Pressure Definition and Examples

Pressure is defined as a measure of the force applied over a unit area. Pressure is often expressed in units of Pascals (Pa), newtons per square meter (N/m² or kg/m·s²), or pounds per square inch. Other units include the atmosphere (atm), torr, bar, and meters sea water (msw).

Pressure Formula

In equations, pressure is denoted by the capital letter P or the lowercase letter p.

Pressure is a derived unit, generally expressed according to the units of the equation:

P = F / A

where P is pressure, F is force, and A is area

Pressure is a scalar quantity. meaning it has a magnitude, but not a direction. This may seem confusing since it's usually obvious the force has direction. It may help to consider pressure of a gas in a balloon. There is no obvious direction of the movement of particles in a gas. In fact, they move in all directions such that that the net effect appears random. If a gas is enclosed in a balloon, pressure is detected as some of the molecules collide with the surface of the balloon. No matter where on the surface you measure the pressure, it will be the same.

Simple Example of Pressure

A simple example of pressure may be seen by holding a knife to a piece of fruit. If you hold the flat part of the knife against the fruit, it won't cut the surface. The force is spread out of a large area (low pressure). If you turn the blade so the cutting edge is pressed into the fruit, the same force is applied over a much smaller surface area (vastly increased pressure), so the surface cuts easily.

Can Pressure Be Negative?

Pressure is generally a positive value. However, there are cases that involve negative pressure.

For example, gauge or relative pressure can be negative. This commonly occurs when pressure is measured relative to atmospheric pressure.

Negative absolute pressure also occurs. For example, if you pull back on the plunger of a sealed syringe (pulling a vacuum), you generate negative pressure.

Pressure of an Ideal Gas

Under ordinary conditions, real gases behave like ideal gases and their behavior is predictable using the ideal gas law. The ideal gas law relates the pressure of a gas to its absolute temperature, volume, and the amount of gas. Solving for pressure, the ideal gas law is:

P = nRT/V

Here, P is absolute pressure, n is the amount of gas, T is absolute temperature, V is volume, and R is the ideal gas constant.

The ideal gas law assume gas molecules are widely separated. The molecules themselves have no volume, do not interact with one another, and experience perfectly elastic collisions with the container.

Under these conditions, pressure varies linearly with temperature and amount of gas. Pressure varies inversely with volume.

Liquid Pressure

Liquids exert pressure. A familiar example is the feeling of water pressure you feel on your ear drums when you dive into a deep pool. The deeper you go, the more water there is above you and the greater the pressure.

A liquid's pressure depends on its depth, but also on its density. For example, if you dive into a pool of a liquid that is more dense than water, the pressure will be greater at a given depth.

The equation that relates pressure in a liquid of constant density to its density and depth (height) is:

p = ρgh

Here, p is pressure, ρ is density, g is gravity, and h is the depth or height of the liquid column.

Sources

• Briggs, Lyman J. (1953). "The Limiting Negative Pressure of Mercury in Pyrex Glass". Journal of Applied Physics. 24 (4): 488–490. doi:10.1063/1.1721307
• Giancoli, Douglas G. (2004). Physics: Principles with Applications. Upper Saddle River, N.J.: Pearson Education. ISBN 978-0-13-060620-4.
• Imre, A. R; Maris, H. J.; Williams, P. R, eds. (2002). Liquids Under Negative Pressure (Nato Science Series II). Springer. doi:10.1007/978-94-010-0498-5. ISBN 978-1-4020-0895-5.
• Knight, Randall D. (2007). "Fluid Mechanics". Physics for Scientists and Engineers: A Strategic Approach (2nd ed.). San Francisco: Pearson Addison Wesley. ISBN 978-0-321-51671-8.
• McNaught, A. D.; Wilkinson, A.; Nic, M.; Jirat, J.; Kosata, B.; Jenkins, A. (2014). IUPAC. Compendium of Chemical Terminology (2nd ed.) (the "Gold Book"). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.P04819. ISBN 978-0-9678550-9-7.