Simple Circuits Curriculum

Section 2: Background Information

Revealing the Causal Models Implicit in Students' Configurations

The purpose of this lesson is to get students to reflect upon and, hopefully, begin revising their mental models of how a simple circuit works. Setting up configurations of circuits that work and don't work, as in Lesson 1, is a way to get students thinking about their models. However, while students realize pretty quickly that a simple linear arrangement doesn't work, they tend to cling to aspects of the underlying Simple Linear Model. There are a number of things that students may say and do that will alert you to whether students truly have a cyclic model or are holding onto their linear models. For instance, some students say that the wires need to be in a circle, but they may think, "the other wire is just a ground." Some students understand the cyclic aspects of the circuit but say things like, "the wire is empty and the electrons travel to the bulb and light it up when they reach the bulb." These students often (erroneously) believe that if you extend the length of the wire, it will take longer for the bulb to light.

In order to prepare for this lesson, it is important that you carefully review the progression of models outlined in the introduction. You will most certainly recognize these models in your students' thinking. Because most students (and sometimes teachers) get stuck at a Cyclic Sequential Model and find it hard to make the leap to the Cyclic Simultaneous Model, this lesson focuses directly on this conceptual leap. If you find that your students are stuck at an earlier point in the progression, you will need to address those models first.

Moving Beyond the Cyclic Sequential Model

You can help your students move beyond the Cyclic Sequential Model by helping them realize that all matter is made up of atoms (which are made up of electrons, protons, and neutrons); and that therefore the circuit cannot be "empty." It is made up partly of electrons. Many students think of electrons as flowing "inside" the wire. Try to make them aware of this and the language that they use to reveal it. Encourage them to see the electrons as part of the metal that is conducting charge. You can also ask students to think about what happens when they flip a light switch. Do the lights on the end of the hallway take a perceptibly longer time to come on than those at the beginning of the hallway? Most students realize that this is not the case. However, don't be surprised if your students patch their current model by saying things like, "the electrons just speed up because they know that they have to go further." It can be difficult to give up a mental model that you strongly believe in!

Introducing the Cyclic Simultaneous Model

The Cyclic Simultaneous Model is an intermediate model designed as a bridge to more complex scientifically accepted models. It draws students' attention to the circuit as a system. How does it work? The wire is made up of atoms, so it already has electrons and protons all along it. They are balanced. Once you hook the battery and bulb up completing the circuit, electrons are repelled or pushed out of the battery on the negative side and attracted or pulled into the battery on the positive side. This makes the whole "circle of electrons" turn. At the particulate level, each electron repels the electrons "ahead" of it on the wire and is repelled by those "behind" it. On the systematic level, the whole circle moves as one, like a bicycle chain. Instead of one thing happening at a time, it happens all at once—it is simultaneous

What is the Role of the Battery in the Cyclic Simultaneous Model?

Understanding this model also depends upon having some knowledge of what the battery does. Later in the module, an in-depth lesson explores the role of the battery to support understanding of an Electrical Potential Model. However, the following information is enough to understand the Cyclic Simultaneous Model.

Why is the push of the battery important if electrons are being repelled and repelling all along the wires in the circuit? Why doesn't this repelling continue to create flow? In the Cyclic Simultaneous Model, something needs to keep the "bicycle chain" turning, so to speak. Without the push of the battery, the electrons would be attracted to the protons and stay at the positive contact. Students may realize that once electrons flow into the positive terminal and accumulate there, they could begin to repel the incoming electrons and stop the flow. Why don't they? The battery addresses this problem by accomplishing the task of moving the electrons back to the negative contact of the battery. The chemicals in the battery do the work of polarizing the protons and electrons to the plus and minus sides of the battery. The "work" of the battery results in an excess of protons on one end of the battery and an excess of electrons on the other. This is work because the protons and electrons are attracted to each other, and creating an excess of electrons (which repel each other) and protons (which repel each other) requires energy, which is provided by the chemicals in the battery.

So what is voltage? The battery is performing work. The negatively charged electrons are attracted to the protons. The higher the concentration of electrons on the negative terminal, the harder it is for the battery to push more electrons onto it. The higher the voltage of the battery, the greater chemical capacity it has to concentrate electrons on the negative contact. Voltage can be thought of simply as a push, or the force that moves electrons. Lesson 8 introduces a more complex way to think about voltage.