Pressure


What is Pressure?

Pressure is a force per unit area and is usually measured in pounds per square inch (PSI or pounds/in2).  You are probably already familiar with measuring pressure.  Every time you weigh yourself, you are measuring the pressure exerted by your body on the scales. 

Let's consider how to convert weight (often measured in pounds) to pressure (often measured in pounds per square inch).  For the sake of simplicity, let's consider the container below which is one foot tall and has a bottom which is one inch on each side.  The container is filled to the top with water.  Not counting the weight of the container, the water weighs 0.43 pounds. That weight is pressing against the bottom of the container, so it is a pressure. 

A container 12" high, 1" wide, and 1" long.

The area of the bottom of the container is found by multiplying the length of two of the sides. 

1 in x 1 in = 1 in2


The pressure is equal to the weight divided by the area.  So, the pressure which the water is exerting is:

0.43 pounds = 0.43 pounds/in2 = 0.43 PSI
      1 in2                                                                        


A dull knife slicing through bread is a perhaps more familiar example of pressure.  Once the knife has exerted a certain amount of pressure on the bread, the bread will break apart.  Once again, think of pressure as a force per unit area.  If you are using a dull knife, then the area of the knife blade which presses against the bread is large.  You will have to use a large amount of force to cut the bread since the surface of the knife is so big.  But if you sharpen the knife, the smaller blade surface will exert the required amount of pressure on the bread when you apply less force.  So, in order to increase the pressure, you either need to increase the force being applied or decrease the area which the force is being applied to. 

A sharp knife has a smaller surface area than a dull knife.

There are two types of water pressure: static and dynamic.  Static is pressure supplied by the mass of water per unit area and dynamic is pressure supplied by mass combined with the energy of motion.



Static Pressure

There are two components of pressure: static and dynamic pressure.  The water pressing down on the bottom of a container is an example of static pressure.  Static pressure is pressure applied by non-moving water as a result of gravity.  Gravity pushes the still water down and applies pressure on the bottom of the container.  

Static pressure is a form of potential energy. Potential energy is energy which is not currently causing any sort of motion, but which has the potential to cause motion.  A ball sitting at the top of a hill is another example of potential energy.  The ball is sitting still, but if it started to roll, it would go all the way to the bottom of the hill without anyone pushing it.  

Potential energy - a ball at the top of a hill.

A reservoir is also an example of potential energy.  The water is sitting still in the reservoir, exerting pressure on the reservoir bottom and on the dam. 

A reservoir is an example of static pressure, a type of potential energy.

Kinetic energy is the energy of motion.  Once a ball begins rolling down a hill, it is moving and has kinetic energy.  Similarly, if the floodgate in a reservoir is opened, then the water's energy becomes kinetic energy and gushes out of the reservoir.  

A ball rolling down a hill is an example of kinetic energy.

Static pressure is what causes water to seek its own level.   When you pour water into a dish, the water quickly spreads out to cover the entire bottom of the container in a flat layer.  Any high points in the water are pushed down by gravity until they are level with the rest of the water.

Gravity presses down on high points in water and makes the water level.

 


Dynamic Pressure

Water gushing out of a reservoir is an example of dynamic pressure which is a form of kinetic energy.

The other type of pressure, dynamic pressure, is a form of kinetic energy.  This type of pressure is applied by moving water on its surroundings.  Water flowing through a pipe exerts more pressure on the pipe than the static pressure which would be exerted if the water in the pipe was standing still.

Water in different parts of the pipe will have different pressures.  This difference in pressure is measured by differential pressure meters.  By reading the pressure at two points in a pipe and taking into account the resistance of the pipes, the flow rate of the water can be determined.  Resistance, or friction, in pipes is caused when water rubs against the edge of the pipe and slows down. 

In a hydraulic system, the pressure of the water at one point against the water at another point causes the water to flow.  If a pressure gauge on a pipe reads 40 PSI, that means that the water is pushing against the nearby water with 40 pounds of pressure for every square inch.  If the pressure in the pipe was greater, the water would push harder against the surrounding water and cause the water to flow faster.