Causality, Again

The first-year students are solving series circuits and explaining what’s happening.  Most are able to connect their answers and thoughts to evidence we’ve gathered this semester.  But most are struggling with the questions about causality.

For each effect they describe mathematically, I ask them to explain what is physically capable of causing that effect. Or, they can choose to explain why the result seems like it can’t be happening.   It doesn’t have to be cannonical, but it must be internally consistent, not circular, and supported by our evidence.  They are struggling most with explaining Kirchhoff’s Voltage Law.  This is understandable — I don’t think I could explain it heuristically either.  However, only one student took the opportunity to say why it doesn’t make sense.

We’ve done lots of practise writing cause statements.  They know what “begging the question” means.  I’ve modelled, and we’ve practised, the importance of saying “I don’t know” when that’s the most accurate thing we can say. Examples of student thinking are below.

I’m tempted to propose a taxonomy of acausal strategies.  Which examples of student thinking do you think fit where?  Would you add or remove categories?   Could you propose some pithy names for them?

  1. It does that because it’s designed to do that
  2. It does that because if it didn’t, this other important thing wouldn’t happen
  3. It does that because there’s a law that says it has to do that
  4. It does that because it does that (begging the question)
  5. It does that just because

My questions are:

“An electron has to use up all its energy that it gets from the battery.  This is caused because if all of the energy wasn’t used, the circuit wouldn’t give accurate results, or work properly.”

“When electrons pass through a component, that causes them to lose energy.  The electrons would have to be able to flow through the circuit in order to keep the current and battery functioning.”

“An electron has to use up all the energy it gets from the battery.  This is caused because if the voltage from the power source is 5V, the electrons have to use up all of their energy, in this case they use up all of it in the resistor (except for the little energy used in the switch).”

“The electrons always use up exactly the energy they gain in the battery because of conservation of energy.”

“It doesn’t make sense that if there’s only one component in the circuit, it always uses up exactly the battery’s voltage.  A higher resistor should be like a steeper hill — harder for the electrons to get past, and requiring more energy.”


  1. Tackling my own questions… they may be answering
    a) how we know that it *is* happening
    b) why it *must* always happen (a very closely-related question indeed!)
    c) what evidence we have that it happens
    d) what makes me think it’s happening (what causes my thoughts, rather than what causes the electrons’ behaviour)

  2. I am thinking that an investigation into measurement is needed very soon. Once they realise that it all boils down to the magnetic effect of a current in a wire they will become more cautious about assigning causes. I often (not that often) think about how Ampere, Volta and Dr. Ohm figured it all out, from a state of ” WTF is going on here ???”. Have they realised that the current in a simple circuit is the same everywhere? Are there any other than handwaving methods to explain resistance? I personally would have left electrons out of it for at least a year!

  3. These are all great questions! It’s interesting to see how they struggle with Kirchhoff’s loop rule. I wonder if they’d feel the same doing just nodal analysis (like how p-spice does it).

    Your questions:
    a) What do the students know? That’s hard for me. All I ever know is what they say!
    b) Why don’t more students say “I don’t know?” In my class I’m afraid it’s because they think (and I suppose they’d be often right) that I’ll just redo the same thing that they didn’t get the first time.
    c) space/time causality: This is a great question! I’m trying to think of things n my own life/experience that I don’t know how or why it works but I’m just happy to say that it does. Nothing comes directly to mind, but I’ll keep thinking about it.
    d) next move? Well, based on the awesomeness of nearly everything you do (or at least what you tell us about), I’m pretty sure it’ll be something I’ll want to emulate (like how I dodged that?)

    #NaBloCoMo #6

  4. Why do so few students say .. “it doesn’t make sense”?

    Because they know it does make sense, just that they can’t make sense of it. I get annoyed when students claim that things are “stupid” or “don’t make sense” just because they don’t understand them. I would have little respect for older students who act in that way.

    At best they could say “I can’t make sense of it”, which is effectively “I don’t know”.

    And why don’t they say “I don’t know”. Well younger ones do. Frequently. Sometimes even when they do know.

    Saying “I don’t know” can be seen as an admission of failure. Most adults aren’t big on admitting failure.

    Your students are who they are. I wonder it you making them “better” by being able to answer the way you want them to is the best use of your energies.

  5. Searching for ” cause” and “effect” is not always very productive in Physics. Often one has to be content with “If I press the switch the light comes on”.
    Is it really the flow of electrons that makes the bulb light up?
    What caused the apple to fall?
    Where is the boundary or overlap between explanation and causality?

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