Re: Poll (& me being boring)

black-dog wrote Wed, 18 Jan 2006 00:02:46 +0000
>
> Message-ID: <43CD67E2.446CA1C1@xxxxxxxxxxxxxx> from Adelaide contained
> the following:
>
> >> Sheesh, you're no physicist are you? Mind you, in your field of
> >> expertise, I guess you are a bright spark.
> >
> >No, I'm not much of a physicist, but I've been telling this (true!)
> >tale to me mates, some of whom are competent in the relevant fields
> >and they've raised no objection. I grant you, my bicycle's seat is
> >intact and it will indeed insulate my gluteus maximi from the frame
> >(thanks, Wm..., I assume you were correct in elucidating bd's
> >objection, I was mystified on my own); I wear boots with thick
> >rubber soles for cycling, ditto for feet; but there is a third
> >alternative. Hint: one hand is on the metal of the lamp. Where is
> >the other one?
>
> Well ok let's do some fizzicks. I'm a border collie, so you know I know
> all about this elastictrickery stuff.
> Remember a guy named Ohm? Clever bloke, he has a law named after him
> and an equation and everything. It goes: V=IR
> Where V is the voltage, I is the current and R is the resistance.
> OK so far? Good. Well yer average bike bulb is probably something like this.
> http://shop.theoldbicycle.co.uk/index.asp?function=DISPLAYPRODUCT&productid=111
> So it takes a current of three watts to light it. Now as you are using
> a dynamo the voltage will vary, but we can work it out by multiplying
> the wattage (3) by the resistance of your body. OK time to get my
> trusty multimeter out and measure my body resistance. Bugger, can't use
> it with paws. Ok lets check on the web.
> http://www.tlc-direct.co.uk/Book/3.4.2.htm suggests that once you get
> past the skin, the resistance of the body can be as low as 500 Ohms.
> Hey, cool, that means you've only got to generate a voltage of 1500 to
> light the bulb.
> How fast can you pedal?
> Apologies to any real scientists out there if I've got this completely
> wrong. I am, after all, only a dog.
> I'd suggest that the reason it lights when you touch it is because you
> restore the broken contact rather than the effect of any electricity
> flowing through your body. Squirt it with WD40.

Thanks, black-dog! We all think of our bodies as being pretty good
conductors (we are mostly slightly salty water, after all), it's
good to be reminded of the numbers.

But -- I didn't get *much* of a glow out of that bulb, she says weakly.

But would I have expected to?

Watts are power, not current; P = V[2]/R [brackets mean 'to the power']

Let's assume my dynamo puts out 6 volts. The resistance of my body
is a bit more problematical, I grant, but according to
http://www.allaboutcircuits.com/vol_1/chpt_3/4.html
both hands wet gripping a 1.5 inch pipe (pretty close to the actual
conditions) = 250 ohms [see 1 below]

36/250 = 0.144 Watts. Assuming my front bulb is rated for 3 Watts
(I haven't checked), would I be able to detect any glow at 5% of its
maximum output? I don't think this would be unreasonable, it is
really bright when it's working properly.

But, conversely, what about the current this postulates went through
my body? Back to P = IV. 0.144/6 = 0.024 Amp = 24 mA; higher up
the same page cited above is a table rating responses vs mA, 24 for
women is between 'threshold of perception' and 'pain'. I would have
expected to have noticed something -- don't remember anything such.
However, the relevant evening was several months ago and I was not
only doing an experiment but also dodging pedestrians along Corn
Exchange Street.

Remember that the lamp had been malfunctioning for a while; I was
*used* to having to shake the lamp to get it to settle down lit.
The gentle touching that worked that night was different; I don't
remember full details now, but since I had been keeping it going by
jarring it into contact I did explore the possibility that this was
what I was doing again this time and concluded probably not.

And using copper wire to connect the lamp body to the bicycle frame
did solve the problem.

However, I'm now triggered to try to repeat the experiment under
more reproducible conditions (minus pedestrians, eg), and if I can
with more than one person, dry vs wet hands etc. It will of course
just my luck if the lamp now refuses to misbehave even without its
copper wire!

Adelaide

[1] The 500 Ohms you cite does include skin, not just body, it says
so (albeit obliquely). Just insides should be a lot less, though I
haven't found a direct quote.
.

Relevant Pages

  • Re: 12 volt 7amp/h versus10amp/h ??
    ... Although it is true the resistance of a lamp changes with applied voltage, ... As the voltage decreases, current ...
    (rec.aviation.soaring)
  • Re: a better incandescent light bulb
    ... Your model shows a net reduction of voltage available to the bulb. ... you changed R1 to a linear resistance. ... But the voltmeter was connected only across the lamp, ... to determine how an unmodified filament behaves when the voltage ...
    (rec.crafts.metalworking)
  • Re: 12 volt 7amp/h versus10amp/h ??
    ... Although it is true the resistance of a lamp changes with applied voltage, it is not an issue since the current flowing through a fixed resistor will change as the voltage decreases. ...
    (rec.aviation.soaring)
  • Re: AC sine wave: What does increasing the frequency do?
    ... >>Which brings up the concept that an incandescent lamp appears to have ... >that changes is the resistance of the lamp filament. ... resistance peaking later than the voltage peak. ... which looks like a capacitive component. ...
    (sci.electronics.basics)
  • Re: Saturation in transformers.
    ... >> the saturation ... Once again, it is not a short circuit, ever. ... >> a voltage across a coil? ... > Er, it might, or might not, depending on the time, the resistance ...
    (rec.audio.tubes)
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