# Tag Info

1

Air pressure exists because if we place something in a gas, then the molecules/atoms flying around will keep banging into it, and in this way produce a net constant force per unit area. As explained by @Chris2807 in the neat formula $P=n k_{B} T$, this is proportional to how many particles there are (since this is proportional to the amount of "banging" in ...

0

Brionius has the right answer, but there is more to be said. Water at room temperature in air will slowly evaporate. Water at room temperature in a vacuum will boil, as is shown here. So these mini torpedos can prevent damage to chemical bonds. Water molecules are polar. The O's are a little negatively charged. The H's are a little positive. The H's and ...

2

In some sense yes. Let me explain a little. If we were to take a sealed container of gas and put it into free space far away from other bodies so that the gravitational force on the box is negligible would you agree that there would still be some pressure in the container? If we assume we have an ideal gas then the pressure is simply given by $$P=nk_{B}T$$ ...

3

In general, air pressure in the Earth's atmosphere is hydrostatic pressure, caused by the Earth's gravitational field. If there was no gravity then there wouldn't be any centripetal force and all the air molecules would just float away into space. This is why there is no atmosphere on the moon - because it doesn't have enough gravity to sustain one.

-1

In fact, they do!! Watch what happens to an ice cube that is left in the air... trillions of particles of its exterior are torn out of their stable arrangement, and soon they cascade down the sides—a microscopic waterfall! So in this case you are right, but it is just the very exterior surface of an object that is exposed to the air and thus affected ...

-1

Another way of looking at this is that things that would be destroyed by the environment (be it heat, light, etc) have already been destroyed (like ice on a hot summer day). The things that you see around you are the ones where the bond energy was high enough that they survived.

2

Things actually do get destroyed by what those air molecules pick up and throw around. Take look at this example [image from here: http://en.wikipedia.org/wiki/File:Arbol_de_Piedra.jpg ] Just like their bigger sized brothers, it's the load of those mini-torpedos that brings the destruction.

60

When you say "why aren't things being destroyed", you presumably mean "why aren't the chemical bonds that hold objects together being broken". Now, we can determine the energy it takes to break a bond - that's called the "bond energy". Let's take, for example, a carbon-carbon bond, since it's a common one in our bodies. The bond energy of a carbon-carbon ...

1

Sounds like constant amount of substance (as in the dimension or physical quantity, "mol" being it's unit). Could also mean mass, but mass is often not strictly constant. If you add energy (e.g. heat), the mass of the gas increases slightly via E=m*c2. Could you provide more context?

0

In thermodynamics physicists and engineers use the concept of 'control volume' that specifies a bounded region of space in which thermodynamic properties of a gas are analyzed. The boundary of the control volume can be used to either isolate or otherwise define a specific flux of energy and/or matter to/from the control volume. I believe the "fixed ...

-1

I have a 600 gallon tank on a 25 foot tower. The bottom of the tank has a 2 inch outlet and pipe that drops straight to the ground. There is also a 3/4 inch pipe on the other side of the tank that drops straight down the ground. At the bottom of the two inch pipe I reduced it to 3/4 inch pipe. The pressure is amazing out of that pipe. The 3/4 inch pipe that ...

0

The pressure of falling water can vary by a LOT - it depends in detail on the shape of the interface between the water and the surface it hits. When a perfectly spherical drop of water hits a hard surface, there will actually be a short moment in time when the contact point between the water and the surface travels faster than the speed of sound in water - ...

1

Each kilogram of water that falls from 10 m will have the kinetic energy of about 100J. I can't tell you the pressure since you didn't specify the area. Shorter falls will have less energy.

1

Yes, your idea would work, and it does save a bit of energy though not much. Suppose we are making water by reverse osmosis at sea level. We have to pressurise the water to around 6.2MPa (900 psi) so the work needed to produce 1 cubic metre of fresh water is 6.2MJ. Now let's do it your way. Let's assume already have a shaft sunk into the sea so we'll won't ...

1

Test data shows that pressure on ground is calculated by dividing the weight of car divided by 4 then divided by patch area which will always be much less that the interior tire pressure. Test data shows that when the load is doubled the patch area only increases by 25%. e.g. under a 1000 pound load on tire the patch area is 100 sq.in while for a 2000 ...

