26 votes
Accepted

Reverse blood flow in an IV

I'm not a nurse, but I think that the pressure into the vein is enough, if you see the blood flowing from the vein to the outside. This should mean that, locally, close to the needle, a small amount ...
basics's user avatar
  • 8,176
20 votes

Is the saying "fluid always flows from high pressure to low pressure" wrong?

No. Fluids in general do not flow strictly from points where pressure is high to points where pressure is low. Even in the simplest static case (which is a reasonable first approximation for blood), ...
Martin 'Kvík' Baláž's user avatar
19 votes
Accepted

How does a bottle pour its contents?

You need an air intake to level the pressure, which would otherwise drop and stop the outflow if you don't replace the lost volume of fluid in the container by air. (And btw. the tea pot sitting on my ...
kricheli's user avatar
  • 3,206
18 votes
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Will just the increase in height of water column increase pressure or does mass play any role in it?

At first, the conclusion seems absurd. You have a tank, which is open at the top of the pipe, that is comfortably holding 1000 L of water. Then you add just 1 L more, albeit in an unusual shape, and ...
Scott McPeak's user avatar
18 votes
Accepted

If a container of water from some depth is brought to the surface, what would be the pressure in the container?

That depends on the compressibility of the medium and the rigidity of the container. Let's first look at a highly compressible medium like air before I answer you question regarding water. Fill your ...
Ralf Kleberhoff's user avatar
16 votes

How does pressure exist inside a drop of water?

The pressure outside a droplet arises from molecules bouncing against the droplet's surface, delivering an inward momentum "kick." Since the droplet doesn't shrink to nothing, there must be ...
Chemomechanics's user avatar
13 votes

How was the Titan submersible implosion time calculated?

We could estimate a lower bound for the hull collapse duration through a scaling argument: Consider the acceleration of a suddenly detached small portion of the hull (with area $A$ exposed to the sea ...
Chemomechanics's user avatar
12 votes

Is the saying "fluid always flows from high pressure to low pressure" wrong?

As you pointed out, the force is directed from the regions of high pressure towards the low pressure, and, according to the Newton's second law, this is where the acceleration is directed. This does ...
Roger V.'s user avatar
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10 votes

The actual proof of $\delta W = P dV$

$$\delta W=F\delta x=F\frac{\delta V}A=\frac FA\delta V=P\delta V$$
Steeven's user avatar
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9 votes

Is the saying "fluid always flows from high pressure to low pressure" wrong?

The question has been answered already but since the OP mentions the Bernoulli equation it is instructive to see how this equation actually explains flow against pressure. Bernoulli's equation governs ...
Themis's user avatar
  • 5,798
8 votes

Liquid pressure at the atomic level

First let me note that technically, even within your argument, the collision velocity could be higher or the collision frequency could be higher. Hence pressure in a gas is proportional to temperature ...
AXensen's user avatar
  • 7,087
8 votes
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Why is pressure at a point inside a static fluid same in all directions?

Suppose that we consider an element of fluid, density $\rho$, of sides $\delta x,\,\delta y$ and $\delta z$ as shown in the diagram below. The arrows adjacent to pressure labels, $P_0$ and $P_0+\...
Farcher's user avatar
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8 votes
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Why is there a pressure node at the open end?

When we are discussing the pressure in a pipe we are referring to the gauge pressure. This is the pressure in the pipe minus atmospheric pressure. So when we refer to a pressure node we mean the gauge ...
John Rennie's user avatar
8 votes
Accepted

Bernoulli's principle at molecular level

if pressure is calculated from the this relation : $PV=nRT$ and we don't observe a decrease in any of those factors that would cause a drop in pressure This statement is not true. $P$, $V$ do change, ...
Themis's user avatar
  • 5,798
8 votes
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Do liquids typically solidify under enough pressure?

The example you have chosen to start your question (water) is an atypical case. The phase diagram has a logarithmic scale for the pressure, somewhat hiding the negative slope of the transition line ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
8 votes

How does pressure exist inside a drop of water?

the reason is that the surface of the droplet acts as if it were a stretched rubber membrane which applies a squeezing force to the water inside. The stretched rubber effect is due to something called ...
niels nielsen's user avatar
8 votes

How does pressure exist inside a drop of water?

