Re: Circuit Question

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

Tina,

        Starting at the end of your message where you
asked, "Doesn't a battery put the same amount of
current out regardless of what is hooked to it?"
Consider this in the extreme case that the
terminals of the battery are not connected to
anything.  I think we can agree that there is no
current.  Inside the battery, the chemical
reaction that transports charges to the anode and
cathode of the battery are stopped when the
electrodes become charged beyond a certain level.
This charging of the electrodes produces a voltage
barrier to the further transport of charge within
the battery.

        In the case of a lightly loaded battery, the
speed of the chemical reaction is limited to the
speed at which charge is removed from the
terminals via the circuit. (Here I am incorrectly
pretending that both signs of charge move in the
circuit.)

        In a zeroth approximation, the battery "knows" to
keep the anode to cathode potential difference
high enough to stop the chemical reaction ( or set
up a equilibrium with the inverse of the "normal"
reaction.  As soon as the potential drops because
an electron has been transported via the circuit,
the battery responses by transporting another ion
to the terminal.

Thanks
Roger Haar

**********************************************************************
Tina Fanetti wrote:
> Hello
> I am getting ready to teach circuits in the next week.
>
> 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 batter "know" that it is supposed to put out more current for different lighbulbs?  Mathematically it makes sense but conceptually it doesn't to me.
>
> I know my students are going to ask me this and I am not wanting to say cause of the little guy that lives in batteries.
>
> Doesn't a battery put the same amount of current out regardless of what is hooked to it?
>
> Thanks
> tina
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429