Science Questions

Why do elecromagnetic pulses interfere with electrical appliances but not electrical impulses in humans?

Sun, 10th Apr 2011

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Question

Lia Svilans asked:

Hi Chris,

I have a question about electromagnetic pulses (EMPs). Why do EMPs damage all electrical equipment but do not interfere with human's electrical systems eg. nerve impulses. I asked Doctor Karl from Australia but he was at a loss. Hopefully you can help.

Thanks

Lia Svilans

Adelaide, Australia

Answer

Dave -   An electromagnetic pulse is essentially a very, very rapidly changing electric and magnetic field.  That very, very rapidly changing magnetic field will induce very large voltages in anything metallic, anything conductive.  Those large voltages will induce very large currents to flow or sparking, and it will essentially just fry electronics.  But Chris, why doesnít this happen in humans?

Chris -   I can only guess that itís a question of the resistivity of the tissue because you can actually induce activity in the nervous system electromagnetically.  We know that because if you take transcranial magnetic stimulation, what that involves is putting a very powerful magnetic field over the head and you can alter the activity of whole populations of nerve cells because the nerves behave a bit like miniature wires.  If you put those wires in a changing magnetic field, you can change the activity of the nerve cell that's connected to them.  So we know that the nervous system is sensitive to things like a big magnetic field.  I can only think though that in this setting, itís because the human brain does not contain physical lumps of metal and therefore, there's not enough of a surge of current, or a big enough voltage produced to do the kind of damage that you would do to a gadget or a computer or any other piece of electrical equipment that would be exposed under those circumstances and basically, blow up.

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Most semiconductor electronics relies on very small highly insulating junctions in which electric fields control the flow of current.  These have relatively low breakdown voltages and once they fail the circuit is damaged.  This makes them more susceptible to electromagnetic pulses than nerve impulses which are much lower resistance current pulses.  Nerves also require repetitive pulses rather than single impulses to affect them and a lot of information passing is based on the rate of firing of nerves.  This is possible using smaller electromagnetic pulses in pain relief (TENS) and some types of brain stimulation. 

Some other forms of electronic circuits  notably thermionic valves, current controlled logic and magnetic devices and logic are also much more resistant to EMP. Soul Surfer, Sun, 10th Apr 2011

I thought it was more about what happens when you put a conductor in a microwave, a current is induced and the sparks fly (don't try this at home!!). The sparks are electrons due to the Compton effect.
We don't use a current as such, but an "action potential" in our neurons which travels at under 100m/s; whereas electric current in conductor circuits travel at or near the speed of light.
The EMP induces a current that fries all the delicate silicon chips as mentioned above but can also overheat lamp filaments and even copper wires.
It's because our bodies don't use free electrons in conducting materials, but heavy ions in solution, that we are relatively unaffected. JMLCarter, Mon, 11th Apr 2011

Lia Svilans asked the Naked Scientists: Hi Chris, † I have a question about electromagnetic pulses (EMPs). Why do EMPs damage all electrical equipment but do not interfere with human's electrical systems eg. nerve impulses. I asked Doctor Karl from Australia but he was at a loss. Hopefully you can help? † Thanks † Lia Svilans Adelaide, Australia What do you think? Lia Svilans , Tue, 12th Apr 2011

Very interesting JML. Didn't know that, but we are conducting too, ain't we? We seem to have a lot of (salt) water in our bodies? Well, except me then. I prefer my single malt undiluted :) Not that I think you're wrong in what you say there.

Found this. "Although the skin has a very large resistance (on the order of MegaOhms), the salt water (electrolyte) solution that is in your body is designed specifically to conduct electricity, so that your nerve impulses can reach the muscles and do those little things we enjoy doing so much, such as walking and breathing.

Normally, dry skin in good condition is insulating enough to provide fair protection against electric shocks up to 120VAC, such as is found in a typical North American electrical outlet. 240VAC is quite capable of knocking you flat on your ass and rendering you unconscious for a while, but still little lasting damage is usually done." yor_on, Mon, 18th Apr 2011

The effects of EMP on electronic equipment are not very different from the effect that a large electrostatic discharge can have on electronic equipment. We've all experienced the situation where we become highly charged after walking on a carpet, then approach some grounded equipment and get a nasty electrostatic discharge (ESD) shock.

