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Re: brightness vrs. power



Michael and I are still at odds. I would never say that a light bulb
follows Ohm's law because it doesn't, and that's a pretty strong
reason. I would say that a resistor follows Ohm's law, and if a
student fries a resistor in demonstrating the contrary that's OK; I
can accept that. Light bulbs obey Ohm's law in the same way, but if
you can see light coming from the bulb then it surely is not in a
regime where it obeys Ohm's law, and it's light emitting bulbs that
we were talking about in the first place.

I've constructed the apparatus and performed the experiment. Using
130 volt, 60 watt bulbs selected for their similarity, and connected
to line voltage of 118 volts:

setup line total illuminance color
current power at 1 meter

single bulb 442 mA 52 W bright yellowish

four bulbs 607 mA 72 W ~ one stop orangish
*dimmer*

The four bulbs are wired in series-parallel, as mentioned in the
previous posting

I used a Polaroid lightmeter behind a translucent screen and got a
difference of 1 EV, which is one stop, or a factor of two in the
illuminance on the screen. I was surprised the difference was that
little; the apparent difference is large. No meter is needed to
detect it. I'm sorry I didn't have a better lightmeter. The color
difference is also evident, either on the wall or looking directly
at the bulbs.

In my view this should put the question to rest. It is a real test
of Ohm's law (V=IR version). The two currents should be the same to
withing experimental tolerances if Ohm's law is applicable here;
they aren't. The two brightnesses should be the same, and they
aren't. The illuminance is not solely a function of the power.

There's plenty to talk about in this demonstration. My most
important point is that Ohm's law is not an important one, and
that light bulbs are an unsatisfactory example of the application
of Ohm's law.

Leigh