Why does a burst of E.M.P disrupt the normal functioning of electrical & electronic devices nearby?

Question

Kindly give your answer based on the physics behind it.

Answer 1

An EMP is an electromagnetic pulse (of a very high intensity) and that sets off a surge of electricity which interferes with the functioning of devices unless they are properly protected. Either the devices trip or burn.

Answer 2

Electro-magnetic pulse (EMP) sets up an electrical field that has a gradient. This means there is a voltage differential between locations in that field. In math talk, we can write E1 - E0 = V; where the E's are the field strength at points 1 and 0, and V is that voltage differential.

Where there is voltage and a conductor between that voltage, there will be current. That follows from V = IR for DC current. Thus we have current I = V/R for a conductor with resistance R and the EMP has set up a current in anything not shielded from it. Zap...all the electronics, which typically deal with micro amps of current, are suddenly overloaded with milli amps or more of current. They burn out.

The military has long been aware of the disruptive nature of EMP. A one megaton nuke, for example, can disrupt communication at especially low frequencies for hours. This means that a task force of Naval ships would be unable to communicate among themselves during that blackout period following an airborne nuke blast.

This is not the same effect I previously mentioned that burns out the equipment. In this case the E field itself is the disruptor because E fields and radio waves are the same stuff...EM waves. Thus, the low frequency of the EMP simply creates a lot of noise; so that the signal to noise ratio (S/N) goes way down...down below a critical level so that the signal cannot be picked up.

The fix for the burnout issue is simple...put an EM shield around the electronics. This can be something as simple as a copper mesh screen. The screen picks up the EMP field and shunts the current off around the equipment; so the electronics don't get burned out. All U.S. Navy ships have hardened electronics that protect their electronics from EMP.

The fix for the S/N issue is not so simple. One thing the Navy can do is ensure they have high frequency radios available. This is because the EMP frequencies are typically low; so they have less effect over the high frequencies of UHF for example. Thus, the blackout of the higher frequency radios will not be so long as it will be for the low frequency radios.

But the higher frequency radios are not totally immune from the S/N issue. When a nuke first goes off, it shoves out a lot of EM energy and, as we all know, E = hF; where h is Plank's constant and F is the frequency of EM waves. Thus, the initial higher energy yields initial higher frequencies.

But the good news, if there is any at all, is that the high energy soon dissipates, leaving e < E energy; so that e = hf < hF = E and f < F. The only thing left after some minutes are the lower frequencies (f). Therefore, the noise frequencies (N) quickly drop to f. And UHF radios and the like filter out the low frequencies in favor of the higher frequencies. So their S/N goes up.

The drawback to using high frequency radios is that their comm range is less than the low frequency ones. Therefore, when a Navy task force needs to communicate, say, with the National Command Center back in the States, it cannot because low frequency is needed for that; and those frequencies will be under blackout the longest. On the other hand, the high frequency comm can be used within the task force to coordinate defenses or attacks.