If we are charged to a sufficiently high voltage (60kV or even more is not impossible) the event can easily disrupt the operation of the equipment, and even cause permanent damage. As Soul Surfer points out, this is because the breakdown voltages in semiconductor devices are low because of their very small geometries.

However, it's not the voltage from the ESD event that directly effects the equipment. It's an electromagnetic pulse event that occurs because of the ESD event. When we discharge ourselves to ground, a current flows from us into the equipment, and, depending on the resistance of the path (some of which is us) the rate of change in the current can be rather stupendous. Essentially, it goes from zero to a very large number of amperes in a very short time.

Because of mutual inductance, that rapid change in current flowing through the case of the equipment can couple significant energy into any low resistance conduction paths in the electronics, and induce a voltage that's large enough to "punch a hole" in the semiconducting material.

In the case of an EMP from a nuclear device, it's not an electrostatic event that effects the equipment, but the electromagnetic EMP event is so powerful that it can induce destructive currents in electronic devices, even at a great distance.

Fortunately, humans are not very conductive. The resistance of our pathways significantly limits the rate at which currents can change in our bodies, so mutual inductance is not an important factor for us. It usually takes a significant amount of power dissipated in our cells to damage us.

(Yoron - Please remember to include references to any quotes you include.) Geezer, Mon, 18th Apr 2011

Human bodies are reasonably good conductors (apart from hair and the outermost layers of  skin). The nerves are inside that mass of conductive stuff (insulated by a fatty layer called myelin).
In effect the bulk of the body acts as a Faraday cage for the nervous system.
It is possible for EM pulses to have an effect on the body, but it's unusual.
Bored chemist, Mon, 18th Apr 2011

BC, a Faraday Cage might take the edge off the highest frequency components of an EMP, but it will be about as effective as a chocolate teapot at shielding the low frequency components. Geezer, Mon, 18th Apr 2011

Faraday's original "Ice pail" experiments worked just fine with practically DC frequencies. Bored chemist, Tue, 19th Apr 2011



Yes, when dealing with a static electric field, however, the "M" in EMP stands for "Magnetic", and

"Equipment sometimes requires isolation from external magnetic fields. For static or slowly varying magnetic fields (below about 100 kHz) the Faraday shielding described above is ineffective."

from http://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding

I doubt that any number of ice pails would be sufficient to prevent an EMP from buckling railway tracks.

EDIT: Hehe! I thought I better find a reference to the buckling railway track effect in case someone asks. I Googled it, and top of the list was this thread!! Would it be OK to site it? I'm still looking.

While poking around looking for a ref. to the buckling effect (which I can't actually find!) I did find that there is a common misconception that you can protect delicate electronic equipment from EMP by putting it in a Faraday Cage. An old microwave oven frequently seems to get the Good Housekeeping seal of approval.

The main snag I can see with this idea is that microwave ovens are jolly good at blocking, would you believe, microwaves. Unfortunately, EMPs mainly consist of monsterwaves. Geezer, Tue, 19th Apr 2011



What's the wavelength of a monsterwave?  :)

Artist's rendition of a cookiemonsterwave:
jpetruccelli, Tue, 19th Apr 2011



Yes they can ...


http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation#Technical_information

but the magnetic field strength necessary to induce currents in your nerves has to be enormous.

RD, Tue, 19th Apr 2011

Back to the Faraday cage question, it's not immediately obvious that it should shield against an EM wave, but assuming it's an enclosed shell of finite thickness rather than a mesh, I think it's ability to shield is related to it's skin depth.  This is essentially the depth to which an EM field can penetrate a conductor.  I suspect if it were a perfect conductor, for example a superconductor, then the skin depth would go to zero and you'd have a perfect shield. 

