# Tag Info

47

Atmospheric pressure is equivalent to supporting a weight of 10 tonnes (about 10 average cars) per metre squared. Put like that, it's not surprising that those metal tanks crumple. However, in the comments you raise the point that you pump your bike tyres to 40 psi (about 3 atm) and yet they don't explode. I think this gets to the crux of your confusion. ...

19

First of all, as mentioned, atmospheric pressure can exert very high loads when integrated over significant areas. As an example, an overpressure of just 2psi is sufficient to destroy many houses and can kill people. That's about 13% of atmospheric pressure. Secondly there is an important scale question. You give an example of a bike tyre: a road bike ...

6

Here's some tennis racket physics from Rod Cross, including links to several Am. J. Phys articles (the physics educators' journal, thus excellent for learning from) and this excellent diagram: There are at least three "sweet spots": The node, at the center of the strings, is a point where the natural standing waves in a vibrating racket don't have any ...

6

Gases in containers at high pressures have those pressures because there are more molecules in them than in the same container at atmospheric pressure, not because there is a difference between the molecular energies. At the same temperature, two containers with different numbers of molecules in them have the same probability distribution of energies. The ...

5

Will a tennis ball go further if i hit it with the side of the racket? No. You want the racket to deform, not the ball. This means using the strings to elastically store energy and return it to the ball. The Ball The ball's deformation upon impact is undesirable because "a tennis ball is required by the rules of tennis to dissipate a fraction of ...

4

The micron used in this way is a unit of pressure. It's short hand for "micron of mercury". It's the pressure that causes the column of mercury in a mercury manometer (pressure gauge) to rise one micro meter. One Torr is one millimeter of mercury, and atmospheric pressure is 760 Torr. 1 $\mu$ = 0.133 Pa.

4

A tank is shaped for pressure from the inside, not the outside. The hull of the tank is convex. Pressure on the inside will cause the hull to assume a shape maximizing the volume per surface which leads to spherical or cylindrical shapes. This does not need much rigidity: balloons come in similar shapes. Pressure on the outside instead will maximize ...

4

Drawing a vacuum in the tank puts the tank walls under a compressive load. The ability of a structure to take compressive load depends on its stability. For a tank car, if we ignore the end caps, compressive loads are acting in two directions - lengthwise and radial. The cylindrical tank will be very stable in lengthwise compression - any buckling forces are ...

3

If you look at the tank from its circular side you could see how it has to perform like an arch to support the load of atmospheric pressure. Let's imagine we cut a section 1 meter long of this cylinder and cut the bottom part off to have a nice round arch and inspect how it works. It is roughly 3 meters diameter so it has to support a load of 3 x 1 meters x ...

3

It's simply inherent to the definition of polytropic processes that they don't allow the system to increase both its pressure and volume at the same time. That doesn't mean you can't increase a system's pressure and volume. You just need a non-polytropic process to do so. For example, it could be a compound process consisting of two polytropic processes with ...

3

First, the reason why the finger becomes more wind-sensitive with some saliva isn't that the saliva evaporates but because the saliva, or water, is a good thermal conductor. The finger has to be warmer than the air so the heat flows from the finger to the air and a good thermal conductor such as saliva helps this flux to take place. Second, because it's the ...

3

This is really three separate questions. Static and dynamic pressure: It is the static pressure that really matters in practical situations. The dynamic pressure is related to the kinetic energy of the fluid which, when it changes, causes a corresponding change in the static pressure. Condenser/evaporator application: The basic Bernoulli equation ...

2

Interactions between the molecules of the gas are not required. In fact ideal gases are modeled as if the molecules have zero interaction. They do however move and interact with the container. That is sufficient to explain the behavior. Imagine that you have a vessel with two identical halves that are connected by a small portal that can be opened and ...

2

Assuming an incompressible liquid, Bernoulli for instationary flow (neglecting friction) is $$\int_1^2 \frac{\partial c}{\partial t} \, \mathrm{d}s + \tfrac12 (c_2^2-c_1^2) + g(z_2-z_1) + \frac1{\rho}(p_2-p_1)=0$$ with velocity $c$, gravitation accceleration $g$, height $z$, density $\rho$ and pressure $p$ and $1$ and $2$ denoting the two positions ...

2

Suppose you do a force balance on the portion of the fluid situated between elevations z and $z +\Delta z$ in the left column. You get: $$p(z+\Delta z)S-p(z)S+\rho g S\Delta z=\rho S\Delta z \frac{dv}{dt}\tag{1}$$where $v$ is the downward velocity in the left column:$$v=-\frac{dx}{dt}\tag{2}$$ The latter equation is correct because the fluid is ...

