Air Pressure Curriculum

Section 3—Lesson 5

Picture of Practice

Linear Versus Relational Causality: An Eighth Grade Science Lesson

This lesson describes how students explore linear and relational causality within the context of air pressure. Here, the bag and jar activity is used as a demonstration, as opposed to being done independently by students. However, you might overhear your students expressing similar ideas to those expressed below. This lesson addresses the idea that a linear model is inadequate for explaining why a baggie sealed onto a jar cannot be pushed into or pulled out of the jar.

Mrs. B. sets up the jar and plastic sandwich bag demonstration in front of the class. The class watches as she seals the bag onto the jar with a thick rubber band.

• Mrs. B: I've sealed this bag to the jar as you can see (she shows the setup to the class). I would like for someone to try to pull the bag out of the jar.

Mrs. B. walks over to Sarah. Sarah puts her hand into the jar and surprisingly finds she cannot pull the bag out.

• Sarah: I can't!
• Mrs. B: Why won't the bag come out of the jar?
• Rachel: It's a vacuum. (Token Explanation)
• Brian: It's a suction cup.
• Mrs. B: What do you mean by vacuum, Rachel?
• Rachel: Well you can't take it out cause it's sucking it in cause there's like air in there…it's weird.
• Mrs. B: How many of you think there is a vacuum inside there? What is a vacuum?
• Kobe: It sucks something up, like a vacuum cleaner.
• Mrs. B: How does it "suck something up?"
• Kobe: I don't know, it just does.
• Mrs. B: Often when we are asked to explain what causes something, we use words such as "vacuum" that we cannot fully explain. What do you think is really happening? Can you explain why the bag won't come out of the jar without using words such as "vacuum" or "suction cup?" Why don't you take a minute to draw out your ideas?

Mrs. B. goes around the room as the class is drawing out their ideas. She notices that Imir's picture is focusing what is happening inside the jar while Kendra's picture is focusing on what is happening outside of the jar. She asks them to put their models on the board for the class.

• Mrs. B: So what do you think of Imir's model and Kendra's model?
• Ned: Imir focused on the air inside and Kendra focused on the air outside.
• Nancy: If Imir's model is correct and the air is applying pressure to the bag, shouldn't it come out of the jar?
• Beth: Yeah, and if Kendra's model is right, shouldn't the bag get pushed into the jar?
• Mrs. B: Good points Nancy and Beth. Do you see what they mean class? Both Imir and Kendra used a linear model to explain their ideas. What do you think I mean by linear model?
• Jarrod: Well, something linear goes in one direction.
• Mrs. B: Right. So looking at their models, how can they be considered linear?
• Jarrod: Well, Imir's model looks at what's happening from the inside of the jar and Kendra's model looks at what's happening from the outside of the jar.
• Mike: Wouldn't you need to look at both to really explain what's happening?
• Mrs. B: Explain what you are thinking, Mike.
• Mike: Well, there's still air left inside the jar and you're trying to pull the bag out but the air's going into the jar too, it's pushing it in so you can't take the bag out.
• Mrs. B: Mike is reasoning about this in terms of a relationship. Many things that we talk about with air pressure involve a relationship. Although Imir and Kendra's models were good, they each only looked at one side of the relationship—they were linear. Sometimes linear models can be used to explain things, but at other times, you need to think about what is happening in terms of a relationship. Was there something you wanted to add, Sarah?
• Sarah: So, the air left in the jar is pushing the bag out and the air outside the jar is trying to push the bag in so that way you can't take it out, right?
• Mrs. B: Is this an equal relationship or is one greater than the other?

Mrs. B. hears a mix of responses from the class.

• Mrs. B: (referring to Imir's drawing) So you could expand this drawing to show that if you took the bag out, say from here to here, we sealed that bag around the jar, right? There's this air inside the jar; when you went to pull that bag out, did the amount of air in the jar change?
• Kendra: No.
• Mrs. B: What happened to it?
• Kendra: It expanded?
• Mrs. B: What do you mean by 'expanded'?
• Kendra: It spread out.
• Mrs. B: What does that cause?
• Yao: It causes less air pressure because there is the same number of molecules bouncing around in more space—so they can't make as much pressure.
• Gwyn: So the air pressure outside is more and prevents you from pulling it out of the jar.
• Mrs. B: Can you draw a model of that on the board for the class Yao or Gwyn?

After discussing Gwyn's relational model, the class moves on to the next demonstration, in which a bag sealed to the outside of the jar cannot be pushed into the jar.