Re: [Phys-l] light bulbs

[brian whatcott <betwys1@sbcglobal.net>]

On 3/8/2011 7:06 AM, Anthony Lapinski wrote:
> I have some incandescent bulbs from Europe. In America, the voltage is
> half as much, so the current is half as much. Thus, the power should be
> one quarter as much (P = IV)
>
> A 60-W European bulb should behave like a 15-W American bulb (4X less
> power). However, when I plug it in, it looks more like the glowing
> filament of a 7.5-W bulb (8X less power). So I am a bit puzzled with this.
> I am wondering if it has something to do with the filament specs. I
> believe the electrical frequency in Europe is 50 Hz, but that shouldn't
> really matter with 60 Hz in the US.
>
> Anyone have experience with this, or a similar demo?
This note was helpful in clarifying my ideas on the link between 
filament power
and voltage. Thanks  to Mike Edmiston and D Mathies, one can now say 
with some
confidence that halving the voltage into a filament lamp does not 
decrease its
input power to one quarter, as it would for constant resistance, but 
decreases
the input power to one third, due to the decreased filament resistance at
 lower temperature, a feature common to metals, but not usually of much
 concern except over considerable temperature ranges, as here.

This makes your observation of steeply reduced illumination even more 
remarkable,
and demonstrates that the reduced electrical energy input is less capable of
 pushing the bulk of the energetic electromagnetic output into the 
visible range.

Brian W