A while ago I saw a show about lightning on Discovery Science, where they showed that lightning doesn’t hurt a car or the electronics inside by zapping a car with a gazillion (well, slightly less but still a lot) volts while the driver was inside it, after which the driver cheerfully started the car and drove off with it. On the other hand, whenever someone talks about an EMP caused by a nuke or a dino-killer type meteor, they invariably mention that this will kill off all the electronics in modern car. Since a car is effectively a Faraday cage, wouldn’t this stop the effect of an EMP as well? Or are those guys on Preppers, who store their radios in Faraday’s cages, mistaken about its protection against EMP’s?
Interesting…
The cops seem to think EMPs really will take out cars
http://www.dailypaul.com/299261/police-arming-with-emp-weapon-to-disable-vehicles
This engineer seems to think a Faraday cage will protect against that.
And in the videos I have seen of lightning hitting cars, the car gets smoked.
So I would guess the Discovery Channel was dead wrong (not the first time)
Hopefully one of the resident physicists will stop by shortly.
An EMP is drastically more powerful than a lightning strike since it essentially comes all at once, and has no shunt path to ground. Lightning is flashy and all, but the actual strike is the last part in a chain of events caused by trying to equalize a potential difference.
A Nuke generated EMP triggers a nearly instantaneous surge field from the Compton Effect in the atmosphere. No modern automobile is shielded enough to survive the cascade, no consumer vehicle even comes close to resembling a Faraday cage. (And, you know it isn’t, you have exposed antennas, light sensors, and wiring all over the external of the vehicle).
One aspect of this has been discussed before, IIRC: When are we going to get a hand-held EMP gun to disable obnoxious car radios? The world needs this very badly.
It’s a Faraday cage with holes, though. So it’s a cage for EM radiation with wavelengths significantly longer than the size of the holes, which is why your AM (wavelength >= 100m) antenna needs to go outside the car, but not much of a cage for radiation with wavelengths significantly shorter than the size of the holes, which is why your cell phone can get a GPS signal (wavelength = 19cm) inside your car.
It turns out that lightning emits most of its RF energy in the low frequency, long wavelength regions of the spectrum, roughly 10 kHz (30,000m) to 1 MHz (300m), so for that kind of radiation, your car is actually a pretty good Faraday cage. (Note: lightning can do damage in many other ways besides electromagnetic radiation, e.g. heat, so let’s imagine we’re talking not about a direct strike, but one a little ways away, so that only the EM radiation is relevant.)
A high-altitude nuclear blast, however, generates plenty of energy at higher frequencies, 10 MHz (30m) to 300 MHz (1m). At those frequencies, your car becomes kind of leaky. Furthermore, any long conducting path (and by “long” here we mean “more than few inches”) can become a halfway decent antenna for the extremely large brief electrical fields of the nuclear EMP. This can readily result in the generation of very brief kilovolt signals in anything connected to these wires. That can damage or destroy electronic equipment.
But note I said “can” not “must.” There are a lot of unpredictables here. Just what is the frequency distribution of power in the EMP? It can vary significantly depending on weapon design and atmospheric conditions (as noted above, the EM signal comes not directly from the fission explosion, but from electrons knocked out of atoms in the atmosphere). Next, what kind of “antennas” do the wires in your electronic provide? Third, where do the large voltages appear? Fourth, do they exist long enough to do damage? EMPs from nukes generate very high fields, but they are also very short-lived, a microsecond or so.
As a rule, old-fashioned discrete circuitry, things made out of transistors, capacitors, and relays, are essentially immune to nuclear EMPs. They can readily stand a few thousand volts for a few microseconds. But newer electronics are much more delicate. On the other hand, they tend to be buried inside of protective enclosures, and the circuit pathways between them tend to be very short – not good antennas. The nasty signal has to come in from the outside, from power and ground and external signal leads. These are always filtered (after all, your spark plugs generate EMPs and the circuitry has to be protected from them) – but will the filtering be up to the bigger but shorter nuclear EMP pulse? It’s extremely hard to know, without…well, getting your particular car and exploding a nuke over it.
There’s a friendly article on the whole subject here:
http://www.ornl.gov/sci/ees/etsd/pes/pubs/ferc_Meta-R-320.pdf
For what it’s worth, in the summary they say:
“Then there are others that think every electronic device in the country will fail, and we will go back to the Stone Age. There are many such exaggerated scenarios; for example, it is doubtful that more than a very small fraction of vehicles will suddenly stop working…”
But in short, the answer to your question is that lightning and nuclear EMPs are not directly comparable, because the EMP also generates a lot of power at higher frequencies (smaller wavelengths) which tends to be more penetrating and to affect electronics with smaller “antennas” (wire leads and such) connected to it.
