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Re: Batteries and Capacitors



Tina,
Your student is partially correct. A capacitor CAN take the place of
a battery. An example is the "Nightstar" flashlight that uses a capacitor
and LED instead of a battery and bulb. (The capacitor is charged by shaking
a magnet back & forth inside of a coil...very cool, even though as a
flashlight it's pretty dim). You're supposed to get about 1/2 hour of light
for every 5 minutes of shaking. Either Educational Innovations or Arbor
Scientific sells the Nightstar; I forget which. It's about $40 and more
effective as a demo if it's "dead" to start with. You can also buy, from the
same vendors, a 1 Farad (yep, a whole entire Farad!) capacitors (also $$$)
which, after charging by a Genecon (more $$$, alas), will keep a flashlight
bulb lit for a few seconds. Hence demonstrating that yes, you can replace a
battery with a capacitor but not for very long.

Think of a capacitor as a "bucket" that holds charge, and a battery as a
"pump" that shoves charge around and keeps it moving. The battery doesn't
store charge. It just makes charge flow.

Vickie

-----Original Message-----
From: Tina Fanetti [mailto:FanettT@WITCC.COM]
Sent: Wednesday, February 12, 2003 10:44 AM
To: PHYS-L@lists.nau.edu
Subject: Batteries and Capacitors


'lo
In my calc based physics class, we are studying circuits and I talked=
about batteries and capacitors. =20
I explained that a capacitor stored electrical energy..yadda yadda

One of my students then asked me what the difference between a capaci=
tor and a battery is. I was like a capacitor stores energy, and a ba=
ttery is a voltage source. =20

He then asked me why you would need a capacitor in the first place, w=
hy not just use batteries. I was like you need to store charge for s=
omethings and a battery can't do that.

It seems there is a piece in here that is important. He thinks somet=
hing about the battery and the capacitor is the same and I am at a lo=
ss to explain what that could not be or how to explain the differenc=
es better.

My guess is that he thinks that a battery has charge and holds the ch=
arge...

Thanks
Tina

Tina Fanetti
Physics Instructor
Western Iowa Technical Community College
4647 Stone Ave
Sioux City IA 51102
712-274-8733 ext 1429
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