Re: Batteries and Capacitors

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

Others have offered the 'physical' differences between capacitors storing
charge and batteries, let me try and offer some practical differences by
citing some uses.

One is a current thread--flash bulbs.  You need to drive a significant
amount of charge through the bulb in a short time to get the high intensity
flash.  A battery can supply the charge--by converting chemical PE--to
charge up a capacitor with the appropriate amount of charge.  The battery
(at least a small cheap battery) can't supply the current quickly enough by
itself, but if the bulb is put directly across the charged capacitor and the
bulb resistance is low, you can get a high transient current.

A digital alarm clock.  Many DO have batteries to keep the time during power
outages.  However, this raises the price ($2 nine-volt battery) and of
course you probably won't know that the battery is dead until you have a
power outage.  Some cheap clocks use a capacitor backup to power the clock
for a few seconds during a transient power outage.  It is cheap and always
works but only handles those outages of a few seconds--the kind we seem get
all the time.  You'll notice that seemingly NO microwave manufacturer has
bothered to provide that convenience for their clock circuits!

Audio power amplifiers.  The power supply of an receiver or amplifier can
usually keep up quite well providing the electrical energy needed to drive
the speakers.  However, strong low frequency notes and very fast transients
in any frequency range can call for an immediate need for large amounts of
power.  Good amplifiers have a couple of large capacitors (on the order of
3000 micro-farads--very dangerous by the way) to store charge to power such
large or quick excursions of the speaker cones.  Even at 'ear damaging'
sound levels, most amplifiers are running at 1-5 watts average output.
However, a sharp transient or strong deep note can call for 100-1000 watts
for several milliseconds.  The capacitor is a good solution to this problem.

fwiw

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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----- Original Message -----
From: "Tina Fanetti" <FanettT@WITCC.COM>
To: <PHYS-L@lists.nau.edu>
Sent: Wednesday, February 12, 2003 11:43 AM
Subject: Batteries and Capacitors


> 'lo
> In my calc based physics class, we are studying circuits and I talked
about batteries and capacitors.
> I explained that a capacitor stored electrical energy..yadda yadda
>
> One of my students then asked me what the difference between a capacitor
and a battery is.  I was like a capacitor stores energy, and a battery is a
voltage source.
> He then asked me why you would need a capacitor in the first place, why
not just use batteries.  I was like you need to store charge for somethings
and a battery can't do that.
> It seems there is a piece in here that is important.  He thinks something
about the battery and the capacitor is the same and I am at a loss to
explain what that could not  be or how to explain the differences better.
> My guess is that he thinks that a battery has charge and holds the
charge...
> Thanks
> Tina
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429