Have the inside walls of the container (or,

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.

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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.

Atmospheres

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.

Converting Among Units

Mercury barometers are not used much anymore because mercury is a toxic liquid, but the unit ‘mmHg’ is still used as a unit of pressure.

When converting among the different units of pressure, it’s important to know how they relate to each other. 1 atmosphere (the force of all of the air ‘pushing down’ on you at sea level) is equal to 14.7 psi (the unit you use to measure the pressure in your car’s tires). 1 atmosphere is also equal to 760 mmHg (or 760 torr). The two most common units used on Ideal Island are atmospheres and mmHg.

Sample Problem

Let’s try a practice conversion. Say you’re hiking up a mountain on Ideal Island and the pressure changes from 1 atmosphere at sea level to 0.

89 atmospheres at the top of the mountain. How many mmHg would this be equivalent to? Now, you may be able to easily figure out the answer to this question using a calculator, but I am going to set it up using dimensional analysis.First, I start out with my given: 0.89 atmospheres. I will then multiply by a conversion factor, which is just a fraction equivalent to 1. My conversion factor is 760 mmHg over 1 atm because the numerator and the denominator are equivalent pressures. I will then multiply the two fractions, canceling out atmospheres, giving me 676 mmHg as the answer.

Lesson Summary

Pressure is defined as the force per unit area, and when a gas fills a container, the particles sometimes collide with the inside walls of the container, which causes the container to have a certain amount of pressure. You may measure pressure in psi, but the two units that are most commonly used when measuring pressure of an ideal gas are atmospheres (atm) and millimeters of mercury (mmHg). An atmosphere is the amount of pressure you ‘feel’ at sea level on a normal day. It is equivalent to 14.7 psi and 760 mmHg (or 760 torr).

Learning Outcome

After watching this video, you’ll be able to explain the units of measurement for pressure (psi, atm, mmHg and torr) and calculate conversions between those units.

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