Obviously the human body isn't a great conductor under most circumstances, so I'm not sure the Faraday cage argument holds up for humans. jpetruccelli, Tue, 19th Apr 2011

"Instead of carrying current like a wire, nerves operate more like a row of falling dominoes. Along the length of the nerve, there's an artificial imbalance of ions (sodium and potassium) that's kept out of balance by tiny pumps within the nerve. This imbalance in ions results in an imbalance in charge, which means there is a voltage difference between the inside and the outside of the nerve. Once the nerve fires, tiny channels in the surface open up, the ions rush through, and the charge flips. This flip causes the next channels down the line to open, and so on and so forth, and the signal is carried down the length of the nerve. Once it reaches the end, it causes the neuron to release chemicals that conduct the signal to the next neuron down the line.

Since neurons carry their signals chemically, rather than via current, (and nerves aren't set up like electrical circuits) an EMP wouldn't have any effect. BUT, since electrical charge is involved in conducting the signals, the nervous system is susceptible to electric shocks. The same is true of muscles, which, electrically, are very similar to nerves. A shock can trigger the ion channels on the surface of the cells to open up, making the nerve fire (or the muscle contract)." From Andymanec.

And "Only an extremely high field pulse would have a noticeable result on the cells of the brain. Considering the baseline or the continuous field power that is existent across the membranes of every living cell when they are presently sustaining a resisting potential to be negative 60 mV, the difference between the voltage would drop considerably throughout the whole hydrocarbon layer in the lipid bilayer that measures 3 nm in width.

This would imply that the power of the resting field sustained through the cell membrane would be 20 MV/m. EMP pulses would have to be almost of this level across the cell membrane to affect living cells. However, this does not rule out the possibility that someone could design an EMP generator that can attain these field strengths." Anonymous source.

EMP (special weapons primer) with some really interesting links too.

yor_on, Tue, 19th Apr 2011



I don't really know much about the effect on humans, but I'm pretty sure most of the energy in the "pulse" consists of relatively low frequency components, and low frequency EM radiation is notoriously difficult to attenuate with any kind of shielding, let alone a Faraday Cage. Perhaps superconductors might be able to do the trick.

(The monsterwave allusion is, of course, a reference to the very long wavelengths of LF EM radiation) 

Geezer, Tue, 19th Apr 2011

The monster wave made me think a bit.  Usually when we think of waves, they don't appreciably interact with objects that are smaller than their wavelength.  Based on that, I'd expect "monster" EM wave to pass right by my MP3 player, for example, without damaging it too much. 

Or is wave theory misleading me here?  Does the monster wave pass on by without being disturbed while still being able to destroy the mp3 player simply because the mp3 player is so small that the induced currents don't do much? jpetruccelli, Tue, 19th Apr 2011



That's a very good point. If I have it right, the energy coupled is a function of the loop area of the receiver, so small circuits collect less energy. On the other hand, it takes very little energy to fry the delicate devices in modern electronics.

I suppose it all boils down to the field strengths. If they are great enough, they do a lot of damage.

Still can't find a reference to railway tracks buckling. If may be bogus information, but I remember reading (many years ago) that EMP could induce enough current in the rails to cause them to move, presumably because of interaction with the Earth's magnetic field. I'm wondering now if I was had. Geezer, Tue, 19th Apr 2011

I can help you with that one (a little bit).  Two parallel wires carrying parallel currents will experience a force towards each other.  They wouldn't need to couple with the earth's magnetic field. 

As to whether it's enough to buckle railroad tracks--I don't know that one. jpetruccelli, Wed, 20th Apr 2011



Ah! Yes, the mists of time are clearing - thanks. That would make more sense. Still, with a separation of over four feet, the currents would have to be "quite large" to produce any sort of mechanical dislocation.

Maybe we should ask Pete Ridley. I've checked up on his background, and I understand he's an expert in the magnetic field (no pun intended).

EDIT: As I vaguely remember, adjacent current carrying windings tend to force each other apart. Is that what you mean by "a force towards each other"? Geezer, Wed, 20th Apr 2011

I'm referring to wires, not coils.  If two straight wires are next to each other, carrying a current in the same direction, they'll attract each other.  See http://theory.uwinnipeg.ca/physics/mag/node10.html, for example.  jpetruccelli, Wed, 20th Apr 2011



I seem to remember doing that experiment - mind you, that was about fifty years ago. Geezer, Wed, 20th Apr 2011

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