Science, Tech, Math › Science Pressure Definition and Examples Pressure in Chemistry, Physics, and Engineering Share Flipboard Email Print Gas exerts pressure on a balloon, causing it to expand when you blow it up. ABSODELS/Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated May 07, 2019 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). 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. Usually, pressure is a positive value. However, negative pressure is possible. 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.