2

A deodorant can contains a liquid hydrocarbon, typically a propane/butane mixture, and the pressure inside the can is due to the vapour pressure of this hydrocarbon. The pressure can be set to any desired value by varying the composition of the propellant - more propane makes a higher pressure while more butane makes a lower pressure. For a deodorant the ...

2

Gold will compress to about half of its volume at atmospheric pressure if you compress it to 2 million atmospheres at room temperature, which is something that I'm sure has been done with diamond anvil cells. For many metals, the atomic lattice will also undergo structural phase transitions from one lattice type to another at certain pressures, but I don't ...

2

There are two questions: "Why does vaccum crush the steel tank?" and Why the tank implode?" lemon's answered the first question perfectly - multiply the 1 atm pressure by the surface area of the tank and you will get the force, that crushed it. The second answer is not that simple. The tank walls are designed to transform the pressure forces (perpendicular ...

2

There are two ways you can change the internal energy of a gas, one is macroscopic, that is, performing work on or by the gas, if the gas either expands or contracts. The other is microscopically through heat. If the compressed gas is at the same temperature than the outside gas, these microscopic collisions will not result in an exchange of energy, because ...

2

In 3000atm it's speed of decompression will be slower because it is facing greater air density and the expanding spring has to move it. There will also be less "resonance" as the denser air damps the spring movement.

1

In a polytropic process other than adiabatic, you are controlling the temperature in tandem with P and V in such a way that n is constant. You can certainly achieve negative values of n by controlling the temperature appropriately. From the ideal gas law, if T and P are expressed parametrically in terms of V, then:$$\frac{P}{P_0}=\left(\frac{V_0}{V}\right)^... 1 The critical pressure is given by$$P_c=\frac{a}{27b^2},$$while the critical temperature is$$T_c=\frac{8a}{27bR}=\frac{8bP_c}{R}.$$The parameter b is related to to the effective volume occupied by the molecules,$$b=4N_0V_0,$$where V_0 is the volume of the molecule and N_0 is the Avogadro number. So at least theoretically you can chose P_c=1\, \... 1 Whether an answer exists depends on your definition of "near" compared to STP. There are a few fluids that have their critical point at a temperature close to STP, but higher pressure. For example, (see http://www.engineeringtoolbox.com/critical-point-d_997.html) material Tc(K) Pc(atm) acetylene 309.5 61.6 ethylene 283.1 50.5 ethane ... 1 I tried to look around but I couldn't find anything. It does seem like supercritical CO2 is very popular in applications because the critical temperature is just a little over 30 Celsius, but it still requires 73 atmospheres of pressure. An interesting thing mentioned on the Wiki page is that Venus may have had supercritical CO2 oceans many years ago. 1 A thermodynamic process is called reversible if an infinitesimal change of the external condition reverses the process. Consider a gas enclosed by a freely moving piston in a cylinder. Let us say it is in mechanical equilibrium with the atmosphere, that is, the pressures on the piston match. If you increase the external pressure infinitesimally the piston ... 1 Since the pressure p is intensive and the volume V and enthalpy H are extensive variables the function p=p(V,H) is homogeneous degree 0 so you always have$$H \frac{\partial p}{\partial H} + V \frac{\partial p}{\partial V} = 0

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re: "Why don't high pressure gases stored in containers lose energy?" They can gain & lose energy: Energy (heat) is lost from a gas as the gas is compressed (whether thru mechanical compression or thru cooling compression (e.g. passing a gas thru a tube that is immersed in a very cold liquid -- like liquid nitrogen). Energy (heat) is gained by a gas ...

1

No, a feather dropped in a vacuum jar will not drop at the same speed as in air. There must be a thousand video's out there of this very topic but my favourite is this one. An object moving through air, or any fluid, will experience a drag force resisting its motion. This force increases as the speed of the object through the fluid increases. It also ...

1

Depends on the wall thickness, for example you can collapse a plastic bottle sucking with your mouth but you can't with a glass bottle. There is an Asme code to calculate the minimum wall thickness of a steel tank. The code for external pressure is diferent for internal pressure because geometry of the vessel is very important. Flat and convexe geometry ...

1

According to the second law of thermodynamics,entropy of an isolated system tends to increase. Considering the high pressure region and low pressure region as an isolated system, its total entropy goes up, making fluid flow from high pressure to low pressure to increase the disorder(entropy of the system).This behavior follows from statistical models of ...

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