higher because the water molecules of hotter water have a higher average kinetic energy ...

0

.052 x 8.34 x height above pump. Will provide you with number of pounds of pressure that your pump must provide to push water to your tank. Of course I can give you the formula for diameter of pipe/ and distance; however I'm not sure that anyone wants to begin calculated process factors for the bends, and turns in a friction on water inside a pipe. I'll just ...

0

The term enclosed is a little misleading. Look at the diagram below: Both containers are of equal size and contain the same amounts of the same liquid. The left liquid is contained by a beaker and a weightless, frictionless piston, the right is contained by a beaker that is simply open to air. Both sit side by side on the surface of the Earth (and thus ...

2

Have you ever blown out a candle, from a few inches away? Have you ever wondered why you can't suck out a candle from the same distance? The motion of fluids like air, through an orifice, is not reversible. On the side where it is leaving the orifice it forms a localized stream, which you feel. On the side where it is entering the orifice it comes from ...

1

The other answer is more detailed and related more specifically to details of your question, but maybe this will create some intuition: It does not take much water to create a great amount of force. For instance, you can float a ship in one cup of water if you make a container that is the same shape as the ship (below the water line) but offset by such an ...

1

The two values are related, but have no fixed relationship between them. The total force applied by the pressure of the contained water on the long wall depends only on the long wall's dimensions (assuming the water is filled to the top of the box). It is completely independent of the width $w$ of the box; $w$ could be a millimeter, or a kilometer; it just ...

3

If air comes into the vehicle then it must effectively also be leaving it, otherwise pressure would be building up, in accordance with the Ideal Gas Law. Trains and buses are hardly hermetically sealed containers. The reason why you may not notice the air leaving is that it will flow through many orifices like imperfect seals, a slightly open window further ...

0

The vacuum of space is only -14.7 psi compared to normal air. You probably wouldn't want to vacation there, but it's not enough to explode you Hollywood style. Medical oxygen tanks are pressurized up to 2200 psi. That's 150 times greater pressure difference than inside your body and the vacuum of space. Obviously, human bodies aren't as strong as oxygen ...

-1

Water in its liquid form is almost incompressible because of the tendency of H- bond not to reduce in length after a certain limit. This can be said in accordance to Hydrogen atom as well. For example,no matter how much you compress the atom, its size will never reduce.

1

This does not work because your friend's theory is wrong, even though it is based on a sound principle. What he claims would be true if the $CO_2$ concentration in the can was indeed in equilibrium with the partial $CO_2$ pressure of the air in the bag but that's not the case. A typical, unopened pop bottle is at about $4$ to $5\text{ atm}$ of pressure. ...

2

As long as the weight is placed slowly, the bowl will remain balanced. The forces on the bowl are Gravity The pole The pressure from the water Note the lack of anything about the added weight. That's because it can only effect the bowl by raising the water level. All three of these will remain centered on the pole as the water level rises, as long as ...

1

Pressure is usually defined as the variation of the displacement from the equilibrium positions for the components of the material we are looking at. In the case at hand, sound pressure should be related to the displacements of the particles present in the air (or whatever else the medium is). If $f(x,t; x_0, t_0)$ describes the amplitude of such ...

15

You have the general idea right, but the following statement is subtly wrong The pressurized fuel/air mixture is ignited and this increases the pressure inside the combustion chamber even more Unlike in a piston engine, the ignition of the fuel air mixture in a turbine engine increases the mixture's volume while pressure stays relatively constant. ...

9

With respect, if you want to fully describe the operation of the modern jet turbine engine, you should consider the greatest change in the last 30 years or so, which has been the development of the large multi-bladed fan at the front of the engine. Most modern jet turbine engines are of the high bypass design, with a large fan placed in front of a central ...

7

Yes - although whether it provides "useable power to a shaft" depends on the kind of jet engine (it is possible but not necessary). For example, a turbojet does not provide "useable" power to a shaft - it just drives the compressor. A turbofan engine has a bypass path: the compressor does not send all the air to the combustion chamber, but some of it ...

2

Yes, your outline is correct. The power extracted by the turbines is used to power the compressor system to keep the system going. 'suck-squeeze-bang-blow' as engineers like to say. Rolls-royce the jet engine book is a good high level overview.

1

There would be no net movement of air in such a straw extending from the surface of the Earth to outer space. Perhaps you're thinking that the air at the bottom of the straw is being pushed into the straw with a pressure of 1 atmosphere (14.7 psi) while the top of the straw is at vacuum (0 psi) and so there is a net force pushing the air into outer space? ...

-1

I fully admit, I am puny and most tight lids defeat me. My solution is to pierce a small hole in the lid, equalise the pressure inside and outside and the lid is then easy to remove. Then, I place a piece of sticky or duct tape on the top if I need to preserve what's left in the bottle.

2

If you hit the bottom of a jar while it is upside down, it loosens the contents and piles them on the lid. If the inside pressure is less than the outside pressure, which should always be the case if the jar was vacuum packed, this may provide a little extra pressure to counteract the outside pressure that is holding the lid on. Tapping on the bottom may ...

