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brightness vrs. power



Hi,

I forgot to mention that I assumed "ideal" bulbs which would have constant
resistance. Of course this is not true but nevertheless it is used in
conceptual exercises, e.g. Dr. Bob Beichner's wonderful DIRECT test makes this
assumption. If this is assumed and internal resistance of the electrical source
is very small then:

For instance when two
similar bulbs are connected in series the net current is (approximately)
halved. The power dissipated in both bulbs is reduced by factor 1/4.

I don't know how you calculated this, but it is incorrect. Light bulbs
do not obey Ohm's law. They are extremely nonlinear.

This is an interesting question; there was a good discussion on this in the
PhyShare list. I would like to state that a bulb filament is an ohmic conductor.
The reason for non-linearity is change in temperature. Definition of Ohm's law
requires constant temperature and this is definitely not the case in bulb
filament. Naturally resistance of most metal conductors increases if
temperature increases (rigorous explanation involves quantum mechanics).

Regards,

Antti Savinainen
Finland