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Are we teaching electricity the wrong way around?

It's the beginning of another school year, and another crop of year 9/10 students is about to not get the hang of what electricity is all about, writes Bernie Hobbs.

Your 14-year-old kids have probably spent as much of their summer holidays on their phones or computers as they have at the beach or moaning about how bored they are.

Yet when they learn about electricity in year 9/10 science this year, it won't be anything to do with the electronics they're immersed in, or the Wi-Fi that enables it. They'll just learn how to build circuits that make light bulbs glow, and how to calculate current, voltage and resistance.

I've got a real problem with this. If we're only teaching about direct current (DC) circuits, we're missing a great opportunity to engage kids in the science behind the technology they actually use.

That would be okay if the analogies and concepts we used to teach DC could be just as easily applied to alternating current (AC) and the transistors, capacitors and photovoltaics behind our favourite technologies. But they can't.

The waterfall analogy — where the height, flow rate and number of rocky obstacles in a waterfall equate to voltage, current and resistance — has no relevance beyond simple battery-based circuits.

Worse still, it — and a lot of our language around circuits — feeds into the idea that moving electrons themselves carry energy from one part of a circuit to another. It's a lie.

The truth is way more spectacular: the energy doesn't travel through the wires at all — it shoots through the space around them, at the speed of light. (Way to bury the lead, science!).

The thing is, there's a conceptual model that explains what's happening in a simple battery/bulb circuit and that works equally well for high-end AC circuits and everything in them. The same concept that actual physicists use (see below). So kids don't need to learn a DC 'waterfall' model of current that they'll have to abandon if they go on to learn about anything more advanced than a torch.

And better still, it builds on a concept that any kid who's played with magnets or seen just about any sci-fi is already acquainted with: force fields. Once you start seeing electric phenomena in terms of electric and magnetic fields, the science behind all appliances suddenly becomes clearer.

I'm definitely not the first to suggest that we teach electricity from a fields perspective. Ian Sefton from Sydney University's Physics Education Research Group was running workshops for physics teachers on the topic over a decade ago.

His paper spells out how the fields model removes the need for both waterfalls and confusion around how the hell energy gets from the battery to the bulb.

It completely blew me away. As did the (many!) discussions I had with the physicists advising me on our new online science game, ABC ZOOM. For me, fields reveal the commonality between all electric phenomena — in technology and in living things — way better than any other model. They're a one-stop mind-picture that links the humble battery and the microwave Wi-Fi signal.

There's just one problem with them — some hard core maths.

Physicists need to be guns at maths, but the rest of us don't. Sefton's paper uses maths-heavy Poynting vectors and Maxwell's theory of electromagnetism to back up his point, which is great for physics teachers and senior physics students.

But I think we can use a simpler, qualitative "Maxwell-lite" version of the story to explain at year 9/10 level just what's going on in that circuit, as well as in more contemporary appliances. (We did just that in our science behind the touchscreen video accompanying ABC ZOOM).

That way the students who go on to study physics will be able to build a proper vector quantity model on to their naive version of fields, and the rest of us can just stick with our simple, but accurate understanding. No waterfalls required.

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Maxwell-lite: fields, current and energy for beginners

The starting point for any story about electricity is charge, so before leaping into circuits, we need to get familiar with the basics — the behaviour of electric charges, and the connection between charge and electric and magnetic fields, and energy.

Remember, I'm not talking about a fully-fledged vector quantity understanding of fields here, just a qualitative notion. We're after a replacement for the waterfall voltage analogy that will work in all electrical situations, and that doesn't have to be un-learned and replaced later on, just "mathsed up".

The main points are:

Separating positive and negative charges creates an electric field with stored energy. Whenever charges are moving in an organised way (like electrons in an AC or DC current), they create a magnetic field. If you've got an electric field and a magnetic field together you've got yourself an electromagnetic field — and energy will flow through that field.

Now applying those points to the battery/bulb circuit, the qualitative story goes like this:

The battery is a bank of separated charge, so it's always got an electric field around it.

When you hook up the circuit, the battery's electric field pushes and pulls on electrons on the surface of all the wires and the bulb filament. You end up with patches with more electrons and patches with less on the surfaces (see diagram below).

That uneven electron distribution on the surface of the wires is a form of charge separation, so it creates another electric field. This second field is inside the wire, pushing electrons in the wire towards the positive terminal. So it's this second electric field that causes the current to flow. And because there's a current flowing (charges moving in an organised way), a magnetic field is generated outside the wire.

Now there's an electric field outside the wire (from the battery) and a magnetic field outside the wire (from the current), so rule 3 applies — energy flows from the sides of the battery through the electromagnetic field outside the wires to the bulb.

So energy isn't carried by electrons or current in the wire, it flows (at the speed of light) through an electromagnetic field outside the wires. That's why the light glows instantly while the electrons move at a glacial pace.

And the current flows because an electric field pushes electrons through the wire in one direction. No waterfalls required.

This qualitative explanation really works for a non-physicist like me. And not just because it does away with the waterfall analogy and current carrying energy misconceptions. Or because it's screaming out for a nice animation to explain it. (Note to self: apply for funding …).

It works for the same reason most good conceptual frameworks work: it's a simplified version of reality as physicists understand it. And a bonus — it makes clear the link between electricity and magnetism from the get-go, as well as the link between between charge, electromagnetic fields and energy.

That alone is worth its weight in conceptual gold.

Thanks to Ian Sefton and Dr Mark Butler for their helpful discussions with me about this idea.

About the author: Bernie Hobbs is an award-winning science writer with ABC Science and former science teacher. Her most recent project — ABC Zoom — is an online science game, where you can zoom down to the cellular and molecular level in people and things and fix the micro worlds you find there.



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