1.2 Hydrostatic Pressure & Blaise Pascal

In the last module we looked at what pressure was. We used Pascal’s ideas to see how pressure behaves - Pascal’s Law is fairly important when working with Submarines and understanding the challenges we will face when underwater.

We found that water could only travel a certain height up a straw, the limit to how high water can go is explained by Pascal’s Law (also known as Pascal’s Principle).

Pascal's principle is defined as

A change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.

Pressure exerted on a fluid in an enclosed container is transmitted equally and undiminished to all parts of the container and acts at right angle to the enclosing walls.

Alternate definition:The pressure applied to any part of the enclosed liquid will be transmitted equally in all directions through the liquid.

(Pascal's law, Wikipedia, 2020)

This law applied with our straw experiment. The water was under pressure from the atmosphere, it was “enclosed” and surrounded by atmospheric pressure.

When underwater we’re almost doing the opposite to the straw experiment. Instead of pulling water up in a column, the water column above our submarine is pushing down on our vessel, increasing the pressure the further we go.

Underwater we will need to consider two types of pressures - hydrodynamic and hydrostatic.

Let’s first consider what they mean. Both start with “hydro” which means we’re working with water. Firstly, “dynamic” means movement, so hydrodynamic pressure probably has something to do with movement, and the second has “static” which means still or stationary, so hydrostatic pressure has more to do with pressure when nothing is moving.

In this module we will consider hydrostatic pressure and Pascal’s findings when it comes to understanding how fluids behave. Before we move on to understanding how our submarine manoeuvres in water, let’s think about how our submarine will interact with it’s environment if it’s not moving at all.

Pascal conducted an experiment of his own to explain his thoughts on how pressure behaves. Pascal’s experiment is a little bit similar to our straw experiment, but upside down.

There is some mystery as to whether or not Pascal actually conducted the experiment but in any case, he is given credit for the discovery.

Watch the video below which recreates Pascal’s Barrel experiment.

Proving scientific ideas is hard, especially when you’re the first. Blaise Pascal was a genius in many different ways. In our submarine, the main thing we need to take away from our understanding of hydrostatics is that the depth of our submarine will determine how much pressure it’s under.

When we conduct testing, we should consider how deep our submarine will need to go, and we need to make sure that our systems are functional both when our submarine is close to the surface, and well below the surface.

Pascal’s experiment had nothing to do with submarines or diving under the sea but they do give us a lot of insight into why things are the way they are. The next video will show the benefits of Pascal’s findings in the real world. Yes, they give a lot of insight into our submarine, but how else is our understanding of pressure used to our advantage?

The video below will provide a further explanation of Pascal’s experiments and of how scientific ideas develop over time. It’s important that we challenge what we believe is true and test our ideas using valid scientific methods.

In science, to test our ideas and justify our understanding of concepts we use a scientific method. Hypothesis testing is critical to success in science, and to success with your submarine. Making a hypothesis means making a prediction of what will happen. After a hypothesis, it’s a scientist’s challenge to design an experiment which will test the hypothesis and either prove it true, or false.

In order to test our ideas we need to conduct experiments that are valid, reliable and accurate.

These three terms are critical when it comes to developing good scientific processes. Using online resources, investigate these three terms in a scientific context and determine what makes an experiment, valid, reliable and accurate.

Hint: an easy way to remember how these terms relate to experiments is by using their first letter. Validity relates to Variables, Reliability relates to Repeatability, Accuracy relates to Apparatus (or equipment).

Using the interactive web page below. Make some hypothesis about pressure at different levels, using different shapes and volumes.

Test to see if our ideas above are accurate using the simulation.

Think:

How does the pressure change at different depths?
Do different shapes effect the pressure at different depths?

How does the pressure change when you remove the “atmosphere”? (hint: use atmospheric units to see this).

In the third configuration, what happens when weight is added on top of a water column? What happens to the water on the open side when weight is added?

https://phet.colorado.edu/sims/html/unde...

A siphon is a very interesting way to move liquid from one place to another. It doesn’t require a pump, or much energy at all, but it works.

Use online resources to research and reflect on how a siphon works.

Watch the video below and investigate a situation where siphons are used in our daily lives.

In order to explain this phenomena a chain inside a cup is often used such as in the video below. With our submarine, creating models and simulating what might happen to our submarine can be very useful, however, it’s not always accurate.

Watch the video below which models a siphon by using a chain. Using online resources, determine how this model might be flawed.

Hint: Wikipedia explains the flaws of the chain model quite well on the “Siphon” page.