If you've ever been in a physics classroom or read a paper in physics education research, you probably have heard students saying that current is "used up" in a circuit.
For example, in a one-bulb-one-battery circuit, a student might think that current is provided from the battery and transported to the bulb, where some of it is used up. Students often say that some of that current is returned to the battery to repeat the cycle, which is evidenced by the fact that you need a return wire. In thinking more specifically about measures of current, a student might say that there is more current before the bulb and less current after, because some of the current was used up. Based on this idea, once all the current is used up, the battery is dead, which explains why batteries die.
It is easy to focus on what's wrong about this:
- The current in a single-bulb-single-battery circuit is the same everywhere, not different. There are different ways to make sense of this, but a physicist often thinks of this in terms of the continuity equation– if charge isn't building up anywhere, the flow must be steady.
- The battery is also not a source of current. It provides the conditions for maintaining a difference in voltage, which provide further conditions for current to flow. For that reasons, batteries don't die when they run out of current; they die when they can no longer maintain a voltage difference. This voltage difference cannot maintained once the battery disassociates all of the ionic compounds inside (which is where the energy in the battery is stored as chemical-potential energy)
- Bulbs also don't use current, and current is not lost there. Current isn't "stuff" you can lose, because it's a process (the flow of stuff). But energy does "leave" the circuit system here in the form or radiated light and heat.
In order to talk about what's so great about this misconception, I want to engage you with this gem. The excerpt below is from a student enrolled my science teaching and learning seminar. This particular student has never taken any physics. They were asked to read a paper about children's mental models of electricity and pick one of the mental models to write about:
"I want to talk about Model C (the current consumed model) , since that is the model I am currently stuck on. I really have no knowledge of how a circuit works and what the circuit does to provide or result in electricity. But to me, it would seem that “energy” would be coming into the light made from the light bulb, and thus the energy leaving the bulb in the form of light would result in less energy leaving the bulb in the wire going back to the battery. Also, batteries die, so one would think that its ability to provide current to the system is lost in that process. It must have gone somewhere, right?OK. What has this student done:
"So Model C (which I suppose must be wrong) is reinforced by the simple ideas that we deal with regularly concerning electricity. We are told to unplug unused electronics to reduce the use of electricity… (but is that really what we are doing?) And how does this make sense: If current travels through a circuit and none is “used up”, why are our electricity bills so high? If we don’t actually use it up, why do we have to keep buying more? Model C just makes the most sense."
- First he identifies a puzzle: If energy is being lost at the bulb, shouldn't less of something being going in than going out?
- Second he identifies a bad solution to that puzzle: Asserting that the current must be the same doesn't resolve the puzzle.
- Third he identifies questions: Why do batteries die? Why do we pay for electricity?
- Fourth he is committed to sensibility (not authority): Despite knowing he is wrong, he is for all for Model C.
His misconception makes more sense than most students' acceptance of the right answer.
I have seen (to good effect) instructors and curriculum use some combination of these approaches toward "fixing" this current-used up misconception:
These approaches can be done together in a variety of ways that are effective, and they each embody something important about science and science learning:
- Engaging students with thinking about empirical evidence that support the idea the current is the same on both sides of bulb.
- Engaging students with arguments for why current cannot be different. (e.g., if it were different, you'd get a traffic jam or build up of charge; or by appealing to the of matter, charge, etc.)
- Engaging students with what current "is" - it is not a "thing" or substance that is used up, it is a flow of charge (a process);
- Engage student in learning about and applying Kirchoff's Laws.
- Empirical evidence - what data could I collect to help address this question?
- Argumentation - In light of evidence, what claims can I support?
- Ontology - what is the nature of this concept or quantity?
- Scientific Knowledge - how do scientists represent, talk about, and use these concepts?
But where do these strategies miss the mark?
They are all built around the idea that students' thinking has more to do with current, than the questions, puzzles, and the desires for explanation from which those ideas emerged.
From my experience, the students I encounter are not really trying to explain anything about current. They are trying to make sense of how bulbs and batteries work: They have ideas about what aspects of the situation are in need of explanation. They also have questions about how it works. They are capable of recognizing puzzles and inconsistencies. And they have some ideas about how they might explain and address those questions. The above excerpt has more ideas about "energy" than about current, and I'm impressed.
But I shouldn't be that impressed. Why? Because the excerpt above is fairly characteristic of the students I encounter in the real world when I talk to them about bulbs and batteries. Of course, if I hand them a multiple choice pretest about current, they look like they have a misconception about current. But when I talk to them away from worksheets and multiple-choice instruments–when they know that I care about what they think and that they will have to put some care and effort into expressing their ideas to me– they talk about evidence, and ideas, and inconsistencies, and questions. And sometimes they bring up ideas like energy, current, voltage, and sometimes they use those words in ways that I wouldn't. But is that a misconception? I'm not so sure.
The questions and ideas that arise in those conversations are way more interesting to me than this question:
Is the current at point C greater than, less than, or equal to the current at point D?So here's the problem:
--> Who asks these questions anyway? No one in the real world asks questions like that. My students are way more interesting and critical with their own questions.
The four strategies to eradicate students' misconceptions do not address any of the questions or issues that perplex students.
By focusing on immediately on current, we are simply trying to correct a detail of their thinking that we are unhappy about. In doing so, we stamp "current must be the same" over the students' interesting questions and insights. And, sure, maybe we even do this in an intellectually honest way so that they really understand deeply why current must be the same. But I fear that along the way we've lost their interest, curiosity, and sense of access to the phenomena. Or we've replaced with a false sense of interest–an interest in superficial understandings.
How I Reconcieve Misconceptions
I will say, in most classes I have experienced, I see instructors spending way too much time trying to eradicate misconceptions. I, instead, try to choose to pursue and explore the questions and puzzles students identify. Along the way, misconceptions seem to corrects themselves through the honest pursuit of students' ideas and questions, because we've been exploring and refining those ideas along the way. And while I may want to monitor misconceptions, I shouldn't be fixated on them or apply too much press on them. The truth is, the more pressure I apply to this misconception, the quicker students learn that I am there to correct them. Once that happens, game over.
So when my students say that current must be different, I don't hear a misconception. I hear an ingenious way of trying to answer the question, "Why do batteries eventually die?" and a good explanation for how bulbs can be a sink for energy.
When a student balks at the idea that "current is conserved", I think, "Great, it doesn't make sense, does it?" When students don't want to accept that answer, I see them as being committed to sensibility (not to authority).
I'm MORE worried about the students who quickly accept the "right" answers. I really am.
And then end of the day, I'm not interested in stamping out misconceptions and replacing them with impoverished ideas, nor am I interested in having students ignore perplexity in the world.