Tuesday, July 28, 2009

Basic electronics - capacitors - and bin lorries!

Countdown timer: 10 weeks 0 days to go

Right, we've done a bit on resistors, so now it's on to capacitors. Here's a photo showing some capacitors, taken from this website.

There are a few dictionary definitions for capacitance, but one is very helpful in bringing us towards a quick understanding of capacitors:

Capacitance: the property of being able to collect a charge of electricity.

Wonderful! So just like a bin lorry collects and stores rubbish (until it's deposited in a rubbish dump), a capacitor accumulates and holds a charge of electricity.

There's a nice quick and easy definition of a capacitor at Dictionary.com:

n. An electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric.

And we read that the capacitor, just like the bin lorry, stores the charge "temporarily" before returning it to the circuit. So far, so good. Just like a squirrel hoards nuts, a capacitor hoards electrons.

Then it gets a tad deeper, with the introduction of electric fields. No, not a new farming method. More like something from Star Wars. Read this from allaboutcircuits.com: "Whenever an electric voltage exists between two separated conductors, an electric field is present within the space between those conductors".

I'm paraphrasing the rest now: Circuits can be thought of as "conductive paths" through which electrons can flow. However, in the case of fields, we're talking about "interactions that can be spread across empty space". Hmmm. Deep. A "field" is an "abstract concept" - it doesn't have mass, and it does not need to exist within matter at all. Toying with two magnets and observing how they either attract or repel each other is a good way to imagine such a "field". (Although that's a magnetic field, not entirely dissimilar to an electric field). Think of Star Wars and "the force" and moving objects around by invisible forces. Field strength is measured in volts/meter, according to the IRTS disk.

Now some further information from electronics-radio.com:

It is found that when a battery or any other voltage source is connected to the two plates as shown a current flows for a short time and one plate receives an excess of electrons, while the other has too few. In this way one plate, the one with the excess of electrons becomes negatively charged, while the other becomes positively charged. If the battery is removed the capacitor will retain its charge. However if a resistor is placed across the plates, a current will flow until the capacitor becomes discharged.


The basic unit of capacitance is the Farad, named after Michael Faraday. Maybe to help us remember this, we could think that a capacitor can only hold charge "for a day" - Far-a-day?

A capacitor has a capacitance of one Farad when a potential difference of one volt will charge it with one coulomb of electricity (i.e. one Amp for one second). (A coulomb, by the way, is six million million million electrons. I learned that last week, and can't believe I actually remember it! PS: That's a lot of nuts to be hoarded by our squirrel !)

A capacitor with a capacitance of one Farad is too large for most electronics applications, and components with much smaller values of capacitance are normally used. Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

* µ means 10-6 (millionth), i.e. 1000000µF = 1F (so 1 µF = .000001 ?)
* n means 10-9 (thousand-millionth), i.e. 1000nF = 1µF (so 1 nF = .000000001 ?)
* p means 10-12 (million-millionth), i.e. 1000pF = 1nF (so 1 pF = .000000000001 ?) (Link)

One more thing to look at is capacitor charging and discharging, but I won't go into that right now because I need to get all the above into my head, especially the units of measurement. For now, I will post this wee image and try to get it into my head along with all the other stuff.

Any questions or observations, just pop me an email or a comment. And remember, I'm not teaching this stuff, I'm learning it! Thanks.

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