Visualising Electromagnetic Fields | Hacker News Reader

Visualising Electromagnetic Fields

lukesturgeon.co.uk

108 pointslelf33 comments

Some years ago, an artist-in-residence at Bristol's Physics dept planted fluorescent tubes under overhead power lines, which then glowed due to the field and could be played with by visitors like a plasma ball:

http://www.bris.ac.uk/changingperspectives/projects/field/

This installation gives a better intuitive sense of field strength, e.g. emissions still strong enough to cause fluorescence dozens of meters away. I'm curious how Sturgeon's visualizations would appear around household appliances with (relatively) high-amperage induction motors like vacuum cleaners.

Depending on the field strength, it can be sufficient to do more than light fluorescent tubes: https://www.youtube.com/watch?v=SD2dfMfgkTw

In high school it was a fun geeky thing to do and drive out into the desert where the power lines from Hoover Dam headed over to LA and light up fluorescent bulbs. There is a story, possibly apocryphal, that was told by one of the professors at USC of a guy who lived out there and used a few hundred feet of 12ga wire to build what would essentially be a secondary coil of an air gap transformer to power his shack.

I found it interesting but not interesting enough to pursue a career in power engineering :-).

That must be pulling a lot of energy out of the power lines huh? I wonder at what point people start complaining...

An explanation of this kind of visualization given in a stanford lecture by Steve Mann, see 35-45 min into video.

Stanford Seminar: 42 years of Phenomenological AR for Natural/Reality/Direct User-Interfaces https://www.youtube.com/watch?v=6IpBVC-p3iU

Example in video https://youtu.be/6IpBVC-p3iU?t=2436

For a basic magnet, there was an old science fair type of experiment where you laid the magnet on a piece of photo paper, sprinkled iron filings, flashed the paper and developed. Of course, these days you can just basically shoot a digital photo of the magnets and filings. Lots of examples online.

The one misleading thing about iron fillings is that they tend to arrange themselves into lines, but the actual magnetic field is continuous rather than made out of discrete lines.

That is, in a sense, just calculus: Building a curve out of a huge number of straight segments is immediately comparable to the method of Riemann sums.

https://en.wikipedia.org/wiki/Riemann_sum

I think it forms a nice bridge from univariate calculus to visualizing the vector fields.

It's not that though. The iron filing change the magnetic field making it denser where the filing are. So they self organize into reenforcing channels of flux. So it's not a true visualization of the field, because the field is changed by the measurements.

Useless pedantic then, it is not a "true visualization" of the B field in free space absent the filings. It is however, an approximate visualization of it.

These are pretty, and I can appreciate them as art. As a visualization, though, it doesn't seem very satisfying because most of these images are pictures of the path traced by the person waving the screen, not pictures of the electromagnetic field. The shape we see is mainly a visualization of whatever way the person decided to move the screen through space, that only incidentally happens to be slightly affected by the field.

Not sure why you're being downvoted, it's a good point. There's a couple of links here that time-lapse the meter across a consistent 2D or 3D voxel space which gives a better idea of the radiation field, but some are clearly just driven by an arbitrary path chosen by the photographer.

I've always wondered about this... if an EM field is composed of photons which have higher energy than light, how come I've never seen image/video captured where the device is 'glowing' or something as a photon source? Is there something I'm missing that prevents us from detecting them similar to the way we detect visible light?

Nope, it is definetly possible to capture images in the non-visible spectrum. We simply need to convert them into the visible spectrum using some transform (usually just translation) so we can see them.

You probably have seen images like this, you just might not have realized. Many of the most beautiful astronomy photographs are actually captured in different spectrums. If you've ever seen a "thermal image" that is also non-visible photons.

Some examples:

https://asd.gsfc.nasa.gov/blueshift/wp-content/uploads/2013/...

https://smd-prod.s3.amazonaws.com/science-red/s3fs-public/th...

http://www.greenbuildingadvisor.com/sites/default/files/Ther...

> how come I've never seen image

Of course you have, it's called an x-ray.

> if an EM field is composed of photons which have higher energy than light

That's not right - visible light is sort of a "midrange" photon energy that's exceeded by ultraviolet, x-rays and gamma rays and preceded by infrared, microwaves and radio waves. Everyday devices emit at low frequencies, with energies significantly below that of visible light. The higher energies (ultraviolet, xray, gamma) absolutely do have images:

- Ultraviolet [https://en.wikipedia.org/wiki/Ultraviolet_photography#/media...]

