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Property of Electricity You Didn't Learn in School

Tuesday, April 19, 2011

Electricity – the cleanest and most versatile form of energy the world has ever known – is not what you were taught in school. (It’s actually much cooler…)

You won’t believe this…

Let’s take the simplest example of electricity I can think of: a flashlight. How does it work? Well, you flip the switch and energy flows from the battery to the light bulb.

Yes, but how does the energy flow? Through the wires, of course! WRONG! In any electric circuit, the energy actually flows through the empty space outside the wires. The wires guide the flow of energy through space, but don’t carry any of the energy themselves.

In a flashlight, the battery sprays energy into the surrounding space, and this energy is then steered by the wires to enter the light bulb. Energy flows into the light bulb’s filament not through the wires connected to it, but from the empty space around it, heating it white-hot. I’m not making this up!

So how can this possibly make sense?

It’ll make sense as soon as you think about how energy flows from the Sun to the Earth via sunlight: Sunlight is waves of electric and magnetic field lines, like the static magnetic field lines that surround a magnet, except undulating. Wherever the electric and magnetic lines in sunlight cross each other, energy flows in the direction perpendicular to both lines.

I really want an image here.

The wires in our flashlight circuit serve to set up these same conditions of crossed electric and magnetic field lines in the space around the wires, and then energy flows by exactly the same principle. (For a fun animation on this, check out the Alice & Bob in Wonderland episode titled “How Does a Flashlight Work?” If you still don’t believe me, check out Eric Giler’s demos on wireless electricity on TED.)

Why didn’t I know this??

If you found this surprising, you’re not alone... 

A guy called Poynting figured this out way back in 1884, when Maxwell’s theory of electromagnetism was still a hot topic, but then the analogy of electricity as water in a garden hose took hold and this knowledge was lost to most of civilization. Now it’s mostly physics-keener-types who keep the torch lit.

So don’t worry, it’s not your fault.

(So what?)

Poynting’s discovery is at the core workings of a staggering amount of technology, from light bulbs, washing machines and electric cars to computers, cell phones and satellite HD.

Related to this is the even more basic idea that electric and magnetic fields store energy in space.

Energy storage is a hot topic these days: One cool example is supercapacitors currently used in some hybrid vehicles that store energy in an electric field for rapid acceleration and regenerative braking.

Another is superconducting magnetic energy storage (SMES) devices that may one day be used by electric power utilities; the SMES device would charge up at night (storing vast quantities of energy in a magnetic field) to meet peak loads during the day.

It all goes to show; sometimes analogies and stories help us see the wonders of nature. At other times, though, nature’s even more fascinating than some of the simplified stories we use to explain its fundamental concepts.


Written by: Richard Epp

Richard Epp has a Masters degree in electrical engineering and a PhD in theoretical physics from the University of Manitoba. He held postdoctoral research positions around the world working in general relativity before becoming Scientific Outreach Manager at Perimeter Institute for Theoretical Physics.