Friday, July 9, 2010

pulsed power basics

Yeah, yeah, I know I haven't written a post in a long time.  I've been busy.  This blog is not my life, it is reportage on one small facet of my life.  Family drama, home life, minor illnesses, fatigue, and actually trying to work on the things I write about here have used up the time I might have used to write blog posts.  So there.


In my travels around the internet, I often stumble across people attempting some of the same things I do.  Some times I stumble across people who seem to have grasped some basic physics, but somehow left big gaps in their education and as a result, are promoting completely ridiculous ideas which they feel are sure to revolutionize some industry or other if only people with lots of money (and incidentally, also lacking a decent high school science education -- a real one, I mean, not like what they teach in high school today -- hell, is science even permitted in the classroom today?) would fund their research.  Okay, sure, some of those are scams, but some of them are more along the lines of, "hEY guys, this seams leik a grate idea, Y WONT this woRk?"

I could provide examples, but that would be rude.

And then there are all the purveyors of refined bovine waste products on eBay who want you to believe that you can get 50,000 watts of power from a battery-powered device that fits in your pocket.  Without it exploding, I mean.  I stumbled across that gem earlier this week, and it just cracked me right up.  FOUR MILLION VOLTS?  Someone call up the guys at all the particle accelerator labs and let them know they've been doing it wrong.

And finally, I know there are a few non-techie types who read my babbling and actually want to understand what I'm writing.

Thus it occurred to me that a very short, simple explanation (more of a review, really) of a few basic physics concepts would be helpful.

First and foremost, this blog primarily discusses "pulsed power". So what is "pulsed power"?  Wikipedia has a concise little definition which I like:

      "Pulsed power is the term used to describe the science and technology 
      of accumulating energy over a relatively long period of time and releasing
      it very quickly thus increasing the instantaneous power"

An excellent example would be the electronic photo-flash for a camera.  The flash's circuitry converts battery voltage to a much higher voltage, slowly charging up a capacitor.  Then the capacitor is discharged through the flashlamp in a pulse which lasts less than 0.001 second (1 mS) but which may have a peak power (most of which is turned into light be heating up the xenon gas inside the lamp) of over 1,000 watts.

That definition uses two important terms which we must thoroughly understand before we proceed: energy and power.

Energy is nothing more than the capacity to do work.  It is a quantity, a value, which we attribute to many things - subatomic particles, fuels, masses in motion, car batteries.

Two basic categories of energy are potential and kinetic.

Potential energy is energy stored in a system that could do work if it were to be released.

Kinetic energy is energy in motion - energy being transferred from one system to another - in general, kinetic energy is energy which is performing work.

Now then, power is not the same as energyPower is the rate at which energy is transferred, moved, used, etc.

Both of these parameters can be measured in different units for different purposes, but they each have a single basic unit of measure which is preferred for science and engineering purposes, and which is derived from the other basic scientific units of measure.

The basic SI (International System of Units) unit of power is -- you guessed it -- the watt.  Watts are not exclusively electrical in nature, although that is the context in which most of us use the term.

The unit (derived) for energy is the joule.  One joule equals one watt of power being delivered / expended per second -- one watt-second.

You can probably see why our power bills don't use joules as a unit of measure.  The joule is not a very large unit.  If a joule = one watt-second, then one kilowatt-hour equals 3.6 million joules for which you pay, on average, around ten cents if you live in the USA.

You may have already noticed that the thing which binds energy and power together into a relationship is time.

A lot of energy expended over a long period of time results in low peak power.  Sometimes that is desirable, and sometimes it isn't.  Likewise when a lot of energy is expended over a very short time the resultis very high peak powers.  Sometimes desirable, sometimes not.

Very large peak power levels are difficult to arrange for long periods of time.  It is very expensive, and requires very large infrastructure to manage.  Just take a look at all the huge cables, towers, generators, substations and so forth required to bring a few paltry megawatts to your neighborhood.

Yet it is not too difficult to generate a short pulse of megawatt peak power in a laboratory setting.  Sometimes, a short pulse is all we need, such as in the strobe light example provided above.

With very, very high peak powers, short pulses are all that is possible for practical, physical devices built by mere human beings.

That should do it for this installment.

My next one will probably discuss how not to be stupid, using my own most recent stupidity as a singularly painful lesson.

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