# Does EMP affect stuff that's been turned off?

Would EMP affect devices that were turned off at the time of the pulse?

“…electronic equipment that is turned off is less likely to be damaged.”

From Page 9, here.
And the footnote, #17, that accompanies this statement starts right out with, “Experts may disagree…” so the best you’re probably going to get is, “…less likely to be damaged”. No guarantees, IOW.

Yes. Just like lightning induced surges can damage anything that is connected to an AC outlet, telephone line or antenna, whether it’s turned on or not. An EMP will induce voltages in any unshielded wiring.

The only 100% safe place is to put the item in a completely shielded container.

Will an EMP destroy the electronics in my car?

Not if you are Tom Cruise

The basic prinipal is to induce a current in a conductor. This is how a generator works, there are windings that move through a magentic field. A EMP pulse is the magnetic field, and any conductor, including a unplugged power cord, internal circuits, even the metal case will get a current induced. This current, if high enough, could overload and ‘fry’ components, or perhaps even melt the conductor. When the device is ‘running’ this pulse could be added to the devices voltage, which may tip the scales between a device surviving if it was unpowered and frying if it was powered.

Probabally a larger factor if plugged into ‘grid power’ a EMP pulse could induce a very large voltage spike do to the long runs of the grid power lines, which would be ‘feed’ to your device plugged into the wall, if unplugged it can’t.

That’s not a sensible question. EMPs come in different total energies based on the source (bomb, microwave generator, etc) that created it. And like any electromagnetic effect, it obeys the inverse square law, where energy & hence damage potential drop off at the square of the distance between you & the source. So you need to answer both those questions to know how you’ll be affected.

If your car is 12,000 miles away from the biggest nuke ever built it will be unaffected. 12 miles away it might well be incinerated, electronics and all. Somewhere in the middle is a distance where some parts will be EMP-ed out but the car will be otherwise undamaged.

If your question is really “Are a modern car’s electronics susceptible to strong enough EMP?”, the answer is yes, definitely.

Old textbooks on semiconductor physics claimed that the gate oxide layer of MOSFETs was suceptible not only to static discharge but also to EMP. Years later when I was teaching semiconductor physics, I passed along this little info tidbit.

Maybe we should have had a lab experiment on it

EMPs aren’t anywhere near as deadly as those horrid shows on the Discovery channel might lead you to believe. First of all, as was previosly mentioned, EMPs follow the inverse square law, so their effectiveness drops off very quickly with distance. The second thing is that whenever you have a metal box it will completely shield whatever is inside the box from the EMP. You can google the term “faraday cage” if you want more details on why this happens.

Would EMP affect devices that were turned off at the time of the pulse?

Depends on the device. For something like an ipod, the headphone wires are going to act like a natural radio antenna and will channel energy from the EMP right into the ipod. Whether the ipod is on or off at the time won’t make much difference.

EMPs also hit power lines, and once a pulse is coupled onto a power line it can travel quite some distance, although it will tend to be attenuated quite a bit by transformers. Still, you might get a decent noise spike coming down the power line from an EMP. It might be just powerful enough to cause damage to your device, but might not be powerful enough to jump the gap between the contacts when the power switch is off, so in that case, turning the device off might save the device.

A lot of devices today though don’t turn completely off. Your computer, tv, stereo, etc. all are still “on” even when you turn them off. They might not have power to all of their circuits inside, but they have power to at least some circuitry so that they can respond to a remote control to turn them on. Old computers with an AT type power supply are switched completely off when you turn the power off. An ATX power supply (which pretty much replaced AT supplies around the time of the first Pentium CPUs) does not switch completely off and is more vulnerable to a noise spike coming up through the AC line.

Noise spikes can also come up through the phone and cable wires, causing damage.

Will an EMP destroy the electronics in my car?

Uh, maybe. The body of a car makes a natural faraday cage to some degree, but since the car isn’t completely surrounded by a metal box, it’s not a perfect faraday cage. A lot of cars have plastic body panels instead of metal too. Police have experimented with EMP weapons to stop cars, and while they have sometimes managed to stop cars under controlled conditions, the EMP just isn’t reliable enough to make a practical system for stopping cars. The radio is more likely to be damaged than the car’s computer. Really old cars that don’t have a computer have much more rugged and simple electronic circuits, and are far more likely to survive the EMP.

Most cars have a “cripple mode”, meaning that they will still run even if the computer gets fried, although they may not get anywhere near optimal performance. I had an old Buick that blew its computer. The gas mileage dropped to about 12 mpg and it sputtered a bit when sitting at a stop light, but it still ran. The “War of the worlds” scenario where all of the cars just died isn’t realistic.

i imagine in the future, there will be a huge ubiquitous magnetic field, due to mag lev trains and roads being so common place, and perhaps our devices will be designed to draw power through induction instead of being destroyed. our cybernetic appendages of course will also use said energy, relying on small batterpacks as buffers. because who wants an unsightly micro reactor strapped to his back? not this borg.

I doubt it. A lot.

A huge and pervasive magnetic field strong enough to power devices (unless they’re really low power) is expensive to generate, wasteful, and would play holy hell with anything not designed with it in mind. Unless you wanted to have to constantly wave your iPod around to charge it, it would have to be an oscillating huge magnetic field. Putting aside the limitations this puts on electronics, humans don’t deal well with strong oscillating magnetic fields. We get dizzy and disoriented. There are probably lots of other reasons why it’s unlikely, but that’s what first comes to mind.

Even if this were to happen (bringing it back to the OP), even a device designed to use energy from a magnetic field could still be fried by a strong enough EMP pulse. It would simply get overpowered. My toaster is designed to be powered off of AC current, but I’m pretty sure it’d fry if I climbed up some high voltage poles and attached the plug to the wires.