Printer Friendly

Pressure

The content and activities in this topic will work towards building an understanding of the effects of pressure in the ocean depths.

Force and Pressure

In the ocean depths, there is much greater pressure than at the sea surface. Pressure describes the amount of force exerted on an object. Hydrostatic pressure is pressure due to the weight of water pressing on submerged objects (hydro- means “water” and -static means “at rest”). A swimmer diving to the bottom of a pool might feel a change in pressure in their ears. Atmospheric pressure is pressure due to the weight of air.

 

<p><strong>Fig. 9.10.</strong> The (<strong>A</strong>) force exerted on your hand from a book is different from the (<strong>B</strong>) pressure felt from the book when it is placed on a pencil.</p><br />

A passenger flying in an airplane might feel a popping sensation inside their ears due to lower atmospheric pressure in their ears. Pressure is related to force. The difference between force and pressure is illustrated in Fig. 9.10. If the palm of a hand is held out palm-up and a heavy book is placed upon it, no pain is felt and it can support the book easily. However, if a pencil is added on the palm of the hand with the eraser end down and the book is put on top of the pencil, it would be uncomfortable. Imagine what would happen if the pencil point rested on the palm of the hand, and the book was placed on the eraser end!


 

Force is the weight of any object, and pressure is the force per unit of area. Pressure is calculated by dividing the amount of force by the surface area on which it rests.

   

This equation can be used to compare the force of the book with the pressure of the book on the pencil point (Fig. 9.10).

 

The average textbook weighs about 2.25 kilograms (5 pounds) and has a surface area of about 480 cm2. Therefore the book resting on your hand has a pressure of about 0.0046 kg/cm2. The same book resting on the pencil tip, with a surface area of about 0.00001 cm2, has a pressure of 225,000 kg/cm2!

 

In each case the same amount of force, the weight of the book, pushes down on the palm. When the book rests on the palm, the pressure is distributed over the entire surface of the hand. But when the book rests on the pencil, its force is applied to an eraser-sized area of the palm, increasing pressure on that area about 200 times. If the point of the pencil had been rested against the palm with the book on top of it, the pressure could have been as much as 4,000 times as great as the pressure of the book alone.

 

Atmospheric Pressure

Both water and air exert pressure. Most people are not aware of air pressure. A familiar example is drinking through a straw. As the pressure inside your mouth is reduced by sucking air through the straw, the increased air pressure on the liquid forces it up the straw in an effort to balance the two. A layer of air more than 550 km thick surrounds the earth. Gravity pulls the air toward the earth. Density within the layer of air gradually increases from least dense at the outer edge of the atmosphere to most dense at the earth’s surface. One atmosphere (atm) is the average pressure on the earth’s surface measured at sea level.

1 atm = average pressure of atmospheric layer (at sea level)

 

At the upper level of the atmosphere, the thickness of the layer of air pressing down decreases, and so does atmospheric pressure. But below sea level—for example in an air-filled mine—atmospheric pressure increases to more than 1 atm. Humans are adapted to living at an average of 1 atm. We are sensitive to pressure changes. People can feel changes in pressure on the air spaces in their ears when they ride in an airplane, drive up and down a mountain road, or go up and down in an elevator in a tall building.

 

Hydrostatic Pressure

Seawater is about 800 times more dense than air. A column of seawater 10 meters high exerts the same pressure as the entire 550 km layer of air above it. The hydrostatic pressure of seawater can also be measured in atmospheric units:

1 atm = pressure exerted by 10 m seawater

 

<p><strong>Fig. 9.11.</strong> As a result of the increased pressure, the lungs (pink) compress slightly as a free diver moves from the ocean surface (1 atm of pressure) to 10 m (2 atm of pressure).</p><br />

The total pressure on a submerged object can be expressed using atmosphere as a unit of pressure. For example, a fish 10 m under the surface of the sea is under 2 atm of pressure—1 atm from the water above it and 1 atm from the air above the water. If the fish swims down to 20 m, perhaps to escape a predator, it is under 3 atm of pressure. Even if the fish leaps into the air at the surface of the water, it is still under 1 atm of pressure from the air above it. Similarly, a free diver experiences 1 atm of pressure at the ocean surface. At 10 meters the free diver experiences 2 atm of pressure. Because of the increased pressure, the lungs (pink in Fig. 9.11) compress slightly as the free diver descends in the water.

