Pressure is force divided by area

It matters how much the force is spread out.

It actually matters very much how an applied force is spread out both in space and in time, but we'll just think about space here.

Here's a little thought experiment. You have a choice of a force of 100 N (about 22 lbs.) being applied to the back of your hand either (1) through the tip of a nail, or (2) through a hand-sized piece of flat wood placed on top of your hand.

In the first case the force is applied to a very small point, and our experience tells us that a sharp enough nail would even penetrate the skin.

In the second, although having that weight on our hands for a long time might get uncomfortable, we wouldn't worry about the board penetrating our skin. The force is more "spread out."

The difference between the two scenarios is the area over which the force is applied. In case (1) the area is very small, and in case (2), it's about as large as it can be relative to the size of a hand.

So we need a way to distinguish this kind of difference between two situations in which equal force is used, and that is pressure: force divided by the area over which it is applied.

Pressure defined

Pressure is force divided by the area over which that force is applied.

Units of pressure

The SI unit of pressure is the Pascal: 1 Pa = 1 N·m-2 = 1 Kg·s-2m-1

The atmosphere (atm) is the pressure at mean sea level on Earth, 1 atm = 101,325 Pa.

Pounds per square inch (psi) are still commonly used in science and industry in the U.S.1 atm = 14.7 psi.

The torr is an older unit of pressure. 1 torr is the amount of pressure it takes to raise a column of liquid mercury by 1 mm. 760 torr = 1 atm. Because of this, 1 torr is also called "1 mm Hg."

The pressure of Earth's atmosphere in Pascals is a little awkward. One bar = 100 KPa is a nicer unit that's often used to measure atmospheric pressure, and it's within 2% of an atmosphere.

Pressure of a gas

Gases are gases because the atoms and/or molecules that compose them have enough kinetic energy to overcome any cohesive forces between them; what were liquids and solids fly apart. Gas particles completely fill any container we try to put them in.

The animation shows a hypothetical gas particle "filling" it's square container. Gas particles bounce off every wall of the container, and if the sample size is big enough, the total average force of all those collisions with the walls is predictable. The sum of all of those forces over time, divided by the area of the walls is the pressure of the gas.

We can do the calculation using just one wall (say the right wall in the two dimensional animation) if we break the velocity vector of each atom down into x- and y-components. The average of the right-ward x-components at any time divided by the area of the right wall would yield the same pressure.

Pressure and vacuum

The chamber shown in this photo is a vacuum chamber. NASA uses it to simulate the vacuum of space. When the round door is closed, pumps can be activated to suck all of the gases out to reduce the inside pressure to nearly zero (choose your own pressure unit).

Photo: Z. Pearlman

When the pressure inside the vacuum chamber is very low, the pressure outside is still 1 atmosphere (atm). That's because the gases in the atmosphere have mass and are pulled to the surface of Earth by the gravitational force. But gases being gases, these forces press in all directions.

It's interesting to calculate the total amount of force, due to air pressure, on such a door. Comparing it to the size of the people in the photo, the door seems to have a radius of about 3 people, or about 5 meters. It's area is about A = πr2= 75 m2, which is about 800 square feet. Now normal atmospheric pressure is about 14 lbs. per square inch, so at 144 in,2 per ft,2, that's a weight of 14·144·800 = 815 tons of weight. The pressure of a gas can be a powerful force.

Pressure under a liquid

A diver under the surface of the water experiences a pressure due to the weight of all of the water above her pressing down. The deeper she dives, the more water is above her and the more pressure she will feel.

Let's create a cylinder of water with a cross sectional area that is 1 square meter (simply for convenience), and with the height of the water above the diver of h.

Now the volume of such a cylinder, in cubic meters is just h if h is in meters.

The mass of water is 1 g per cm3 (ml), or 1 Kg per liter. And because there are 1000 liters in every cubic meter, the mass of the water in our cylinder is

Now the force of that mass (its weight) pushing down on the diver is that mass multiplied by the acceleration of gravity, g = 9.8 m·s-2, which gives us a force, in Newtons, of

This force is equal to the pressure because we chose an area of 1 m2. Here are some typical pressures that a diver would experience at depths between 1 and 100 meters below the surface of the water.

Notice that 1 atm has been added to each result because the air, and it's weight is still up there. The pressure due to the water is in addition to the air pressure we all live under all the time.

You might notice from the figure, and you might also have some experience with this, that the bubbles that a diver exhales expand as they rise. That's because the pressure pushing in on them from the outside is lower as they rise. It's the same thing with bubbles that form in the bottom of a glass of soda and rise. They expand a little.

Only the height of the column of water is relevant in calculating pressure. In the example above, if the circular area of the cylinder were doubled, the mass would be doubled, too, and the two effects would cancel, yielding the same pressure.

Example 1

Glacial ice has an average density of 850 Kg·m-3. Calculate the pressure, in atmospheres, of 50 m of glacier ice on the rock below it.

Solution: Take a chunk of ice with a cross-sectional area of 1m x 1m, or 1 m2, and height h.

The volume of such a chunk of ice would simply be the height in meters, therefore we're working with a sample of ice of volume 50 m3, and mass

The force of gravity on this mass of ice is

Because the area is 1 m3, the pressure in Pascals is 416,500 Pa, and here is that pressure in some other units.

These forces account for the inexorable, slow, grinding force of glaciers on rock, capable of carving deep valleys and moving ground-up rubble like a bulldozer over the eons.


Studying extremely high pressures

In the laboratory, it is possible to study extremely high pressures. A typical way to do that is the so-called "diamond anvil cell," shown here.

The small sample (red rectangle) is captured between the sharp points of two diamonds. Those diamonds are carefully aligned to avoid the points slipping past each other, and then screwed together. The pressures involved can be tremendous because

The cross-sectional area of the applied force is very small – the size of the pointed end of a diamond, and

Diamonds are very difficult to deform or crush. They are one of the hardest substances of which we know, so all of the force is transmitted to the sample.

Pressures in excess of 7-million atmospheres have been generated with diamond anvil cells.

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