Physics question: Emanation of magnetic fields vs. electric fields

According to this link, magnetic fields ‘form a kind of bubble, which stops growing after four feet’, whereas electric fields ‘waft out like ripples in a pond until they reach an antenna.’

How can this be so?

Do magnetic fields not fall off with the square of the distance as do the RF signals I am familiar with?

If not, what causes magnetic fields to emanate less futher than electric fields?


The article talks about very short distance communication via magnetic induction. This is nothing new, as it’s basically just an air-core transformer. But it does have quite a few advantages over an electromagnetic wave (wherein you have orthogonal electric and magnetic fields traveling through space), as the article mentions.

Since a static magnetic field can only be generated by a dipole, where there is a North and a South, the strength of the field depends on your distance compared to the distance between the North and South poles. When you get farther away, their effect is almost zero because from your point of view, they cancel each other out.

With the electric field, you have charges, which decrease with the square of the distance, so are strong farther away. If there were such a thing as a magnetic monopole, it would also extend out with more strength.

But what causes the advantages?

Is it simply that the weaker magnetic field is far enough down that the noise swamps it at four feet?

I posted too soon – thanks CurtC.

I think the idea is to use a static (or quasi static) magnetic field as you would find from a bar magnet or solenoid. Now instead of the propagating electric/magnetic field we normally use for wireless communications you rely on the fact that a static magnetic field will change when another coil is introduced. The change in magnetic flux leads to a voltage change which can be used to drive a communication signal.

Since there is no propagating electric/magnetic wave the strength of the magnetic field drops off quickly. Same thing happens with static electric fields too.

I guess we can now make these coils small enough to be placed anywhere. Neat idea.

On preview CurtC said it better.

Having glanced over the linked materials, I would say that the writer of the “popularized” article has lazily exaggerated a few words of hype from the Aura people.

Yes, a magnetic field does fall off much more rapidly–more and more much-more, if you will–than EM oscillation propagation. However, I believe it never literally “falls to zero”; it’s just that, with regard to some particular system, there will come a nearby point at which it’s so weak that you can’t do anything with it.

Always doing my bit to give credit to my now-long-forgotten mentor, I want to note that Tom Swift Jr., 18 years of age, had a home security system based on this exact principle, down to the little coils. And that was in 1954! God DANG that guy was smart!

You may think it a real stretch to attribute such a thing to a teenager, but I see the inventor of the Aura device is a 9-year-old kid! Sometimes I think the toddler next door is spying on me…

It’s wrong. Magnetic fields and electric fields behave essentially the same (forming the classic dipole pattern, and vibrating in and out.)

It’s EM RADIATION that’s the different one. Radio waves waft out like ripples, while magnetic or electric fields act like a bib bubble which expands and contracts.

Right, magnetic or electric fields fall off much faster (by 1/R^3 if I recall, but I might be wrong, it might be even more.) In other words, a transformer made from two coils is fundamentally different than a radio transmitter/receiver using loop antennas.

Here’s an MIT physics class page with an animation of a radio antenna.

Electric Dipole Radiation (quicktime)

Hit “play” repeatedly and watch what happens in the region near the antenna. See the “bubble” which grows and shrinks? That’s the antenna’s magnetic (or electric) field. The rest of the stuff is electromagnetic waves which are being “shed” by this vibrating field. If the antenna is extremely small compared to the radio wavelength, then there won’t be any radio waves, just the expanding/contracting bubble in the center. The device then becomes a transformer, not a radio transmitter.

bbeaty, either electric or magnetic dipole fields fall off as r[sup]-3[/sup], but of course as was noted above, there are also electric monopole fields which follow the more usual r[sup]-2[/sup] law.

That said, electric dipole fields are the ones in which anything interesting actually happens, and in particular you need a dipole field for EM radiation to start happening.

Good point. Of course, it’s difficult to make a tiny portable transmitter that emits a vibrating monopole field.
Also: the field from a coil or from a pair of wires falls off as 1/r^3, but the field from a radio antenna falls off as 1/R^2… yet a coil or a pair of wires is a radio antenna.

How can it act in two different ways? Easy: a coil or a pair of short wires acts like an “aperature” for electromagnetism, and if the aperature size is too small, no waves can get out. The coil has to be larger than around 1/4 wavelength. If you try to transmit with low frequency, you’ll only create a vibrating “magnetic bubble” which becomes extremely weak with distance.

But keep your coil the same size while cranking the frequency higher and you shorten the wavelength, and eventually the wavelength will be short enough that the coil starts sending out radio waves. And then the fields remain much stronger at great distances.

But any accelerated charge will radiate. Does an accelerated charge have a dipole moment?

I should have said, “does a linearly accelerated charge have a dipole moment”?

Eh, I meant in terms of stuff like antennae and so on. Accelerated charges, of course, are still monopoles. Whether there’s a dipole or not depends on the charge; some have them, others don’t, and in neither case is it a particularly large contribution to the field.