- Xray https://en.wikipedia.org/wiki/X-ray#/media/File:Radiograf%C3...

- Gamma https://en.wikipedia.org/wiki/Gamma_ray#/media/File:VACIS_Ga...

Now, the photons with lower energies than visible light form an interesting case. First off there's infrared, which is fairly familiar:

- Infrared https://en.wikipedia.org/wiki/Infrared_photography#/media/Fi...

Now, as the photon energies get lower the wavelengths get longer. This introduces a problem for lower-energy images: when the wavelength is around the size of the aperture, wave-like things will happen involving the aperture. This tends to blur the images, meaning that for longer and longer wavelengths larger and larger cameras are required to achieve the same level of detail. However, these images still exist - but they are rarely taken of everyday things. Note that the imaging you are talking about (to capture the glowing of everyday electronic devices in low frequencies) would have to happen in the microwave and radio bands - because that's around the frequency at which electronics operate. The result is that a reasonably-sized camera would not be able to take a meaningfully sharp image of a router's "glow."

- Microwave https://en.wikipedia.org/wiki/Microwave_imaging#/media/File:...

- Radio Waves http://www.gb.nrao.edu/epo/PageMill_Resources/galaxy.jpg

The radio wave picture depicts a galaxy. Radio imaging setups are usually very large, like this one: http://mstecker.com/pages/astroVLAa15vlad-2c1.htm

So does this mean infrared wavelengths are around the lower limit for taking images with a reasonable size camera? I've played around with the thermal IR camera attachements for smartphones (i.e https://www.amazon.com/FLIR-ONE-Thermal-Imager-iOS/dp/B00VIL...) and they're pretty small.

The law for diffraction-limited systems is that the blurred spot size increases linearly with wavelength. (Double the wavelength, all else being equal, means twice as blurry.) Engineering concerns (for example, how big can you manufacture a CCD before it becomes prohibitively expensive?) determine what constitutes an "excessively large camera." Although, because infrared covers a 1000-fold range between visible and micro, I feel like it would be safe to say that cellphone-sized cameras might never be made for microwaves.

[0]https://en.wikipedia.org/wiki/Diffraction-limited_system

In addition to other replies, you can easily "visualize" IR light from say a remote control. Just point your phone's camera to the bulb on the end of a remote that you point at the device it controls. You'll see a flash on the display because the IR flash from the remote saturates the CCD in your phone.

Light is an EM vibration, but the fields here are essentially stationary potentials (similar to a bar magnet, or static electricity.) So it is not light, and not composed of phitons (that last part might be a simplification.)

This is cool and reminds me of avalanche rescue training as you have to learn to follow flux lines during searches: http://beaconreviews.com/transceivers/rescue_searching_coars... edit: better link

Excuse my question. I'm curious about a basic formula to calculate magnetic flux at a given distance from an AC conductor at a given power level?

http://www.softschools.com/formulas/physics/magnetic_field_f...

the strength of the magnetic field is equal to the (current times the permeability of the medium) divided by (the distance times 2 pi).

magnetic flux is the dot product of the magnetic field and the area vector representing the area through which you want the flux. theres another way to express flux as a ?derivative of voltage? but I can't find a reference and the exact relationship escapes me at the moment.

thanks

Another take on this (Light painting WiFi by Timo):

https://vimeo.com/20412632

Hard to believe. What's the secret to see them ?

A simple coil of wire could act as a receiver. And attached to a light emitter, the strength of the signal would translate to a brighter light.

Just a guess though.

They have a video you can watch

The video doesn't really explain what they're doing though. Can anyone explain how they're visualizing the EM fields here?

It looks like they have a metering device that glows different colors based on the strength of the field detected. They then appear to be systematically moving the meter across a grid with time lapse photography. The lights photographed from different positions are blended together to create a voxel-like view of the field.

Thanks for explaining that makes sense. What kind of metering device will glow based on these kinds of EM fields?

Similar to this project:

https://hackaday.io/project/4329-wifi-power-mapping

Also see this video which goes over the concept:

https://www.youtube.com/watch?v=aqqEYz38ens

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