0

Most bottles (plastic ones, at least) I've seen attach to their caps with a screw like mechanism. I think a screw like mechanism can get jammed easily if one of its components is out of place. Whacking or knocking on the cap could help dislodge the bottle/its cap from an out of place position to one where it is naturally supposed to be, which would make it ...

0

Whacking on the lid of a bottle, e.g. with the handle of a knife, works because it disrupts the seal that is holding the cap on.

0

As @Goobs says, the pressure force is $0$ at the top of the water line and increases to $\rho~g~y~dA$ on a surface of area $dA$ at depth $y$. Since this pressure increases linearly from $0$ to $\rho~g~y$ the average force on the wall is the average of the start and end: so, it is half of this value, and the total pressure is $\frac 12 \rho g h (h \ell).$

0

Even if your seals are perfect, you have to consider that all liquids start to boil as your pressure approaches a perfect vacuum. Some liquids do better at this than others, but they all eventually boil because they will constantly adjust to achieve a gas/liquid equilibrium for the pressure. Thus you will find that you can never actually get down to a ...

2

The surface wave formed in a Rayleigh-Taylor instability is caused mainly by surface tension. Like i mentioned before, a liquid tends to minimize its surface area and $n$ droplets of volume $V/n$ have more surface area than a liquid column of volume $V$. Initially, the film is uniform and surface tension will minimize the area by starting to form waves. The ...

2

Your misconception is that the water particles are moving very slowly or are stationary because they are not escaping. In fact, they are moving very quickly and are constantly bouncing off each other and the walls of the container. The pressure is basically how many collisions occur over a given time period. As you squeeze the piston, you are increasing the ...

5

One answer is that atmospheric pressure doesn't weigh down on us, it presses in from all sides, including up from under us (since there are air gaps under your shoes and feet). So standing under the atmosphere isn't at all like standing under a 10-ton weight. The long answer, which addresses tree new issue of "why doesn't atmospheric pressure crush us like ...

6

Let's simplify things down to the barest minimum: one dimension, one particle, and a wall. O | The particle moves to the right, hits the wall, and rebounds, perfectly elastically. If the wall is fixed in place, the particle will leave the collision with exactly the same kinetic energy as it came in with. But what if the wall is moving to ...

-2

The problem of your conclusion lies in a flawed premise, which I think is a very common one. None of the other answers have addressed this. Kinetic Theory of Matter does not explain by itself all macroscopic behavior. This means that although we can understand much of the properties of macroscopic matter from the point of view of Kinetic Theory of Matter ...

22

The question isn't silly. The speed of each molecule in the liquid is much higher than the speed of either the piston or the water shooting out from the nozzle. At room temperature, for water molecules the average is on the order of 500m/s. And yet, the speed of sound in water is three times higher than that, which implies that pressure can propagate in ...

22

Adjacent molecules in a liquid all repel each other because of the electron clouds that surround the nuclei that they contain. In that sense these molecules never even 'touch' each other (at least not in the intuitive sense of the word). When you apply pressure to the liquid you're squeezing them into a (very slightly) smaller volume, thereby increasing the ...

1

Try picking a tiny hole in a milk cardboard container. Squeeze the container. You only press is slightly, but milk is poured fast out of the tiny hole. This system is simply redistributing the total force that you provide onto a much smaller area. Same force on a smaller area equals larger pressure: $$p=\frac{F}{A}$$ The pressure on the particles at the ...

3

Pressure inside the container: zero Effective pressure on the container: proportional to$\frac{N^2}{r^4}$ (where $r$ indicates the size of the container) The situation: There are $N$ protons inside an otherwise empty container. Assumptions: The container does not interact with the protons except for completely stopping them from moving outside (essentially, ...

0

Very likely the gas in the can is not air at all but rather some non-flammable hydrocarbon gas that's environmentally safe by today's standards. Freon was once used, but that's no longer considered safe for the ozone layer. I believe this to be so because of the comment you made - that there is a difference when the can is inverted. For the typical aerosol ...

4

A large number of positive ions without any negative charges cancelling out the charge is extremely difficult to achieve in practice, which is why no one ever talks about it. You certainly can't use PV=nRT. An important consideration you left out is where are the negative charges? I understand that they are not in the container, but they are somewhere, ...

0

Somehow missed Frisbees comment, thanks for that! I think problem is now solved: I found bulk modulus of water. Then I used relation between force F=pA=kx, (-> x=pA/k), put them to potential energy equation E=1/2kx^2 and assumed that all stored potential energy goes for heating of liquid. Í was asking how much temperature raises when high pressure is added. ...

0

This is a matter of putting a few bits together. Pressure is a scalar quantity, so it has no direction. You can find the pressure at any given depth by dividing the weight ($mg$) of water above some surface area by that same surface area. Because the mass is proportional to the surface area, the area will cancel out (so it doesn't matter what area you ...

0

Here is a proof on wikipedia if any one else wants to follow along. The proof states that the pressure in the water is zero (I will take atmospheric pressure to be zero) after it has exited the hole. This is because there is no longer fluid on top of it after it goes out the hole. However, the proof does not say that the pressure is zero immediately inside ...

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