Re: Batteries and Capacitors

["John S. Denker" <jsd@MONMOUTH.COM>]

Emigh, David wrote:

> A charged capacitor has a potential difference when it is just
> "sitting around"

OK....

> A battery has no potential difference unless there is a current
> through it.

Is there any evidence to support that assertion?
Why can't I measure the battery voltage using
a field mill, which draws no current?
Even if I use a voltmeter (which does draw an
infinitesimal current) how can the voltmeter
tell the difference between a battery and a
large capacitor?

> IF I take a capacitor and drop its temperature to 5 K say, it will
> still have a charge to discharge. A battery at 5 K is dead, the
> reaction will not happen.

So don't do that.

I know lots of things that don't work at 5K.
Including bipolar transistors.
That doesn't mean they're not transistors.

> In a capacitor I can discharge as fast as I want depending on the
> circuit,

No, you can't.
Real-world capacitors all have frequency-response
limitations.

> A capacitor's potential difference changes as the the charge changes.
> A battery delievers a specific potential difference is a given
> circuit as long as there are chemicals left to react.

In the real world, battery voltage depends on
how fully bicharged the battery is.