Re: brightness vrs. power

[Robert A Cohen <bbq@ESU.EDU>]

Leigh,

   Could you describe your series-parallel configuration?  I'm assuming
you are comparing a single light bulb...

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

with the following series-parallel configuration...

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

If R was the same in each case, then the total current would be the same
and the current through each light bulb would be halved, correct?  Due to
the dependence of R on temperature, there is actually less resistance in
the second circuit and thus more total current.  Is this your point?  It
seems you are implying something about how the brightness of the bulb
might be (naively) expected to be brighter, the same as, or dimmer than
the single bulb alone, but I didn't catch it.  Perhaps I am still lacking
your insight.  Please illuminate.

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

On Wed, 3 May 2000, Leigh Palmer wrote:

> 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