From: Geoff Ruth (gruth@leadershiphigh.org)
Date: Fri May 24 2002 - 10:31:57 PDT
Message-Id: <a05010402b91428eac40f@[192.168.1.25]> Date: Fri, 24 May 2002 10:31:57 -0700 From: Geoff Ruth <gruth@leadershiphigh.org> Subject: Re: pinhole battery questions
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.
>>
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>
>
>
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