Wikipedia:Reference desk/Archives/Science/2021 October 5

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October 5

Physics

What are the the reason for observing proper dressing while in a laboratory — Preceding unsigned comment added by 197.248.176.86 (talk) 03:30, 5 October 2021 (UTC)

What do you mean by "proper dressing"? ←Baseball Bugs ^{What's up, Doc?} carrots→ 03:36, 5 October 2021 (UTC)

That it is improper to appear in an improper dressing? (I think this also applies to weddings, football matches and salads. Not exactly rocket science.)  --Lambiam 04:30, 5 October 2021 (UTC)

What did the oil-and-vinegar say when someone opened the refrigerator? ←Baseball Bugs ^{What's up, Doc?} carrots→ 05:02, 5 October 2021 (UTC)

While it depends on the laboratory, the appropriate garments and gear is needed to prevent or reduce injury in the event of a mishap. A lab coat is basic protection against chemical spills, and goggles may be necessary for eye protection. Personal protective equipment might be a good starting point.-gadfium 05:19, 5 October 2021 (UTC)

You may also have to be careful not to get hair in your experiment or dirt off your shoes into your equipment. When measure magnetic fields, it will be helpful not to have any metal objects on your person (eg keys, watches, phones, jewelry). So your experiments/equipment may need protection. Graeme Bartlett (talk) 10:15, 5 October 2021 (UTC)

See also cleanroom where special clothing is required to prevent contamination. Alansplodge (talk) 11:50, 5 October 2021 (UTC)

Why doesn't water fill the container if I do this?

If I submerge the container upside down, then water doesn't fill the container. Look at my image for more clear idea. Rizosome (talk) 07:07, 5 October 2021 (UTC)

The water can't fill the container because the air in the container has nowhere to go. If you made a small hole in the bottom of the container you will see the air bubble out the hole as water fills the container. 41.165.67.114 (talk) 08:02, 5 October 2021 (UTC)

This is precisely how an open-bottomed diving bell functions.--Shantavira|^{feed me} 08:34, 5 October 2021 (UTC)

See also Can pockets of air exist underwater?. Alansplodge (talk) 11:47, 5 October 2021 (UTC)

This is the mirror situation of an upright container filled with water. Why doesn't it spill out? In both cases the centre of mass would become lower by the change, so that is the preferred situation, the one with the lower potential energy. However, in either case the centre of mass needs to get temporarily higher before it can get lower, which is why the change does not happen. In the quantum realm it is possible by tunnelling, but not in the macroscopic world.  --Lambiam 20:53, 5 October 2021 (UTC)

As you push the container into the water the air gets compressed into the upper part of the container. The boundary of the region the air is in, is determined by the depth of the container, the surface tension of the water-container interface, the surface tension of the water-air interface, and the line tension of the water-container-air interface. Depending on the values of the surface and line tensions, what can happen if you push the container deeper and deeper into the water, is that the shape of the air inside will become concave, sagging at the container boundary and end up leaking out of the container from the edges when pushed deep enough into the water.

If this does not happen, the air will still gradually leak out by dissolving into the water. The concentration of air dissolved in the water is in thermodynamic equilibrium with the concentration of air at the surface. If you push the container into the water, then the air inside will be pressurized and therefore no longer in thermodynamic equilibrium with the dissolved air in the water. This will cause the air to gradually dissolve into the water. The concentration of air in the container will then increase, while the air pressure decreases. But no equilibrium will be reached where the pressurized air can remain. The larger concentration of air in the container compared to the air in the water outside of the container will cause the dissolved air inside the container to diffuse out of the container. So, in equilibrium, the concentration of the dissolved air inside the container will have to be the same as outside the container, which means that non of the trapped air can remain in the container in equilibrium. Count Iblis (talk) 21:43, 5 October 2021 (UTC)

But how does air escapes the container if I tip it to the side ? Rizosome (talk) 00:21, 6 October 2021 (UTC)

It has to slope a bit upward, the buoyancy force on the air then as a component in the direction toward the opening. Count Iblis (talk) 05:13, 6 October 2021 (UTC)

I am asking how air escapes in first case but not in the second case? see this image. In first case, I can see bubbles coming out of the container as water fills in. Rizosome (talk) 08:18, 6 October 2021 (UTC)

In case ① the air can move straight up to escape the cup. In case ②, it first has to move downwards to get across the rim before it can start moving up. See my answer above.  --Lambiam 09:02, 6 October 2021 (UTC)

In both cases, air is surrounded by fluid then why both cases are not equal? Rizosome (talk) 14:53, 6 October 2021 (UTC)

In the case of an upright cup filled with fluid, the air is not surrounded by fluid. The cases are then equal if you swap air (which has the lower density) and fluid (which has the higher density), and at the same time flip up and down. If you do the same swaps to image ①, the fluid will flow out.  --Lambiam 18:15, 6 October 2021 (UTC)

Just to be picky, air is a fluid, meaning it flows. Liquids and gases are both fluids. --Trovatore (talk) 17:57, 7 October 2021 (UTC)

The point is illustrated in this image, which is your ① | ② image except that I have swapped air and liquid and flipped it upside down. If you can understand why the liquid escapes on the left but not on the right for these liquid-filled cups immersed in air, you should be able to understand the same for the air-filled cups immersed in water.  --Lambiam 08:48, 7 October 2021 (UTC)