Re: Circuit Question

[Brian Whatcott <inet@INTELLISYS.NET>]

At 02:45 PM 2/7/02, Tina Fanetti wrote:

> I was looking at a question that has a battery hooked up to a 5 W bulb
> and then the battery is hooked to a 10 W.  It then asks which case
> has the greater current and then which light bulb has the greater
> resistance.
>
> Now comes the silly part.
>
> How does the battery "know" that it is supposed to put out more current
>  for different lighbulbs?  Mathematically it makes sense but
> conceptually it doesn't to me....

> Tina Fanetti


There is an interesting and educational question to be answered here.
A purist might wail about the anthropomorphic turn of phrase, but the point
remains that a messenger is sent from the battery, and when it returns, the
battery is instructed to act appropriately.

I will specify that the leads which connect to the lightbulb have some
appreciable length, so that when connected (say by a switch) - there is a
wave of potential which progresses along the wires accompanied by a current.
The battery is faced with some effective resistance constituted by the
thickness of the wires, and their spacing and this determines the dynamic
resistance to which it supplies a current - batteries usually being fairly
modelled by an ideal voltage source in series with some internal resistance
- until at the end of the wires, a filament sinks a greater current than
the battery has initially been supplying, so that a wave of reduced
potential returns to the battery which is thereby induced to
supply more current.

You will see some discussion of a similar topic on the trumpet thread - but
you will notice that the transmission line approach (which is what is being
demonstrated here,) is not in common use.




Brian Whatcott
  Altus OK                      Eureka!