What is Pressure? | Definition, Types, Formula & Examples
What is Pressure?
In physics, pressure is defined as the force acting on a given surface. With equal forces, the smaller the contact area, the greater the applied pressure. The formula used to describe and calculate pressure is:
Pressure = Force ÷ Area
or
P = F/A
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How is Pressure measured?
The standard unit for measuring pressure is the pascal, abbreviated as “Pa”. It is also newtons per square meter as given by the above formula. Other units used for pressure include pounds per square inch (psi), bar, and standard atmospheres (atm).
Example problems:
If a block weighs 60 N and lies on a surface with an area of 2 m x 3 m, what is the pressure exerted on that surface?
Pressure = Force ÷ Area
P = 60 N ÷ (2m x 3m)
P = 60 N ÷ (6m2)
P = 10 pascals
If the same 60N block is now on top of its 2m x 0.5m what is the pressure?
Pressure = Force ÷ Area
P = 60 N ÷ (2m x 0.5m)
P = 60 N ÷ (1m2)
P = 60 pascals
Air or Atmospheric Pressure
An important type of pressure is the pressure exerted on objects from the air or the earth’s atmosphere. It is actually a measurement of the weight of the gas above an object on a given surface. The higher the altitude, the lower the atmospheric pressure exerted by the air acting on the object less.
Liquid Pressure
It is also important to determine the pressure under water or in a liquid. Underwater pressure increases with depth. The equation for the pressure under the liquid is:
Pressure = D * g * h
where D is the density of the liquid, g is the standard gravity (9.8 m/s2) and h is the depth of the object in the liquid.
Pressure and the State of Matter
Pressure also affects the state or phase of matter. We often think that the state of matter changes from solid to liquid or from liquid to gas as a function of temperature, but pressure also has an impact on that state. In most cases, the higher the pressure, the higher the temperature required to change state.
An example is the boiling point of water. At higher altitudes, where atmospheric pressure is lower, water boils at a lower temperature.
Interesting Facts about Pressure
Pascal is named after the French physicist and mathematician Blaise Pascal.
One pascal is a fairly low pressure. It takes 101,325 new pascals to equal one atmosphere.
Some objects, such as the tip of a fingernail or the edge of a knife, are designed with a very small surface area to maximize the pressure exerted by the force.
Submarines must be specially designed to withstand the high pressures of being in deep water.
Barometric pressure is usually measured with a device called a barometer. Most barometers today measure atmospheric pressure in millibars.
Changes in barometric pressure are important to forecasters because they can indicate changes in weather.