# Tag Info

107

Your derivation is composed of correct statements and indeed, if something is known to act as a lubricant, we want the viscosity to be as low as possible because the friction will be reduced in this way. For example, honey is a bad lubricant because it's too viscous. However, your derivation isn't the whole story. The second condition is that the two ...

27

A good lubricant tends to effectively minimize direct contact among components of any device that need it Keeping this in mind, viscosity is not the only factor involved. Grind a graphite pencil lead, and it makes a mighty fine lubricant. It might be that in the case of water placed between two surfaces, a water droplet which was supposed to act as an ...

19

The parallel plate situation that you describe is not the typical condition encountered in practical lubrication operations. In addition to facilitating the surfaces sliding over one another, the lubricated bearing must also support a normal load. To do this, the gap between the surfaces varies with location along the bearing. For example, in a journal ...

13

The most immediate answer would seem to be that a great variety of different crystal phases can exist because their long-range order makes it possible to classify them based on the different symmetries of their lattice structure. Since the liquid (or amorphous solid) phase only has short-range order and the gaseous phase doesn't even have that, it seems ...

12

Why Oil is Slippery Explaining why oil is slippery requires a look at its chemical properties. First, oil is non-polar, which means it does not have a positive or negative charge. Some molecules, like water, have a “charge distribution,” which means the molecule acts almost like a battery, part of it has a positive charge and part of it has a ...

6

Ice can be denser than water for certain values of $P,T$. Look at these two pictures taken from here: The darker areas in the second picture denotes areas of greater density. So you can clearly see that when pressure is increased, ice becomes denser than water along the coexistence line. For example at $T=400$ K ice VII is clearly denser than water ...

5

The water molecule is neutral on overall basis, i.e: the water molecule as a whole has no net charge. The water molecule is not linear rather it has a bent shape with two hydrogens on the same side. This happens because of the lone-pair-bond-pair repulsions. The oxygen has is a more electronegative element than hydrogen, i.e: oxygen has high electron-...

5

From the original assumptions: So far, my understanding of water evaporating is the following: The higher the temperature, the higher the vapor pressure, therefore the faster water vaporizes. The rate of water boiling, assuming a constant boiling temperature, is dependent on the rate of heat transfer to the water, not the vapor pressure. At sea level, ...

4

@tbf is right; lubrication, and tribology in general, is complicated. That's why there is that high effort to understand it and to design advanced materials. There are several phenomena that cause the friction force exist and the ones you have neglected causes that oils are superior to water in most industral applications. In dry sliding we can identify ...

4

Nothing in the laws of thermodynamics forbids multiple liquid phases for a single substance. The only limit is the simultaneous coexistence of at most three phases (at triple points). Water has a solid-liquid-gas triple point and several soid-solid-liquid and solid-solid-solid triple points; see the phase diagram of water and ice. In addition, although not ...

3

It is an optical illusion that uses stroboscopic light. It is the same principle that allows you to see still pictures into a movie. For instance, when you see the drops floating still, the drop is not the same, the light frequency is synchronized with that of the the falling drops, so that every time the light is on it shows you the image of a different ...

3

The answers boils down to yes, the larger the rate of of heat (assuming you can transfer it at any rate you want), the larger the vaporization rate. The rate of change in internal energy at constant volume is $U=U_0+\dot Q\Delta t$, where $\Delta U$ is the total internal energy necessary to change the phase, and $\dot Q$ is the rate of heat transfer. Thus $\... 2 My answer is based on our everyday experience. The simple answer is no. Suppose you have a hot pan, now put a drop of water over it and you will see that it will sustain for a while. This is because the steam makes a insulating layer between the pan and water drop. This phenomena is always present which (may not be the only reason) is one of the reasons ... 2 Water vapour can be said to be one of the the most important contributor to the greenhouse effect. To estimate its potentiality is a bit complex exercise as the absorption ranges of wavelengths in the infra-red region overlaps for different green house gases. In some of these overlap regions , the atmosphere already absorbs 100% of radiation, ... 2 This is a more general answer. Atoms and molecules are quantum mechanical entities. This means that the "shape" of atoms depends on the solution of quantum mechanical equations, which give probabilities for locating in space the electrons that are bound to the nucleus of the atom with the electric potential provided by the protons of the nucleus. The ... 2 Water can not bear normal loads as well as oil. Water is bound to escape from high pressure bearings to lower presser places in an open lubrication loop leaving bear contacts. Water can create bubbles around cavities and corners and break the laminar flow which will compromise the separation of moving parts. Water will react chemically with surfaces. There ... 2 I only find the expression "proper liquid" (in this context) in works from the 19th century. An obvious reason would be the highly non-linear expansion curve (note that the picture below doesn't show that curve itself, but rather its derivative). For mercury, the secant volumetric expansion coefficient varies very little, from$0.18165*10^{-3}$at 0°C to$0....

2

If water in some volume $V$ becomes gas (the attraction between water molecules is switched off instantly), then the pressure of this gas will be enormous. This is how explosive works.

1

Using Bernoulli's equation and the momentum conservation equation, we can show that water flowing out of a pipe with cross-section $A$ at speed $v$ exerts a force $F$ on a wall (at 90 degrees), acc.: $$F=\rho Av^2$$ With $\rho$ the density of the water. But your specification of "8" pipe with 500psi stream of water exiting it and hitting a wall at 90 ...

1

No, there is not because, contrary to your last sentence: The amount of water and the container itself I am not concerned with, strictly some sort of equation I could use to get a measurement of time. the boiling time is not a well defined quantity unless you take these things into account carefully. To calculate the rate of temperature rise you need ...

1

Yes. It's is not so much the water is the beach sand reflecting light back to you like a parabolic mirror. The droplets of water on your skin can form more surface area to catch light creating a magnifying effect focusing light on your skin as well. The random texture in the beach sand will also give you even tan. Most sand is white in color even if not the ...

1

Water is the people's most popular liquid but it is also one that differs from almost all other liquids in some almost qualitative ways. The most important reason why water is considered an "improper liquid" is that its density is higher than the density of its solid phase, the ice. For "proper liquids", it's the other way around – the solid phase is ...

1

There is actually only one disordered phase - from a physicist's perspective, the liquid and the gas are actually the same phase because one can continuously vary the external parameters (temperature and pressure, in this case) to get from the liquid to the gas without passing through any phase transition, because the phase transition line terminates within ...

1

Yes. It's is not so much the water is the beach sand reflecting light back to you like a parabolic mirror. The droplets of water on your skin can form more surface area to catch light creating a magnifying effect focusing light on your skin as well. The random texture in the beach sand will also give you even tan. Most sand is white in color even if not the ...

1

I entirely agree with @Vintage in his offered answer to @BarsMonster's question. It seems to me that the question isn't how to make the most accurate, error free measurement for water. Rather, what's a practical method that can give a 1st order, believable result? Placing two clean metal plates reasonably close together where d/A >> 1 and then measuring the ...

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