Air Pressure Curriculum
Section 3—Lesson 8: What is Charles' Law and How Does it Involve Relational Causality?
Background Information
Charles' Law Describes the Relationship Between Volume and Temperature When Pressure Remains Constant
Understanding pressure-related phenomena involves being aware of different variables and the relationships between them, and analyzing a given situation systematically. Scientists analyze the behavior of a system to see what rules govern its behavior. Scientists have discovered several laws that nature consistently follows in relation to air pressure. The last lesson explored Boyle's Law, which states that at a constant temperature, the pressure times the volume of an enclosed gas remains constant. When one increases, the other decreases to maintain equilibrium. The activities offered a number of examples of Boyle's Law; for instance, the bag and the jar activity from Lesson 5 and the stoppered flask in the three flasks activity from Lesson 7.
What happens when the variable of temperature is introduced into the equation? This lesson addresses Charles' Law. According to Charles' Law, in a closed system, an increase in temperature (T) results in an increase in volume (V); or a decrease in temperature (T) results in a decrease in volume (V) to maintain a constant pressure (P). In formulaic terms:
Taken together, Boyle's Law and Charles' Law completely describe the relationships between the pressure, temperature, and volume of a gas in a closed system. A "closed system" refers to a system that is not open to input or exchanges with the outside environment. (While no system can ever truly be closed, systems can be closed along certain parameters or to a certain extent. For instance, it is possible to construct a container that is air-tight so that it does not exchange gases with the outside environment, or to minimize temperature exchanges with the outside by insulating a container.)
Charles' Law and Relational Causality
A relational causal model nicely demonstrates the relationship that Charles' Law describes. Charles Law stipulates the relationship between temperature, volume and pressure. If temperature increases, then either the volume or the pressure (or some combination of the two) will increase. The opposite is also true. If temperature decreases, then either the volume or the pressure (or some combination of the two) will decrease. Pressure will only increase if the volume is held constant. In a flexible container, volume will increase (so pressure remains constant). In a closed and rigid container, volume stays constant and pressure increases instead. (This is in an ideal world with an ideal container. In the real world, there is some combination of change in pressure and change in volume to accommodate the temperature change.)
This makes sense from a molecular point of view: increasing the temperature of a gas causes the molecules to move faster, hitting the sides of the container or closed system more frequently and with more force. In order to maintain constant pressure, and knowing that pressure is defined as force per unit area (or P = F/A), the area that the gas is in contact with must increase as much as the force of the molecules hitting the container does. This results in an increase in volume. A flexible container, such as a balloon, illustrates this principle well. If the gas were kept in a rigid container with a fixed volume, an increase in temperature would result in an increase in pressure—more force from the molecules hitting the container, without any increase in area.
In this lesson, students do an activity that involves heating the air within a flask. This causes the molecules to move at a faster rate, hitting the sides of the container more frequently, which increases the force on the container walls. The volume of the gas increases, and a balloon on the flask inflates to maintain the air pressure inside the flask until it is at equilibrium with the outside air pressure. When the flask is removed from the heat source, the particles cool and slow down. This causes the balloon to deflate until the air pressure is again at equilibrium with the outside air pressure.
