Why is water pressure increased in a plastic bag in a bucket?

[sophiecentaur]

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B.

The medium surrounding the bag is the same for both (water columns). The pressure gradients will both be the same. It's the pressure in the bag that counts and the value will depend on the nature of the bag material but the bag will deform in shape. Assume no stretching of the membrane. It will depend on the pressure on its outside but that air pressure will be the same throughout. The bag will deform to balance the outside pressure of the water which will correspond to the normal pressure gradient. The bag will be squeezed in at the bottom of the air space (even wrinkling) and up at the top and the air pressure will be higher than ambient by an 'unspecifed' amount - somewhere corresponding to the head of water in D but the membrane will modify (support) that to some degree, everywhere.

I imagine that any resulting air pressure at the surface could be achieved just by tightening the neck of the bag and doing away with the bucket.

 

[A.T.]

The vertical pressure gradient in the medium surrounding the bag, is greater in D than in B.

The medium surrounding the bag is the same for both (water columns).

Look at the diagram again.

 

[Herman Trivilino]

It's fascinating to me that even with that 1' there's a gradient and that such a small amount could still be enough.

It's smaller than that!

We don't have 1 ft of pressure gradient. The 1 ft is the amount of pressure, but the height of the bag, just a few inches, is the amount of pressure gradient.

 

[sophiecentaur]

Look at the diagram again.

I did and apologies - I was comparing C and D as the difference in those two situations is less 'obvious' than comparing B and D.
Whilst correct, your description uses pressure 'gradient' when it's really the pressure on the surface of the clay that counts. That would have made the situation clearer. After all, that's what would cause absorption of air components into the clay.

We don't have 1 ft of pressure gradient.

I think there must have been a units problem. The use of ' and " for feet and inches is foreign to SI users. My opinion is "When in Rome ...." and Physics is a land of SI, in nearly all cases.

 

[ImaginaryTango]

Because the density of water is greater than the density of air.

I could be losing track of the factors, but with that in consideration, I get that the gradient is greater in D than B, but what about D compared to C? And why?

I'm beginning to think a critical part of this process is making sure there is some air in the bag and, possibly, even making sure the clay in the bag is exposed to the air in it.

 

[sophiecentaur]

I could be losing track of the factors, but with that in consideration, I get that the gradient is greater in D than B, but what about D compared to C? And why?

I'm beginning to think a critical part of this process is making sure there is some air in the bag and, possibly, even making sure the clay in the bag is exposed to the air in it.

Air in the bag seems very relevant. Option D will ensure that the air pressure on the clay surface is increased by the extra head of water in D and it provides a large surface area of air / clay interface. However, if there's a higher air pressure, how could that affect the transport of air molecules through the clay to lower layers and to affect the bulk properties?
Would D be any better than just squeezing the bag a bit? OR is there some detail in the process that hasn't been introduced to the discussion?

 

[A.T.]

... what about D compared to C?

See the posts by @jbriggs444

 

[sophiecentaur]

See the posts by @jbriggs444

I've been thinking, rather, that air was needed inside the clay but I was re-examining what @jbriggs444 had to say. Could be - but would it need an almost dry surface on the top? Does anyone use a detergent sprayed on top? But then, what would be the need for extra pressure in D?

The requirements for clay suitable for pottery are a bit special (potters are quite fussy about what they like and probably with good reason)
If the idea is to expel air then vibrating (as with concrete) could achieve that. Alternatively, a moderate vacuum works well to get smooth plaster of Paris for casting.

 

[haruspex]

What about the possibility that capillary action is responsible?

By itself, capillary action will tend to draw water into the clay. This will tend to expel any entrained air. If the clay is entirely immersed, there is nowhere for the entrained air to go. It is trapped and will be under some pressure. At best, it can attempt to bubble out the top. But surface tension will resist bubble formation. If, on the other hand, the clay has a surface exposed to air, then the entrained air has an exit pathway. Capillary action can then fully displace the entrained air.

The plastic bag is helpful because it ensures that air inside the bag will reach 100% relative humidity. The exposed clay will not dry out.

That's one of the only two ideas I had. The video shows the water being poured over the top, but there could still be dry patches.
The other is that the bag may exert shear forces on the surface that help to open up the channels.

Posted versions of the trick have some variation, which should help to narrow down the options.