The atmosphere and the surface tension forces both squeeze the water droplet which generates a pressure more than the atmospheric pressure inside the drop
Aarush Saharan's user avatar
7 votes
Accepted

The actual proof of $\delta W = P dV$

For a general material volume $V$, you can write the power of surface force acting on its contour $\partial V$ can be written as $\dot{W} = \displaystyle \oint_{\partial V} \mathbf{t}_n \cdot \mathbf{...
basics's user avatar
  • 8,176
7 votes
Accepted

Is temperature held fixed in this derivative for pressure?

Strictly speaking, as discussed in Themis' answer, $-P$ is the partial derivative of the internal energy with respect to volume at fixed entropy. However, in the case of the electron gas in typical ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
7 votes
Accepted

Would different sounds of the same intensity but different frequencies have the same harmful effect on the human ear?

Introduction I don't think this is so simple as to be answered with a "yes" or "no". The hearing system is rather complex on its own and comprises of both mechanical and "...
ZaellixA's user avatar
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7 votes
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What is pressure energy in a closed system?

This increase in pressure seems to store energy, like a compressed spring. It seems to, but for an ideal gas, it doesn’t (store internal energy, that is); it stores negative entropy in the form of a ...
Chemomechanics's user avatar
7 votes
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Help me understand extreme pressure

TLDR: To understand it, don't think about pressure. Think about energy. The best approach here is to think in terms of energy/work, instead of pressure. There's lots of situations that become more ...
ksousa's user avatar
  • 274
6 votes

What is the underlying concept in case of finding pressure inside a balloon kept at some height in atmosphere?

If the volume of the balloon is not changing then the net force acting on any part of its shell is zero. If we neglect the forces applied upon the part of the shell by the nearby shell parts (the ...
Alex Sveshnikov's user avatar
5 votes

Is the saying "fluid always flows from high pressure to low pressure" wrong?

I don't know much about blood circulation and just give some basics of steady state fluid mechanics. Poiseuille was a physilogist who was interested in the circulation in blood capillaries. In the ...
Vincent Fraticelli's user avatar
5 votes

Is temperature held fixed in this derivative for pressure?

Begin with the fundamental equation $$dU = T dS - P dV + \mu dN$$ which implies $$ T=\left(\frac{\partial U}{\partial S}\right)_{V,N},\quad -P=\left(\frac{\partial U}{\partial V}\right)_{S,N},\...
Themis's user avatar
  • 5,798
5 votes

Why chemists usually only deal with enthalpy and not internal energy?

The internal energy is a thermodynamic potential, that gives the intrinsic energy content of a system at equilibrium with a given particle number, entropy and volume: $$ U(S, V, N) $$ Enthalpy on the ...
Sebastian Riese's user avatar
5 votes

Help me understand extreme pressure

Yes, you are right that we can calculate the force on the sub by adding up the total weight of everything above the submarine (the water), this force is "Force" = "Area of Sub" $\...
Dast's user avatar
  • 1,786
5 votes

Will just the increase in height of water column increase pressure or does mass play any role in it?

The 1 liter pipe only encompasses a tiny fraction of the lid area of the tank. But is has caused a pressure of 972 kPa at the lid, and the lid is pushing downward on the liquid inside the tank with ...
Chet Miller's user avatar
  • 33.3k
5 votes

Why is pressure at a point inside a static fluid same in all directions?

The issue you are having is a problem of scale. You are completely correct that if you were to take a small “box” of the fluid from the bulk, the pressure exerted on the upper and lower portions of ...
Matt Hanson's user avatar
  • 2,495
4 votes

The actual proof of $\delta W = P dV$

IMO the answer by @Steeven is the most direct one. So the purpose of this answer is to support his. The expression for work can be shown in various forms. The most basic is force times displacement. ...
Bob D's user avatar
  • 70.4k

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