 

Because of pressure, living in the ocean differs from living on land. Air pressure changes very little with vertical movement; water pressure changes rapidly. If someone were to walk down the stairs in a building from the third floor to ground level, a vertical distance of about 10 m, the atmospheric pressure exerted on them is essentially the same. But if someone were to dive from the surface of the ocean to a depth of 10 m, the atmospheric pressure exerted on them doubles from 1 to 2 atm. If they dive to 20 m, the pressure is 3 atm.

 

Activity

Activity: The Effects of Pressure

Simulate the effects of pressure on a free diver’s body by using a syringe system.

 

Pressure and Gas Volume

Boyle’s Law is a predictive statement describing the effect of pressure on gas volume (Fig. 9.13). This law states

If the temperature of a gas does not change, its volume decreases as the pressure increases; conversely, as pressure decreases, its volume increases.

 

<p><strong>Fig. 9.13.</strong> Boyle’s Law demonstrates that (<strong>A</strong>) as volume increases, pressure decreases and (<strong>B</strong>) as decrease volume, pressure increases.</p><br />
<p><strong>Fig. 9.14.</strong> Diagram of the volume of air in a tube at 0 m and 10 m depth. At 10 m air is compressed to half its volume due to a doubling of the pressure.</p><br />


Boyle’s Law can be applied to a tube opened at one end and filled at the surface with 1 L of air. When the tube is lowered into water with its opening down, air is trapped inside by the upward pressure of the surrounding water. At a depth of 10 m, the water pressure is 2 atm, double the surface pressure. The hydrostatic pressure at 10 m compresses the air inside the tube to 0.5 L, half its original volume. Air compresses in the tube until the air pressure inside the tube is equal to the water pressure outside the tube. Thus, at 10 m the air pressure inside the tube is 2 atm (Fig. 9.14).

 

Pressure and Diving

Just like with the air space in the tube, water pressure affects the lungs and other air-filled spaces in our body. These air-filled spaces include the middle ear, which is connected by the Eustachian tube to the throat and the nasal sinuses, which are connected to the nasal passage (Fig. 9.15).

<p><strong>Fig. 9.15.</strong> Air spaces in the head include the middle ear, the Eustachian tube, the nasal sinuses, and the throat. These areas respond to changes in pressure.</p><br />


 

As divers descend, they quickly feel the effects of increasing pressure as air in the body compresses. Pressure on delicate membranes in the ears and sinuses can cause sensations that divers call “squeezes.” The eardrum, between the middle ear and the outer ear, is particularly sensitive to pressure changes. To avoid pain and damage to their eardrums, divers must equalize the water pressure outside and the air pressure inside their throat and ears. To do this they yawn, swallow forcefully, wiggle their jaw, or pinch their nose closed and blow gently. This opens the Eustachian tube and lets air from the throat enter the middle ear. The sensation divers feel when “clearing the ears” is like the popping a person feels when they travel up a mountain or take off in a plane.

 

Barotrauma (baro- refers to pressure, -trauma to injury) injuries are caused by pressure differences. If the pressure is not equalized and the Eustachian tube remains closed, a diver may suffer pain, bleeding in the middle ear, or even a rupture of the eardrum—injuries that can lead to infection or even permanent hearing loss. Pressure imbalances can also affect nasal sinuses blocked by congestion.

 

Question Set

Question Set: Pressure

Further Investigations: Pressure

Table of Contents:

Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.