Unraveling capacitors

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By Collin Cunningham
Posted 2009/03/13 @ 5:16 am
Category Electronics

From the NYCResistor crew comes this action-packed crash-course in capacitors – complete with a cliffhanger ending! Here’s hoping the saga continues with a miniature rescue mission via shrink ray (they must have one in their somewhere, right?)

If after that, cap functionality still seems a bit fuzzy, check out this rather helpful little web demonstration from Molecuar Expressions.

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5 Responses to Unraveling capacitors

wbeaty on March 13th, 2009 at 1:11 pm said:

Awwww, the Molecular Expressions simulation is wrong.

Rats.

In reality, the wires are *always* full of electrons, and the battery just pulls electrons out of one capacitor plate while pushing an identical amount into the other plate. The amount of electrons inside the capacitor is never changed. (Many books get this fact wrong, so I guess a common mistake can leap off the pages to infect java applets!)

Put simply: capacitors don’t store electric charge, instead they store electrical energy. They store joules, not coulombs.

A good capacitor analogy is a water-filled tank with a rubber sheet down the middle. If you suck water from one side of the tank while pumping water into the other side, the rubber sheet gets stretched, and you create a rising pressure-difference. (A constant current creates a rising voltage.) But the amount of water in the tank won’t change. The tank gets “charged up” with energy, not with water.

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The Universal Dilettante on March 13th, 2009 at 2:29 pm said:

Aside from Molecular Expressions getting it wrong, I also think that you’re really doing a disservice to the purposes of caps. Yes, you covered capacitors to the level of a high school physics class, but most of the reasons you use capacitors have nothing to do with the basic experiments shown here.

Please, please, please cover capacitors as filters, since that’s essentially their overwhelming use case.

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Collin Cunningham on March 13th, 2009 at 2:34 pm said:

@wbeaty – aah, now that you mention it that explanation makes sense. Electrons from one plate wouldn’t run by others on their way to opposite plate – they’d just all shift over.
Perhaps someone along the line thought the above above description was easier to communicate (?)
thanks for chiming in!

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Collin Cunningham on March 13th, 2009 at 3:06 pm said:

@Dilettante – the experiments were intended to be just that – basic, approachable, and accessible. I’m happy to hear you believe the vid reaches high school physics – not bad for 8 minutes. I’d love to delve into more common applications and hopefully will in the future. Thanks for the feedback

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Tim on March 13th, 2009 at 6:08 pm said:

Woa, loads of very dodgy explanations of how capacitors work here!

The first video says that the reason the charge is retained without a voltage applied across it is because the +ve and -ve charges on each plate are attracted to each other! The is of course rubbish. The real reason is that the charges have nowhere to go. As soon as you add a wire they’ll happily flow around it.

wbeaty: I can see your point, but I think it is equally valid to think of each plate storing a +ve or -ve charge (where +ve = lack of electrons). Of course you could build a capacitor that actually does use positive and negative ions. Also grounding one of the plates would make it literally store charge.

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