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wikipedia says that Electromagnetic pulse As with any electromagnetic signal, EMP energy may be transferred in any of four forms:

  • Electric field

  • Magnetic field

  • Electromagnetic radiation

  • Electrical conduction

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

I know that the four forms of energy are around us and we use them in our daily life, For example: Electrical conduction are used in transformers and we use Electromagnetic radiation to listen to our radio stations and TV shows. They are not destructive for electronic devices. So:

What makes an E-bomb destructive?

I believe it may affect signals for a short time and then all electronic devices will work properly again.

As for my second question:

How can it achieve a very wide range (big radios) of damaged electronic devices? How can it damage all the devices of a town or big area?

Note: I ask this question because I'm curious and just to learn, I don't want to destroy and thing :)

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The pulse creates an electromagnetic wave or series of waves with very large field strength. As those waves propagate outwards and encounter electronic devices, the intense field causes either over-voltage failures or over-current failures in the devices. I would guess that over-voltage failures are the more likely outcome. – mkeith Jan 7 at 17:27
    
What makes it destructive? The nuclear explosion that you need to cause EMP. – Tomáš Zato Jan 8 at 15:12
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"I believe it may affect signals for a short time and then all electronic devices will work properly again." - that happens too, to stuff that is farther away - see the burglar alarms in the quote below... – rackandboneman Jan 8 at 15:44
up vote 16 down vote accepted

The reason such a pulse is so destructive is because it has a broad frequency range, and can therefore affect things with a wide range of physical dimensions, and the large amplitude of the pulse.

Every bit of conducting material acts like a antenna. In normal experience here on earth, ambient RF radiation is so low that the resulting currents and voltages in those conducting materials is so low as to cause no harm. In fact they are so low that radios deliberately intended to pick up these signals have to amplify them to get to useful levels.

The resulting voltages and currents are proportional to the strength of the field that causes them. Some level of voltage and current will damage something, so therefore some level of field strength will cause damage. This is a several orders of magnitude more than what we normally experience here, but that's exactly what the right type of nuclear bomb can produce.

If a circuit is intended to work with the microamp signals coming from a antenna, it should be no stretch to imagine that amps can damage something.

This is not as far fetched as it may seem. Large coronal mass ejections from the sun have taken out sections of the power grid before. In this case, the power lines are really long antennas, and the fields caused by the movement of so many charged particles caused enough excess current to trip breakers in the system.

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Starfish Prime (whose results were visible here for days achieved: " .... in July 1962 the Starfish Prime test damaged electronics in Honolulu and New Zealand (approximately 1,300 kilometers away), fused 300 street lights on Oahu (Hawaii), set off about 100 burglar alarms, and caused the failure of a microwave repeating station on Kauai, which cut off the sturdy telephone system from the other Hawaiian islands. ...." - Wikipedia EMP and High altitude explosion – Russell McMahon Jan 7 at 21:01

Two things make an EMP weapon destructive:

  • the tremendous amount of energy released
  • the very short duration of time over which it's released

Another smaller, but similar example is lightning. A lightning strike might release 500 megajoules of energy in just a few milliseconds. For comparison, a 50 kW AM broadcast station releases the same amount of energy every 2.8 hours. But a lighting strike to your house will probably cause some damage, whereas 2.8 hours near an AM transmitter will not.

You might gain some intuition into why a short pulse is more destructive by looking at the definition of inductance:

$$ v(t)= L\frac{\mathrm di}{\mathrm dt} $$

Where:

  • \$v(t)\$ is the voltage at time \$t\$, in volts
  • \$L\$ is inductance, in henrys,
  • \$\frac{\mathrm di}{\mathrm dt}\$ is the rate of change of current over time, in amperes per second

An EMP weapon will create very rapid changes in the magnetic field, which will induce very rapidly changing currents in any conductor. Consequently \$\frac{\mathrm di}{\mathrm dt}\$ will be very large, and even working against a small inductance, this can result in voltages high enough to damage electronics, especially sensitive things like MOSFETs (the basis of modern digital electronics), many of which can be damaged by applying as little as 10 volts to their gate.

