Simple Circuits Curriculum

Section 2—Resources

Picture of Practice

Comparing Cyclic Sequential and Simultaneous Models for Circuits: An 8th Grade Class Discussion

The following picture of practice describes a lesson in which students discuss the behavior of electrons and protons in a circuit, and the way in which the interaction within a wire and battery results in the flow that causes a bulb to light. Through this discussion with their teacher, students learn that there are two ways to think about the cause of flow (or current). One is a cyclic model that occurs in a sequential fashion, where a cause leads to an effect. The other is a cyclic model that occurs simultaneously, where effects also act as causes within the system, resulting in simultaneous cause and effect reactions.

• Ms. Hughes: Class, today we're going to discuss why the flow of current in a circuit causes the bulb to light up. Before we begin, are there any questions from last time?
• Sara: Yeah, I have one. You know how the protons and electrons are still going through this part here and back into the battery (pointing to the wire), but wouldn't the neutrons be attracted to the protons here, so then wouldn't there be a bunch of neutrons in the top of the battery?
• Ms. Hughes: Actually, neutrons don't have a charge, they aren't part of what is involved in attraction.
• Sara: I meant the protons and electrons.
• Ms. Hughes: Oh, okay, say it again then?
• Sara: Wouldn't the electrons be attracted to the protons when they go back into the battery?
• Ms. Hughes: Yes!
• Sara: Then wouldn't there be a bunch of electrons in the positive part of the battery?
• Ms. Hughes: Yes, they get attracted back in and then the chemical pushes them to the negative terminal. That's why the battery's doing work. And that's why batteries die. People think that batteries are like a little container of energy and they just send it to the bulb and the bulb is just eating it up. That's not quite how it works. There is a chemical in the battery that has the ability to do work or push apart these protons and electrons that are so attracted to each other that they want to be together.
• Sara: I have another question. Aren't there protons on the wire? What are they doing?
• Ms. Hughes: Um, the protons, let's talk about the protons all along this wire (pointing to the mat on the floor). The protons don't move. Only the electrons move along the path...
• Sara: ...so the electrons are attracted?
• Ms. Hughes: The electrons are moving to the next proton partner. They're pushing the one in front, who's pushing the one in front, who's pushing the one in front. And the whole thing is pushing at once like this (demonstrating with hand-turning motion), so it's turning at once because all of them...its not like one is the cause and one is the effect. Its like, one is the cause of the next one and that's the effect, but then that one is the cause of the next one, is the cause of the next one, is the cause of the next one. So as more get pushed along the wire, every single electron is repelling the one in front of it. Okay? And that's why you get this constant flow. We're going to come back and talk about this a little bit more. First, we'll talk about different kinds of cyclic causality. Let me talk about two different types. Now, some people when they think about circuits what they think about is this whole thing empty (pointing toward the mat with the circuit drawn, wire and battery). And they think when you hook the battery up, electrons start marching along and they get up to the bulb.
• Justin: That's wrong.
• Ms. Hughes: How is it wrong? How do you know?
• Justin: Well, everything is made up of atoms and atoms have protons and electrons. And the wire's copper, it's made of atoms, so it already has protons and electrons in it, so when you connect it, it is already a cycle.
• Ms. Hughes: That's right. What would you see when we hooked up the bulb to the battery if it took a while for the electrons to get up there—if this were empty? What kind of evidence would we look for to determine if that is how it works?
• Emma: There'd be a delayed reaction.
• Ms. Hughes: Yes, there'd be a delayed reaction. So if there were no electrons in the wire, and you had to wait for them to get to the bulb every time you turned on a light switch, you'd be standing there waiting for the electrons to get through the circuit. You'd have to wait for them to get all the way to the light. But because it's already filled up, it happens almost immediately when you flip the switch, the flow starts moving. So it's not a cyclic "one-at-a-time," cyclic sequential thing, where it is taking a little march along the wire. It's a cyclic "all-at-once" or simultaneous event.