A question about EMP -

[hedgehog]

I think I understand how a pulse might disrupt an active alternating current - I'm thinking that the frequency is disrupted. I'm not so sure about a DC current.

I can follow that some components might be fried by a power surge.

But if a wire will carry a current today, is exposed to an EMP, what keeps it from carrying a wire tomorrow? How would an EMP cause every chemical battery exposed to completely discharge?

I can see a grid being knocked out, but what keeps it from being brought back up?

[ProdigalJunkMail]

1. the wire itself would be fine

the problem is the frying of the electrical components attached to them...everything CAN be brought back up; it would just take potentially a long time to do so.

sP

[jeff47]

2. The problem is current generated by the EMP

The problem caused by an EMP (speaking nuclear-weapon-scale EMP here) is that it generates a very large current when the EMP hits a wire. That current fries the devices attached to the wire.

Assuming the EMP isn't so large that the current melts the wire, the wire will still carry current tomorrow. But the stuff attached to the wire has already been destroyed. That's what keeps "the grid" from coming back up - they have to replace a lot of melted/exploded/burned equipment.

[hedgehog]

3. ok - so would that mean , for example, that if the wires on a motor are thick enough,

the motor would run, otherwise it would be burned out?

Light bulb filaments?

LED lights? ( I don't know how those work)

I figure older cars could be made to run, but not cars with transistors or other computer components.

[jeff47]

4. Depends if the EMP hits the devices or not.

If the devices are shielded from the EMP, it would be kind of like a lightning strike on a power line. There's a gigantic surge of current. It will destroy some devices and not others, based on the design of the device.

There isn't going to be a hard-and-fast rule about which devices survive and which die. There's too many variables.

Motor - would depend on the wires as well as if it was running. That big surge can cause mechanical damage in a motor.

Any electronics (including LED lights) are likely to be destroyed....but that will depend on the design of the power supply attached to the electronics.

Standard light bulbs - depends on the lightswitch.
-If the switch was on and it was destroyed quickly enough, the bulb may survive because the switch's destruction blocked the surge. If the switch takes too long to be destroyed, then the bulb will be destroyed. At the same time, being on means the filament is at its most fragile.
-If the switch was off, it depends if the terminals within the switch are close enough for the current to arc. If they are close enough, the current will hit the light bulb and destroy it.

But all this assumes that something blocks the EMP from the devices in question. If the EMP hits the devices, then it will generate a ton of current within the devices, most likely destroying them.

[Xipe Totec]

5. It would melt high tension cables about an inch thick

The voltage differential caused by an EMP is proportional to the length of the cable. The longer the length, the higher the voltage. Very long lines are much more susceptible than short runs, but even small metallic objects are vulnerable.

We're talking megavolt differentials.

If you want to see the effects of an EMP at smaller scale, look at the effects of metal objects when placed in a microwave oven.

Any object containing pointed metal can create an electric arc (sparks) when microwaved. This includes cutlery, crumpled aluminum foil (though some foil used in microwaves is unsafe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape.[36] Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter (3×106 V/m).

http://en.wikipedia.org/wiki/Microwave_oven#Metal_objects

[hedgehog]

6. thank you all for taking the time to explain this to me!

[dimbear]

7. If by any chance you should survive, catch the latest news on your Atwater Kent.

Instead of shorting out, vacuum tubes just merrily arc for a while and then keep on trucking.

[bananas]

8. P = I^2 * R

Power = Current Squared times Resistance

EMP induces a huge current.

Power is proportional to the square of the current.

That power is mostly dissipated as heat, causing circuits to evaporate in a puff of smoke, transformers to explode, batteries to explode and catch fire, etc.

Here's an example of a battery fire: a Tesla Motors car after an accident (no one was hurt, the firewall protected the passengers):