19.0 - Introduction

We live in a vast ocean of gas called the atmosphere. Deeper than the seas, it

provides the oxygen we breathe, guards us against harmful radiation from space,

helps to regulate the temperature, and makes flight possible.

In addition to supporting life, gases are used in many engines, and this is an

important topic of study in physics. Transferring heat to a gas can cause it to

expand. As the gas expands, it is able to do useful work. To understand how

engines work, it is necessary to understand the behavior of gases.

In this chapter and elsewhere in this textbook, we focus on ideal gases. An ideal

gas can be modeled as a number of small, hard spheres (atoms or molecules)

moving rapidly around a container and colliding with each other and the container

walls in perfectly elastic collisions. This aspect of the study of gases, which focuses

on the motion of gas particles, is called the kinetic theory of gases.

To begin your study of gases, try the simulation to the right. In the container, each

particle represents a gas atom or molecule. A gauge in the simulation provides a

readout of the gas pressure inside the container. The pressure is a function of

various factors including the volume of the container and the speed and the number of the particles.

The pressure measured in the simulation is caused by the collisions of the particles with the walls of the container. Each time a particle strikes

a wall of the container, it exerts a force on the wall and the wall exerts an equal amount of force on the particle. The greater the speed with

which the particle strikes the wall, the greater the force the particle exerts on the wall.

As detailed below, you can change three properties of the gas. The changes you make will be reflected solely in changes in the pressure of the

gas. Other properties of the gas could also change, but in this simulation we hold those other properties constant.

Clicking the up and down arrows for volume causes an external mechanism to raise or lower the container’s lid. As the volume changes, the

gas particles continue to move at the same average speed. How do you think gas pressure will relate to the volume of the container? Will

decreasing the volume increase or decrease the pressure? Try it. Does the simulation help you see why there is a relationship between volume

and pressure? Consider how it alters the frequency of the collisions.

You can also vary the temperature of the gas. The volume will not change as you do this, since the lid’s position remains fixed. If you increase

the temperature of the gas, you increase the average speed of the particles in the gas, and if you decrease the temperature, you decrease their

average speed. Considering how the force and frequency of the collisions affects pressure, what do you think is the relationship between

temperature and pressure when the volume is constant? Again, run the simulation to confirm your hypothesis.

The final controller allows you to add or subtract particles from the chamber. The particles you add will move at the same average speed as the

gas particles already there. Do you think adding particles will result in more or less pressure? To test your conclusion, run the simulation to add

particles and observe the resulting pressure.

When you have finished answering the questions above, you have experimented with some of the essential properties of a gas. The simulation

should enable you to see relationships between gas pressure and the volume, temperature, and quantity of a gas.

19.1 - Ideal gas

Ideal gas: A gas that can be modeled as a set of

particles having random elastic collisions. The

collisions are the only interactions between the

particles.

The concept of an ideal gas is used frequently in physics, both in the study of gases

and later in the field of thermodynamics, where ideal gases play an important role. Real

gases tend to behave like ideal gases at low enough densities (where quantum

mechanical effects are not important). This makes ideal gases a good model for

analyzing gas behavior. Here, we summarize the major properties defining an ideal gas.

Some gases, like carbon dioxide (CO 2 ) and oxygen (O 2 ), are composed of atoms

bonded together in molecules. Other gases, like neon (Ne) and helium (He), are made

up of particles that are individual atoms. For the sake of brevity, we will simply say that

gases contain molecules (or particles), rather than repeatedly writing “atoms or

molecules.” When we analyze a gas, it is one made up of a single substance, such as oxygen or carbon dioxide.

An ideal gas consists of a large number of molecules. They are separated on average by distances that are large relative to the size of a

molecule. They move at high speeds and collide frequently.

Ideal gas

Molecules widely separated

Interact only in elastic collisions

(^360) Copyright 2000-2007 Kinetic Books Co. Chapter 19