When's there's a nuclear blast, how does it create the EMP?
When a nuclear blast occurs, a number of things happen at once. Many
high-energy photons (x-rays and gamma rays) are produced. These photons
collide with electrons in the bomb debris or the surrounding air and strip
them from their nuclei. This causes a movement of the electrons away from
the atomic nuclei. This separation of charges generates an electric field,
and the motion of the charged particles (electrons) also induces a magnetic
field. Magnetic and electric fields that change with time are all you need
to generate electromagnetic radiation.
Because of the high energy of a nuclear explosion and the high temperature
of the fireball, these electromagnetic pulses pack quite a whallop. The
frequency of the radiation in an EMP is fairly low, just in the range that
electronic devices are sensitive to. Susceptible electronic circuits act
as receivers, and pick up damaging voltage and current surges. The
electronic components overheat, and that's the end of the device.
Richard Barrans Jr., Ph.D.
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Update: June 2012