From: pauld@exploratorium.edu
Date: Fri May 24 2002 - 12:38:08 PDT
Message-Id: <200205241938.g4OJc6p01105@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole battery questions Date: Fri, 24 May 2002 12:38:06 US/Pacific
Hi Geoff
In a pure battery, the electrons which flow out of the battery are generated by
chemical reactions as they are needed.
In a pure capacitor the electrons are already free and are stored on metal
plates.
The flow out of the battery is limited both by the maximum reaction rate and also
by the flow of charge within the battery from one electrode to the other. These
two effects combine to give the battery an internal resistance.
(I say pure battery and pure capacitor because recent creations blur the
difference.)
Paul D
> Algis,
>
> This is very helpful. I'm confused about the distinction between a
> capacitor and a battery: it sounded like from what you wrote that the
> limiting factor was the resistance of the light bulb and not the
> galvanic cell. So in a flash, wouldn't the difference be in the
> resistance within the flash bulb, and not the resistance within the
> battery/capacitor?
>
> I guess I am also confused about why a battery has its own internal
> resistance. We made galvanic cells using zinc and copper metals and
> solutions in class, and I couldn't identify what item was increasing
> resistance and decreasing discharge. What would that be, if we used
> really heavy gauge wire to connect the metals?
>
> Thanks again for your help!
>
> - Geoff
>
> >I don't have all the answers, but here are some:
> >
> >One reason a battery can't discharge all its charge at once is
> >because it has its own internal resistance. I guess this is why if
> >you short circuit a battery, not only will the wire heat up, but
> >also the battery.
> >
> >The battery does "try" within the limits of its own resistance to
> >discharge all at once into the light bulb, but the light bulb's
> >resistance is so much greater than the battery's that it limits the
> >current at a pretty steady, slow rate.
> >
> >There is a device that can discharge all its charge "at once". It
> >is a capacitor. They are used to store charge for a flash (it takes
> >time to charge up) and it lets the charge out all at once, which
> >allows a small battery to create such a bright (though brief) flash.
> >
> >Algis Sodonis
> >Urban School of SF
> >
> >
> >pinhole@exploratorium.edu writes:
> >>One of my brilliant students came up with some questions that I don't
> >>understand.
> >>
> >>If you directly connect the terminals of a battery with a wire, it
> >>will start to shortcircuit and heat up. Why, though, doesn't the
> >>battery immediately discharge when it's shortcircuited? Is it because
> >>the wire has a resistance and therefore can only carry a certain
> >>amount of amperage at once?
> >>
> >>Here's another question: if you hook up a battery to a small
> >>lightbulb, it can power the lightbulb for hours, until the battery
> >>runs dead. Why doesn't the battery immediately "try" to discharge in
> >>a massive burst of current, and burn out either the wires or the
> >>lightbulb filament?
> >>
> >>In other words, it seems like something is controlling the rate of
> >>discharge of the battery.
> >>
> >>Both these questions came up through studying electrochemistry, not
> >>through the physics side of e/m.
> >>
> >>---------------------------------------------------------------------------
> >>
> >>To unsubscribe from pinhole, send an email to requests@exploratorium.edu
> >>with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
> >>of the email.
> >>
> >>To subscribe to the digest and only get 1 combined message a day, send an
> >>email to requests@exploratorium.edu with the words 'subscribe digest
> >>pinhole' (without the quotes) in the SUBJECT of the email.
> >>
> >>Check out what your colleagues have written on Pinhole in the Pinhole
> >>archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
> >>---------------------------------------------------------------------------
> >>
> >>
> >
> >
> >
> >---------------------------------------------------------------------------
> >
> >To unsubscribe from pinhole, send an email to requests@exploratorium.edu
> >with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
> >of the email.
> >
> >To subscribe to the digest and only get 1 combined message a day, send an
> >email to requests@exploratorium.edu with the words 'subscribe digest
> >pinhole' (without the quotes) in the SUBJECT of the email.
> >
> >Check out what your colleagues have written on Pinhole in the Pinhole
> >archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
> >---------------------------------------------------------------------------
>
>
> ---------------------------------------------------------------------------
>
> To unsubscribe from pinhole, send an email to requests@exploratorium.edu
> with the words 'unsubscribe pinhole' (without the quotes) in the SUBJECT
> of the email.
>
> To subscribe to the digest and only get 1 combined message a day, send an
> email to requests@exploratorium.edu with the words 'subscribe digest
> pinhole' (without the quotes) in the SUBJECT of the email.
>
> Check out what your colleagues have written on Pinhole in the Pinhole
> archives at: http://saturn.exploratorium.edu/ti/alumni/pinhole.html
> ---------------------------------------------------------------------------
>
>
>
--------------------------------------------------
This message was sent using Exploratorium web mail
http://www.exploratorium.edu/
This archive was generated by hypermail 2.1.3 : Mon Aug 05 2002 - 09:21:42 PDT