Lia Svilans asked:
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.
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.
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.
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.
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.
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.
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).
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
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.
"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.
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.
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.
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