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Re: lightbulbs

Let's see. For Ohmic devices:
Power = V^2 over R
and therefore
R = V^2 over power

If I were designing a lightbulb, the resistance would depend on the rated
power as well as the rated voltage. I have here a bulb (rated 12v) which
is the headlight for a HO-scale model car. It puts out a tiny fraction of
a watt. I also have here a bulb (also rated 12v) from a real car. It puts
out over 100 watts.

One might even say
R = (rated V)^2 over (rated power)
and
(actual power) = (actual voltage)^2 over R
where R depends on the ratings as above, hence
(actual power) = (actual voltage over rated voltage)^2 * (rated power)

So if you try to plot R as a function of rated voltage, you will get
ill-determined results unless they all have the same rated power.

==========

And by the way, the foregoing is a huge oversimplification because light
bulbs are enormously non-Ohmic. The resistance you measure with a digital
multimeter has very little correspondence to the ratio of actual V over
actual R under operating conditions.

==========

I have no idea why someone would write such a foolish experiment into a
textbook. Obviously they neither tried it nor even thought about it very hard.




At 05:34 PM 11/9/99 -0500, Yvon Jean wrote:

Here's a simple experiment I'm having trouble with. Our grade 9 science
text suggests experimenting with different voltage miniature screw
ligntbulbs operated at the same voltage. The purpose is to find the
relationship between the resistance of the bulb and the illumination
produced by the bulb.

I tried 3 bulbs (1.5 V, 3.8 V, 6V) sequentially wired to the same 1.5 V
drycell.
The least light is obtained with the 6 V bulb and the most light with the
1.5 V bulb, as I would have anticipated. But here is where it gets
complicated. I would expect that the greatest the voltage rating of the
bulb, the highest would be its resistance and lowest its current.
The current and resistance values were obtained with a digital
multimeter. Here are the results:
1.5 V bulb 3.8 V bulb 6 V bulb
1.6 ohms 2.9 ohms 1 ohm
0.22 A 0.18 A 0.28 A

The results were consistant with different bulbs from the same batch, and
were triple verified. Please enlighten...