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Re: brightness vrs. power

On Thu, 4 May 2000, Leigh Palmer wrote:

Ohm's law is not a law of Nature. It is not even unique, as we have
seen in this discussion. It is not applicable to light bulbs except
in a perverted form of very limited applicability.

I'm still confused about why I can't use V/I as the definition for
resistance.

Unless I'm mistaken (if I am, I'm sure someone will correct me), Ohm's law
is called a law not because it is true but because it was developed
empirically (like Newton's Laws). I admit that this confuses students,
but rather than avoiding the terminology, I think we need to emphasize
what "law" means. Otherwise, students will continue to think "a law is a
proven theory."

In any event, defining R to be V/I is not the same as saying that V/I must
be constant. The latter is Ohm's law. Okay, so Ohm's law isn't exactly
true - is there anything wrong with defining R to be V/I? (or sigma to be
J/E, or rho to be E/J, etc.)?

you are comparing a single light bulb...

---------------X-------------

with the following series-parallel configuration...

/-----X------X-----\
----- -------
\-----X------X-----/

[snip]

That's right. If the power dissipated is greater, the beginning
student should expect the four lights to be brighter. They aren't.

Actually, I thought beginning students would guess (assuming constant
resistance) that the total current in each case would be the same and so
each light bulb in the second circuit would be half as bright as the lone
light bulb in the first circuit. Consequently, a qualitative examination
of the set-up (brightness) would support their prediction. Only a
quantitative check of the currents (with ammeters) would reveal a
potential weakness in their model.

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| Robert Cohen Department of Physics |
| East Stroudsburg University |
| bbq@esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
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