Thank you very much for your excellent answer, I was never too good at physics but I can actually understand this!
Good answer Carl, if I could up vote it I would. Thanks.
In undergraduate school we used digital I.C.s originally designed for military standards. They were “hardened” to be less susceptible to an EMP. 
With a lightning bolt (a few million volts, btw) the energy tends to hit high on the car, usually on the roof or a radio antenna, and then travels along the body of the car, down the sides, and hops the short distance from the car’s body to the ground. Still, sometimes part of the lightning will jump through part of the interior of the car and/or may enter the car’s electrical system in other ways (through the headlight wiring, for example), and it is quite possible for the car’s electrical system to get fried. I’ve seen the aftermath of a parked van that got struck by lightning. The bolt went through the windshield and down through the steering column, where it not only fried the van’s electrical system but it also started a fire that burned the dashboard and other internal bits.
If a car’s body were a perfectly closed box, it would be much more immune to lightning damage, but then it would be fairly difficult to drive (no windows). As it is, cars can survive lightning strikes, but it is a bit hit or miss. Sometimes the lightning will leave the electrical system unharmed but may blow out the tires.
An EMP designed to work against a car is a completely different beast. You have a couple of different types. You have the ones that are just aimed at the car, and those don’t tend to be all that effective for the same reason that lightning also doesn’t cause damage. The energy tends to travel across the car’s surface and doesn’t do much to the car’s electrical system. There’s also the type that goes under the car, either a thing that can be fired from another vehicle and slides under the car, or something that is deployed in front of the car like spike strips. These are a bit more effective because they are designed to avoid the car’s body and send the pulse into areas where the wiring is much more exposed. They also aren’t as easy to use.
EMP weapons were first proposed all the way back in WWII, where they were promised to knock out the enemy plane’s engines and make all of their planes just drop from the sky. While the EMP weapons performed well in controlled tests, they didn’t prove to be very effective in the real world.
EMP weapons meant to disable cars have been promised since at least the 1970s (when i remember seeing shows about them). They suffer from the same general problem that the EMP weapons of WWII had. Metal around the outside of the target tends to shield the sensitive electrical parts from the EMP. While EMP weapons have stopped many vehicles in controlled tests, in the real world they just haven’t proved to be very effective. Ever since the 70s, though, I keep hearing about how this “new” technology will be used to stop cars, and they show a demonstration, and then they talk about how they have a few difficulties in the real world that they need to iron out, but the things will be available “real soon”. Every few years the exact same story repeats, and every few years it’s a repeat of “real soon”.
Modern cars are computer controlled, which makes them more sensitive to an EMP, and some companies have put out EMP car stoppers. They just don’t work very well, though. Sometimes they’ll stop a car, but they aren’t the miracle super high tech weapon that they are always advertised as being. They kinda suck in the real world.
Even though a modern car’s electronics are more sensitive to an EMP than an older car, the same things that the auto designers have to do to protect the car’s electrical system from things like cell phone interference also protect it from an EMP.
The effect of an EMP on modern autos is greatly exaggerated. Unless the EMP were powerful enough to disable all of the electronic components of the car, it isn’t going to prevent it from running.
When the PCM, power control module, or whatever your cars brain is called, quits getting information from vital sensors that may cause damage to the engine, it will just place the system into “Limp Mode.”
This is a reduced power and functionality mode that will allow continued driving at a reduced power level and let you ‘limp’ back home. Your modern car will even run in limp mode if you unplug various vital sensors.
The PCM or car’s computer recognizes the illogical input from the faulty sensors and puts the car into limp mode.
EMP bursts stopping every electronic everything is just Hollywood Science. Instead of all the cars being stopped on the freeway you will have a freeway full of cars that run like shit, but still are drivable.
Here is a brief, readable explanation of “Limp Mode.”
This is incorrect.
A sufficient EMP will destroy the electronics (the microprocessor). Without the µP, the car will not run - limp mode or not. The µP is doing all the engine timing.
Lightning and EMP are very different phenomenon. Lightning is a discharge (current flow) along a conductive path. EMP is free electrons propagating as a wave. EMP induces large currents in all exposed wiring. Lightning can do this to a small extent, but not nearly as much as EMP.
Note that “real” EMP requires a nuclear explosion outside the atmosphere. A ground burst will cause more damage by blast than the EMP.