Capacitance is similarly defined, being the dual of inductance:

$$ i(t) = C \frac{\mathrm{d}v}{\mathrm{d}t} $$

This means a rapid change in voltage can result in damaging currents.

Of course an EMP weapon is likely to generate both magnetic and electric fields, but from these two equations you can see why the pulse part of electromagnetic pulse is key to its destructive power. Roughly speaking, releasing the same amount of energy in half the time doubles the currents and voltages that might be produced, with a consequent increase in destructive power.

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Megajoules? Don't be ridiculous; everyone knows the energy of a bolt of lightning is measured in jiggawatts! – Mason Wheeler Jan 7 at 19:24
    
@MasonWheeler Watts measure power, not energy, i.e. energy per unit time. Conversely, given the power in watts you get the energy by multiplying by the time over which that power is exerted. So, for example, a 100GW bolt which lasts for 5ms will deliver a pulse of 100GW x 5ms = 500MJ of energy. Therefore the figures given by Phil are perfectly in line with yours. – Lorenzo Donati Jan 7 at 19:53
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@LorenzoDonati Whoosh... – Mason Wheeler Jan 7 at 19:55
    
@LorenzoDonati youtube.com/watch?v=I5cYgRnfFDA – Phil Frost Jan 7 at 20:02
    
@PhilFrost Man! You made my day! I didn't remember that scene. I'm starting to see why Dave L. Jones from EEVBlog likes "Back to The Future" so much! :-D – Lorenzo Donati Jan 7 at 20:22

The nuclear bomb EMP pulses can affect such a large area because the bomb contains such a large amount of energy.

Referring to https://en.wikipedia.org/wiki/Nuclear_electromagnetic_pulse#Starfish_Prime and https://en.wikipedia.org/wiki/TNT_equivalent, the high altitude "Starfish Prime" which caused electrical damage in Hawaii, about 1,445 kilometres (898 mi) away from the detonation point, knocking out about 300 streetlights, used a 1.44 megaton bomb: that's equivalent to half of all the conventional explosives used in WW2 in a single explosion. 6 petajoules of energy.

That's equivalent to running the 50kW AM radio station in Phil Frost's answer for 3800 years.

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The 1962 Starfish Prime high altitude (space) H bomb test whose results were visible here (in New Zealand) for days and which allegedly caused some damage from EMP affects achieved:

  • " .... in July 1962 the Starfish Prime test damaged electronics in Honolulu and New Zealand (approximately 1,300 kilometers away), fused 300 street lights on Oahu (Hawaii), set off about 100 burglar alarms, and caused the failure of a microwave repeating station on Kauai, which cut off the sturdy telephone system from the other Hawaiian islands. ...." From:

Wikipedia EMP

and

High altitude explosion

Mechanisms & consequences:

Effect of the FAST NUCLEAR ELECTROMAGNETIC PULSE on the Electric Power Grid Nationwide: A Different View
Written ~= 1987.

  • Abstract: This paper primarily considers the potential effects of a single high-altitude nuclear burst on the U.S. power grid. A comparison is made between EMP and natural phenomena such as lightning. This paper concludes that EMP is no more harmful to the power grid than its counterparts in nature. An upper limit of the electric field of the very fast, high-amplitude EMP is derived from first principles.
    The resulting values are significantly lower than the commonly presented values. Additional calculations show that the ionization produced by a nuclear burst severely attenuates the EMP.

How stuff works - ebomb


70+ pages - anti-war focus but large amounts on emp.

Nuclear weapons test effects: debunking popular exaggerations that encourage proliferation


40 page + compilation - much on emp included.

Effects of nuclear weapons: the factual truth compiled from declassified reports

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