Pressure: Definition, Units, and Conversions

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  • 0:05 Introduction: Pressure
  • 2:04 Atmospheres
  • 2:52 Millimeters of Mercury
  • 3:59 Converting Among Units
  • 4:42 Sample Problem
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: Kristin Born

Kristin has an M.S. in Chemistry and has taught many at many levels, including introductory and AP Chemistry.

Have you ever wondered what pressure is and how it gets measured? In this lesson, we are going to define pressure and explain some of the units that are used to express measurements of pressure.

Introduction: Pressure

Johnny Dalton and his family have just arrived on Ideal Island, where all gas particles behave ideally. They move rapidly and randomly, they don't interact with each other, they have elastic collisions (meaning they don't lose energy when they collide), and they are point particles (meaning the individual particles don't have any volume).

Just like when you travel to a foreign country and have to use a different currency or different units to measure things, Johnny must use different units on Ideal Island. Today, we are going to discuss pressure and the pressure units that are used here on the island.

I want you to think about the last time you measured the pressure of something. It may have been the pressure of the air in your car tires. Do you remember what it was? The air in my car's tires is 32 psi. Psi represents the pounds per square inch, which is a common unit for pressure. Let's look a little more closely at 'pounds per square inch.' What does a pound measure? If you said weight, you are correct. Weight is just the force of gravity's pull on an object - we measure it in pounds. Now, the square inch is just an area. If we put the two parts of 'psi' together we get the definition for pressure: the force per unit area.

Pressure in a tire is caused by the air molecules hitting the sides of the tire.
Pressure Form Molecules in Tire

So, now we know what pressure is, but what causes it in our tires? To answer this, I want you to picture the gas particles flying around inside of your car's tires. They're in there flying past each other, hitting each other, and hitting the walls of the tires. The pressure comes from each time they hit the inside walls of the container (or, in this case, it would be the tires). The more times they hit the inside walls, the higher the pressure in the tires. So, if you keep adding air to the tires, you're going to increase the number of particles that hit the inside walls of the tires and increase the pressure.


Let's go back to the unit that we use to represent pressure: psi. This is one of many different units that can be used to measure pressure, and it's probably the one that you use most on a daily basis in the United States. But, on Ideal Island, different units are used, so it's important to know what they are and how to convert among them. On the island, the most common units of pressure that are used are atmospheres (atm) and millimeters of mercury (mmHg). 1 atmosphere is often abbreviated as atm, and it's really just the weight of all of the air above you 'pressing down' on you while you stand at sea level. Now, if you were on a mountain, there would be less air 'pressing down' on you, so the pressure would be lower (less than 1 atmosphere).

Millimeters of Mercury

The unit millimeters of mercury (mmHg) goes back to the device used to measure pressure, the barometer. In the barometer below, we have an inverted tube containing a column of mercury with the chemical symbol Hg, and it's sitting in a pool of mercury. As the atmosphere 'presses down' on the mercury pool, the liquid extends up into the inverted tube, and the height is measured in millimeters (or some other length measurement). So, if the weather is changing, resulting in an increase in the barometric pressure, the increase is measured by how far up the mercury extends in the column. At sea level, the mercury extends about 760 millimeters up the column.

A barometer is used to measure the atmospheric pressure created by weather changes.
Barometer Diagram

The unit 'millimeters of mercury' can be pretty confusing because it seems as if it's a unit for length, not pressure. Sometimes you may see the unit 'torr' used instead. A torr is equivalent to a millimeter of mercury, and it's named after Evangelista Torricelli, the first person to use a barometer to experiment with and document the pressure of the atmosphere.

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