Lightning bolt power and Back to the Future

buddyholly9999

This is the second time I write this post because the first time it got deleted...so I must say that I am not going to go into all the detail that I went into the first time I wrote this..not to mention that I'm a bit p.o.'d.

In back to the future, doc brown says that a lightning bolt can generate 1.21 GigaWatts of power...I read another website that said that an average lightning bolt produces only hundreds of MegaWatts, which is clearly not greater than or equal to 1.21 GigaWatts...would I be correct in assuming that the lightning bolt would not have be able to send Marty back to the future?
http://hyperphysics.phy-astr.gsu.edu...ightning2.html

Also, would there even have been a cable that would have been able to handle that much power back in 1955? We all know that if the cable couldn't have handled it, it would have burned up, thus the power never reaching the deLorean.

A superconductor (such as mercury below 4 Kelvin) is an example of an "ideal conductor"? And by "ideal"..I mean no resisitivity. Is there such a conductor found naturally and applicable in normal terrestial temperatures?

chroot

The 1.21 gigawatt figure was simply made up. The actual "power" of a lightning bolt depends on many factors, including the total charge differential and the thickness of the plasma channel. Not all lightning bolts are the same.

The total amount of heat dissipated by a cable depends not only on its resistivity, but also on the time period over which current is applied. If the current is passed for only a tiny fraction of a second, even standard household wiring can support enormous instantaneous currents.

And no, no naturally-occuring high-temperature superconductors exist. Some man-made ones are getting close, though. The race is on.

- Warren

Rach3

http://en.wikipedia.org/wiki/Lightning

Using order-of-magnitude numbers from wikipedia, we have peak 100kA currents at >10^8 volts lasting for tens of microseconds; this translates to peak power of 10^13 watts (10 terawatts). The total amount of energy dissipated is ~500MJ (says wikipedia). For comparison, the world's largest capacitor bank stores ~50MJ, a tenth of that (http://www.rheinmetall.de/index.php?fid=1805&lang=3).

Rach3

Obviously though, most of the power dissipated would go into resistive heating of miles of atmosphere, rf radiation, x-rays, etc., rather than the short length of conducting cable. That's why trees struck by lightning aren't vaporized.

buddyholly9999

On the subject of the movie, since it now seems that the bolt of lightning could have produced that much power and the wire could have handled it for instantaneous amounts of time, how about the electrical breakdown around that cable...surely couldn't have been good for Doc Brown's health.

Rach3

Movies take great license when portraying reality... usually they get it dead wrong. Makes it easier for them to write the script. Don't take any of this stuff literally...

russ_watters

As long as the cable was connected, Doc Brown would have been fine. The electricity finds the path of least resistance, and it ain't through the good doctor's arms.

Rach3

I've mentioned that the energy dissipated in a conductive cable is insignificant compared to what's dissipated over many miles of resistive atmosphere... look at lightning-protection cables (link), they're quite thin and they don't even start fires! (we hope)

buddyholly9999

If my original post hadn't been deleted, you might have read my disclaimer that I realize it's "just a movie" but still...fun to think about stuff

Rach3

I'm skeptical that that would apply at lightning frequencies (rf or so), inductive reactance becomes as important as resistance.

pallidin

You are correct. Though most of the current will seek a path of least resistance, a phenomenon happens when the current is extremely high:
When the least-resistant path is "saturated", secondary emissions along previously high resistance paths can occur.

Also, inductive reactance is a real and serious attribute of sudden discharge DC, especially with regards to lightning bolts.

pallidin

Also, keep in mind that lighting is DC, not AC, so their is no "frequency" per-se. It is a sudden discharge event, not cyclic. However, the sudden displacement of the many millions of electrons causes the brief but powerful RF(and other EM) radiation characteristic of radio "cracking" during a thunderstom.

Rach3

There are multiple discharges, at microsecond scales. Not sinusoidal AC by far, but still at RF timescales. There is a huge deltaI each time (up to 100kA), so magnetic effects should be profound.

pallidin

Indeed, which accounts for the potentialy severe inductive reactance you mentioned and most people forget or do not think about.

pallidin

Rach3, you brought-up a potentially good clarifying point. Would it be fair to extend that lightning, though DC, is expressed as a "bastardized" sawtooth DC waveform?

ravisastry

can we use this lighting energy for domestic purposes ? how can we harness it ?

Rach3

No. It's basically a static discharge, following the path of least resistance. Note that the 500MJ are distributed over several thousand meters of atmosphere - pretty dilute. It's also quite rare, at any fixed location. We've discussed this question before:

http://www.physicsforums.com/archive...hp/t-4742.html

Note the comment that at ~700MW globally, it's not enough to even be worth harnessing.