What makes E-bomb or Electromagnetic pulse (EMP) destructive?
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.
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.
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.