Richard Notton wrote:
> > Not true. It's called the inverese square law... and you can't change physics.
> >
> Not wholly true either in the stated instance. You can't use your eyeball to
> _measure_ light, its not linear, its almost logarithmic like your ears, plus the
> iris compensates by opening and closing, visibly 4 times less light LOOKS like
> about half, you need a professional light meter.
True, BUT this has nothing to do with the inverse square law. The inverse square law
applies to radiation intensity and distance from the source. If you put a lamp bulb
up on a thin pole, ideally it radiates in all directions. If you surround it with an
imaginary spherical surface, like a balloon, and measured the light intensity at
that surface and compared the intensity to that a surface of twice the radius, the
larger surface would have an intensity of one quarter. This comes from simple
goemetry: The area or a sphere is 4*pi*R*R (4 pi R squared)... If you double the
radiur, the area is 4 times as large. The amount of light is constant, so the
intensity is 1/4 as much. That's the inverse square law. Period.Why stop with DC ?
AC circuits are the same without any impedance, (L or C),
> What it is, is a temperature dependant resistor and it makes a big difference.
Right.
> I have just run out to the workshop and found a 12V 5/21W stop/tail bulb, using
> only the 21W stop filament I connected this to my variable, stabilised bench
> supply via a digital ammeter with another digital volt meter across the bulb
> contacts so I could set voltages _exactly_ across the bulb; since the digital
> ammeter necessarily has a tiny, but significant sense resistor in it. (these
> things actually measure voltage across a very low value resistor for current and
> do the I = V measured/known internal R for you).
>
> At 12V applied it draws 1.78A giving 21.36 watts and therefore a hot filament
> resistance of 6.742 ohms.
>
> At 6V applied it draws 1.24A giving 7.44 watts and therefore a not so hot filament
> resistance of 4.838 ohms.
>
> As you can see the power variation for a nominal 21W _filament_ lamp from rated to
> half voltage is not 4 times but only 2.871 times, and you can see the filament
> resistance has not remained stable but changed by 28.24%.
>
> Worse still the cold filament resistance actually measures 0.6 ohm so it changes
> by a whole decimal order from cold to hot ! Try measuring a domestic light bulb
> cold and doing the sum, you will not find it is anywhere near the printed rating.
GOOD FOR YOU. I was too lazy to go do the experiment.-John
> Similarly, for a diesel heater glow plug you can also expect the resistance to
> change from one voltage to another, but probably to a lesser degree.
>
> Richard
> Southampton - England
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