Physics

A little immersion in the world of physics

Pennywise talked about underwater and diving and… did you know? There are a lot of physical laws linked to this topic!

Let’s dive in a sea of physical principles then! 😉

Galileo discovered that air has a weight and Torricelli measured it: he used a glass tube which was open at one end and closed at the other end and  filled it with mercury; he put it into a vessel full of mercury and noticed that the mercury in the 100cm tube dropped 76 cm. So he understood that the weight of the tube was balanced by another force: atmospheric pressure or, in other words, the atmospheric mass on our head.

Another type of pressure is  hydrostatic pressure: it’s exerted by a fluid in balance because of gravity.

After this short introduction, I’m going to list the main physical principles that come into play during an immersion.

– Pascal’s law states that the  pressure exerted anywhere in a fluid is transmitted equally in all directions throughout the fluid. Don’t worry, an example will better explain the concept! Consider a “pierced” vessel full of water and provided of a piston: a force acts on the piston and if the force increases, the water comes out of the vessel with bigger intensity. So the rise in pressure is the same anywhere in water.

When we’re under water, our body is subjected to Pascal’s law because we’re mainly made up of water, which is subjected to the outside pressure and it’s redistributed on the mass of the body.

– Charles’s law, or “law of volumes”, says that a gas expands as temperature increases. We can see that during the recharge of a depth charge: when we finish the recharge, the pressure gauge  reads 200 atm; the same device will sign a lower value after some minutes because of the cooling of gases. Indeed this change is caused by the superheating of the molecules of compressed air, so their volume and the pressure (read by the gauge) increase.

– Archimedes’s principle states that any object, immersed in a fluid, is buoyed up (kept afloat) by a force equal to the weight of the fluid displaced by the object. This means that only if the weight of a body immersed in the water is smaller than the weight of the displaced fluid, it will float. So we say that the fresh water is heavier than salt water because its weight is lower and, at a constant volume, buoyancy is smaller.

– Dalton’s law states that the total pressure exerted by a gaseous mixture is equal to the sum of the partial pressures of each gas. For example, nitrogen represents the 78% of the air that we breathe and it exerts a partial pressure of 0,78 atm (unit of measurement of pressure). If we dive deeper, pressure increases and the partial pressures of gases increase, too; this could cause some problems, like nitrogen narcosis or oxygen intoxication. Nitrogen narcosis can cause feelings of euphoria or sadness, as if the diver is drunk!

– Henry’s law states that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid. An everyday example is given by sparkling water: before the bottle is opened, the partial pressure of the carbon dioxide is higher than atmospheric pressure; when the bottle is opened some of this gas escapes giving the usual “hiss”. Some of the dissolved carbon dioxide comes out of the solution in the form of bubbles, because the pressure of this gas above the drink is lower. The same phenomenon happens in our body during an immersion: the nitrogen dissolves in our blood. If we go up too quickly, the gas returns to the gaseous state and it can cause the formation of “bubbles” in the blood (emboli).

– Boyle-Mariotte law states that at a constant temperature, the pressure and the volume of a gas are inversely proportional. Doing skin-diving, if we fill a balloon (that is, our lung) at -10m (where the pressure  is higher than atmospheric pressure) and then we go up, the volume will increase until the balloon bursts. This can cause serious problems, like lung over-distention: the breaking of the alveolus that could cause the passage of the air in the pleural cavity and the lung can collapse!

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