| From: | Michael Bourne |
| Date: | 1 Feb 2000 at 12:06:08 |
| Subject: | Re: 'O'-level physics (was "PFS3") |
From: Michael Bourne <michael@circleware.org>
Hello JmW,
On Tuesday, you wrote:
> From: "JmW" <John@wasilewski.co.uk>
> -----Last Message-----
> From: Michael Bourne [mailto:michael@veil.circleware.org]
>> From: "Michael M. Rye" <mrye@execpc.com>
>>> From what I remember, lightning strikes are high _voltage_ ,
>> not high amperage.
> To put it simply:
> It's both. A very high voltage is required initially to ionize
> the air but the current which then flows is very high indeed, as
> stated.
> Its current (rate of flow) that vaporizes copper etc not
> voltage... that's just "pressure" and does nothing but, by
> analogy, "push" the coulombs/sec (amperes) through whatever the
> conducting medium happens to be.
> -----Comment by JmW-----
> Sorry, chaps, I can't resist the urge to intervene in this
> erudite debate. MB was right in his assertion that "its both,"
> and he earned quite a few marks by his "pressure" analogy.
> However, as every - erm - /civil/ engineer knows, MB then lost
> marks in claiming that the current does the work.
True, it's the product of the "pressure and rate of flow" :)
> It is the POWER (no analogy).
Very true.
> Direct current
> POWER (watts) = VOLTAGE (volts) x CURRENT (amps)
Exactly.
> For Alternating current, the same applies except that it varies
> continuously so it is usually averaged over the AC cycle. AFAI
> can remember, a typical sinusoidal AC cycle has Avge POWER = 2/3
> x (PEAK Voltage) x (PEAK current).
Not quite, average is 0.637, but RMS (Root Mean Square) - 0.707 -
values are a more accurate measurement of real power for sinusoids
in *resistive* circuits. It becomes more complex where reactive
components are involved.
> And in case anyone is in any doubt about it, although the
> ultimate bearing capacity of a driven or bored pile in building
> foundations equals the sum of the total shaft resistance and the
> total end-bearing capacity, the two components are not mobilised
> in equal proportions under normal service loading.
:)
> John Wasilewski
No future in keeping it "simple" <grin>... this is getting out of
hand :)
Michael Bourne
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