# Why can't we use the hydrostatic pressure gradient to do work?

• Mohankpvk
In summary: It depends on your definition of 'energy'.In summary, hydrostatic pressure gradient can be harnessed to generate energy, but it does not work the way people think it does.

#### Mohankpvk

Is there any way to harness the hydrostatic pressure gradient to generate energy?
The pressure at the surface of an ocean is atmospheric pressure.As we descend down the ocean, the pressure increases .After a point, the pressure will be very high.Why can't we use this pressure difference to do work?

That's a fair question, but you can answer it yourself. If you put a soda straw in a glass, the pressure at the bottom of the glass is bigger. How fast does water flow through the straw? It ought to make a fountain, right?

Mohankpvk
anorlunda said:
That's a fair question, but you can answer it yourself. If you put a soda straw in a glass, the pressure at the bottom of the glass is bigger. How fast does water flow through the straw? It ought to make a fountain, right?
I haven't done it before, but I think the water will stop at the top of the glass.It will not rush out like a fountain.I think its because, the weight of the column is the reason for the pressure, so it can't lift itself.But initialy the water rushes to fill the straw, can this be used to do work?.Something like water rushing through a hole in a ship.Can this rush be used to do work?

Mohankpvk said:
But initialy the water rushes to fill the straw

No it does not. The water doesn't move at all. The end of the straw goes from top to bottom. The water at the top of the column is the same water that was on top before the straw.

Edit: but if you plugged the end of the straw first, then push it into the glass, then open up the end, water would rush in and you could extract energy, but first you had to put in the same energy (or more) to push the air-filled straw down.

Chestermiller and Mohankpvk
(I see Anorlunda has made my main point while I was writing.)
Mohankpvk said:
... initialy the water rushes to fill the straw, can this be used to do work?.Something like water rushing through a hole in a ship.Can this rush be used to do work?
Yes they could.
But, thinking about the straw, you would have to do work to push an empty straw (sealed) to the bottom of the glass against the buoyant force. When you open the straw, the inrush of water is just returning (some of) this energy.
Rather than making holes in the ships (and sinking them), you could seal them up, then pull them to the bottom with strong strings (hawsers.) When you want your energy back (some of it) you can allow them to float to the surface doing work by pulling on the strings. This is easy to repeat.

It is more or less (hedging his bets!) equivalent to lifting some water up and later letting it fall again.
In UK the electricity supply companies do something similar. They pump water up from a low pond to a high pond when they have spare energy, then let it run down again through turbines, when they need the energy. I suppose the advantage is that they can have a big difference in the heights of two lakes, rather than having to have one very deep lake and lots of large ships on hawsers.

Mohankpvk
Thank you guys.And, the idea of storing excess energy as water potential energy is really nice.I have learned that A.C. power cannot be stored.So this is a good way to store it.

Mohankpvk said:
Thank you guys.And, the idea of storing excess energy as water potential energy is really nice.I have learned that A.C. power cannot be stored.So this is a good way to store it.
We already do that with hydroelectric dams.

Thinking laterally, would an air-lift suction system qualify, as used by underwater archaeologists and artisanal gold dredgers ??

Nik_2213 said:
Thinking laterally, would an air-lift suction system qualify, as used by underwater archaeologists and artisanal gold dredgers ??

It depends on your definitions. You could extract energy from the air lifted column, but you need to provide more energy to compress the air and get it to the bottom. So the net is negative. How would you count that?

Yes, you are using air-power, but it leverages a previously static system to do useful work by creating a powerful suction which entrains otherwise intractable stuff.

Slightly further off-topic, I'm reminded of the 'permanent soda fountains' now warily de-gassing those African lakes with volcanic CO2 etc saturated bottom layers, potentially lethal should they 'overturn' again...

Nik_2213 said:
Yes, you are using air-power, but it leverages a previously static system to do useful work by creating a powerful suction which entrains otherwise intractable stuff.

You have to be careful how you say that. It may be very useful, very valuable, but in terms of energy you can't get out more than you put in. Using the word leverage implies over unity gain which is impossible, and a forbidden topic on PF. But that's not what you meant, right?

anorlunda said:
You have to be careful how you say that. It may be very useful, very valuable, but in terms of energy you can't get out more than you put in. Using the word leverage implies over unity gain which is impossible, and a forbidden topic on PF. But that's not what you meant, right?
After a few hours of thinking about it, I think it would make an interesting homework problem:

Of course, the friction is always the booger in the perpetual motion scheme of things.

BTW, I just spent a couple of hours doing experiments on my very ancient pressure measuring device, to determine how accurate it would be if I were to do such an experiment.

ps. I decided that the maths was quite a bit too difficult, and I therefore cheated, as usual.

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No over-unity gain, that's a given.
Sorry, it is such bedrock Science, I sometimes forget about woo-fringe ijits' sad tenacity...

## 1. Why can't we use the hydrostatic pressure gradient to do work?

The hydrostatic pressure gradient is the change in pressure of a fluid due to the difference in elevation within the fluid. While this pressure difference can be used to do work in some situations, it is not a reliable source of energy as the pressure difference is typically small and unstable.

## 2. What factors affect the hydrostatic pressure gradient?

The hydrostatic pressure gradient is affected by the density of the fluid, the acceleration due to gravity, and the difference in elevation within the fluid. Any changes in these factors can alter the pressure gradient and make it difficult to use for doing work.

## 3. Can we harness the hydrostatic pressure gradient to generate electricity?

It is possible to use the hydrostatic pressure gradient to generate electricity, but it is not a practical or efficient method. The pressure difference is typically small, and it would require a large and complex system to convert it into usable energy.

## 4. Are there any other limitations to using the hydrostatic pressure gradient for work?

In addition to the small and unstable pressure difference, there are other limitations to using the hydrostatic pressure gradient for work. These include the difficulty in controlling and directing the fluid flow, the potential for leaks and other mechanical issues, and the limited applications for which this method can be used.

## 5. Is there a more efficient way to do work using fluid pressure?

Yes, there are more efficient methods for harnessing fluid pressure to do work, such as using hydraulic systems or turbines. These systems are specifically designed to convert fluid pressure into useful work and can be more reliable and efficient than relying on the hydrostatic pressure gradient.