Re: brightness vrs. power

[Leigh Palmer <palmer@SFU.CA>]

> 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:

I want to deplore this practice. Nothing undermines the goal of teaching
physics quite so thoroughly as such nonsense. Physics is a cosmology; it
is a way of formulating one's world view which is better than other ways
in an important aspect; it is quantitative. Revealed religions, for
example, fail miserably in describing the universe quantitatively.

This unrealistic model of the response of light bulbs can only lead the
perceptive student to distrust physics. It is just plain wrong, and
since the correct physics is well understood, why introduce this "ideal"
light bulb? You may feel that, somehow, the student should be gradually
challenged with more detail, but this is clearly not the correct problem
to start on. It is quite evidently wrong as soon as the student starts
to investigate it in the laboratory. I'm not talking about small errors
here. I use light bulbs in series as demonstration examples in my
introductory university physics lectures. They are not ohmic devices,
even "conceptually".

> > 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).

I don't know who Dr. Bob Beichner is, but he is wrong headed to approach
Ohm's law in this way. He must feel that light bulbs are to Ohm's law as
the orbit of Mercury is to Newton's law: corrections can be made later,
as they are needed, and as the student grows more sophisticated. Well, I
couldn't disagree with him more. The use of light bulbs to illustrate
Ohm's law is the sort of thing that gives "conceptual physics" a bad
reputation. No student will ever believe the lie that two bulbs in series
produce half the light of a single bulb connected to the same source. If
she does, she is not yet ready for physics. If she doesn't, she will look
elsewhere for enlightenment.

Leigh
he's just WRONG.