Electromagnetic pulse

[matrixscarface]

What would happen if you had an electromagnetic pulse.. and it hit metal like a wire or bar, it would induce electric current.. what current would it be? dc or ac?

[Qfwfq]

A pulse of current!

[matrixscarface]

Originally Posted by Qfwfq

A pulse of current!

i understand that.. but (ac) alternating current or (dc) direct current!?!?

ok let me ask this, when america tested the nuclear bomb.. more specific the trinity test. at this test, an large electromagnetic pulse was produced because of the blast and then traveled downward to the dummy city.. at this dummy city a strange thing occurred.. The electromagnetic pulse came in contact with the electrical wires and anything metal that could conduct electricity and produced a very strong current.. Now I need to know what current was that. Ac or dc? That current traveling through the wire had to be one or the other.. I am having a very hard time trying to find this out

[Boerseun]

As far as I can see, it can only be dc.

Ac operates by flipping the polarity several times each second (depending on the frequency). An EMP is a single burst of electromagnetic energy, lasting only a few millionths of a second. I can't see a mechanism that would cause it to reverse polarity in a conductor during that time.

So, yeah - I'd say dc.

[Jay-qu]

yeah it would be DC - to be AC the magnetic field has to reverse, since its only a pulse you would get one pulse of current.

[Erasmus00]

DC usually refers to a current supplied by some constant voltage (hence the constant current if connected across a constant resistance)

AC usually refers to pure sin wave type current.

The pulse you speak of does neither. It's voltage starts off low, gradually gets high, and returns to 0. The current doesn't alternate (the voltage only has one sign). However, inductors won't behave as closed circuits and capacitors won't behave as opens (which is characterstic of DC).
-Will

[Qfwfq]

Originally Posted by matrixscarface

Ac or dc? That current traveling through the wire had to be one or the other..

No, it doesn't have to be one or the other. AC and DC are both special cases. Strictly, they are ideal cases that can be described simply and have a constan regime. Interference generated by various sources is usually neither fish nor fowl, it doesn't really make senes to distinguish between these to categories. I wouldn't be sure that the pulse you're concerned with didn't have any reversal of direction.

If you really want to go in depth, any signal, variable in time, can be analysed by Fourier transform into a spectrum of frequencies. This is especially complicated when talking about a pulse of limited duration, you have a distribution of frequencies far from being a discrete set of them.

[He's Dead Jim]

will an emp have any effect on electronic parts or boards that are not hooked up to power??

[Jay-qu]

Yes, this is called electromagnetic induction. When there is a changing magnetic field, a current can be induced in any conductor such as wires, whether they are hooked up to power or not.

[CraigD]

Welcome to hypography, HDJ!

Originally Posted by He's Dead Jim

will an emp have any effect on electronic parts or boards that are not hooked up to power??

Practically speaking, yes – an electronic device, such as a PC, an uninstalled computer chip, or a automobile fuel/ignition computer, doesn’t have to be powered on, or even connected its usual power source, to be damaged by an EMP.

Though what generates a large EMP can be varied and complicated, the pulse itself is simple – a rapid, extreme change in magnetic field strength. Like the moving magnetic field produced by the moving magnets in a simple electric generator, this changing field induces electric current in conductors. If the change in strength is rapid and large, the current will be large.

Precisely speaking, if you could disconnect sensitive semiconductors from the conductors that connect them to one another, I suspect they could be made very resistant to damage by EMP. However, the conductors in such components – the etched metal printed circuits, wires, etc. - can’t be removed or disconnected. Even though they’re very short, and the currents induced small, semiconductors are very sensitive to damage from excess charge, making practical electronic components vulnerable to damage by EMP, whether produced by a weapon, a non-weapon, intentionally or accidentally. Such devices as large generators have been known to damage sensitive electronics by inducing currents in them with their magnetic fields – stories about such incidents - typically involving the crippling of an auto parked too close to a large generator - have gained urban myths status, but are based on real events.

Electronics can be “hardened”, or shielded, against EMP by enclosing them in conductive cases - Faraday cages. These can be as simple as the cardboard boxes covered in aluminum foil described on this alarmed-sounding survivalist webpage. Practically, such shielding poses a problem, as an unshielded conductor (such as a power cord) needs to be shielded, too, as does the wire supplying it, and so on – so making electronics that can simultaneously be used and not be damaged by an unexpected EMP can get pretty complicated, involving fast clamping switches and other techniques commonly associated with surge supressors. However, it can be and is done, so the scenario of a massive EMP destroying all the electronics in the world is unrealistic. In fact, many disaster=planning experts believe that damage to large power transformers from current induced by EMP is a greater risk than damage to sensitive electronics.