2

You assume that the screen is perfectly absorbing when you use this field. By uses this field you are assuming that the electromagnetic field pass through the material without reflection. Furthermore, you are assuming that you don't have field in the another side of the surface. $$\textbf{E}=E_0e^{i(kx-\omega t+\delta)} \hat{\textbf{y}}, \ \ x<0$$ $$... 1 The very famous Newton-Laplace equation is a relation between the speed of sound and the pressure of an ideal gas. It can be written as:$$ v = \sqrt{\gamma P / \rho} $$where v is the velocity of sound in the given medium, P is the pressure, γ is the ratio of the heat capacities for the medium and ρ is the density of the medium. The Newton-Laplace was ... 1 There is a model described in Main's Vibrations and Waves in Physics dealing with the speed of sound variations you might consider useful. Sorry, I would just comment that, but I don't have enough reputation. The other way might be to derive the speed of sound not from the ideal gas laws but from van der Waals equation, but to be honest, I've never tried ... 0 If the vessel thickness t is made of a material of elastic modulus E, Poisson's ratio  \nu  the bi-axial strain as well as radial strain is$$ \epsilon = \Delta R / R = \dfrac{p\cdot R ( 1-\nu)} { 2\; t E}$$and radial dilatation:$$ \Delta R = \dfrac{p\cdot R^2 ( 1-\nu)} { 2\; t E}$$EDIT1 This is how the equation is derived: Considering a ... 3 The boiling point of any substance may be defined as 'the temperature at which the pressure of its own vapors become equal to the external pressure'. So for example in the case of water, at 100oC, the vapor pressure become equal to approx. 1 atm. Now if you increase the pressure further, you would need to heat the sample more such that the new pressure is ... 2 Others have explained the physics. I just want to add that those of us with low body fat can become negatively buoyant at the surface if we breath out and empty our lungs. I can sink down and sit on the bottom of a swimming pool. So it is very possible to dive to a depth where you have negative buoyancy but what depth that will be will depend on the density ... 0 Pressure-sensitive paints? Is that what you need? http://en.wikipedia.org/wiki/Pressure-sensitive_paint 2 Assuming the oxygen and lead vapour can be treated as ideal gases then yes the partial pressures of the two will be the same. This happens because the pressure is proportional to the momentum of the atoms multipled by their velocity (shout if you want me to explain why this is):$$\begin{align} P &\propto pv \\ &\propto mv \times v \\ ...

0

Seems to me the electrical analogy is a good one. The heart system is trying to maintain a certain current I (to ensure adequate O2 flow). Raising any resistance, serial or parallel, increases R, thus raising V. There is vascular smooth-muscle compliance, but that just complicates the analogy.

1

I guess the force due to pressure on the hemispherical portion at the base is equal to the weight of the fluid. You can just calculate the volume of the fluid and multiply by $\rho g$.

1

Suppose you have a box filled with water in a uniform fashion. Now if you try to stretch the box in the $z$-direction, say, while keeping the other dimensions constant, what is the energy required? Well if the water distribution remains uniform, you can approximate this by the Hookean law $E_\text{elastic} = \frac{1}{2}k(z-z_0)^2$. Note that the constant $k$ ...

1

'Negative ' pressure is strictly a relative state; relative to what one may wish to define as zero pressure, and here on earth we chose to define that as one standard atmosphere of pressure which is about 760 mm Hg absolute pressure. If you are capable of removing all gas particles from a space, then you will achieve -760 mm Hg gauge pressure, but you cannot ...

1

The joule Thompson coefficient is negative as the gas is coming out through the small hole, for negative coefficient the tempr. decreases and for positive coefficient the tempr. increases.

4

A reasonable sub woofer at sound power level of 130 dB would produce pressure fluctuations of 60 Pa. Compare this to the ambient pressure of 100'000 Pa and you will see that related temperature fluctuations would be negligible. It extinguishes fire because it pushes the air back and forth. For the small fire in this video you could take a small air blower ...

2

A gas (like air) will tend to diffuse through any material, and the rate of diffusion will be roughly proportional to the pressure difference between the inside and the outside. It so happens that it tends to diffuse through the rubber of ordinary bike inner tubes fairly rapidly. In addition, the inner tube wall is quite thin, and the thinner the material ...

3

Yes, there is a force, because the air pressure inside the tire is larger than the pressure outside, so air will try to leave from any hole it finds, however small. It might be through some micro cracks on the tire or, more often, through leaks from the air valve.

-3

whenever external force is applied on the object automatically a restoring force is developed inside the object to restrict the deformation of the object.The ratio of restoring force perpendicular to the surface to the area is known as stress.The ratio of external force perpendicular to the surface to the area is known as pressure. for example if you press ...

-1

layman, It takes more PSI to suspend a heavier load with equal square inches. Or the same PSI with more square inches. Rubber tires will mostly just flex to give more square inches. Foot print of tires x psi = weight of car

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