Capacitors

capacitors

A few capacitors – lots of different sizes.  Big ‘swiss-roll’ ones mean greater parallel plate area and smaller plate distance thus larger capacitance. Some have to be connected the right way round – called electrolytic capacitors. Here’s why. One way to make a large capacitor is to take two long strips of aluminium foil (=large plates), put strips of isolating materials between them, and make a nice compact swiss roll. Capacitors up to around 1MF can be made this way, but they are physically big, so if we want even higher capacity, we need to look for other things than plate area. It happens that we know of a very thin and very voltage resistant type of isolation material: Aluminium oxide. If we cover a strip of aluminium foil with a thin oxide layer, we have one plate and a very thin dielectric. Problem now is to make the other plate come close enough to the other side of the oxide layer. The thing that comes really close is a liquid, so if we submerge our oxide covered plate in a conducting liquid, the liquid forms the other plate, and we can make a very large capacitor. A conducting liquid is called an electrolyte, see?

Nothing comes for free, so this type of capacitor has its drawbacks. Some have practical solutions: Instead of having liquid sloshing around inside the capacitor, an electrolyte-soaked paper is used, some modern types are even virtually solid. Others become restraints we have to live with:

The oxide layer is made by an electrolytic process; the foil is submerged in some liquid and current is passed through the liquid into the metal, forming the oxide layer. This is an advantage and a disadvantage: The good news is that the dielectric layer has self-healing capabilities, so if a weak spot occurs, the resulting leakage current will more or less rebuild the isolation. This is the reason electrolytic capacitors can regenerate if you raise the voltage over them slowly. The bad news is that the process is reversible! If you reverse the polarity, even the slightest leakage will begin to tear the dielectric down, resulting in more leakage, which tears away more dielectric, which… well, you get the picture. This is the reason you need to observe polarity strictly when using electrolytic capacitors.

Here’s a little applet to show how a capacitor charges and discharges for those who like it simple..

Here’s a bottom-up capacitance tutorial. Almost everything we need to know and more is here.….pick and choose what you need. No integration is necessary, but, if you can follow the maths, so much the better.

This link can be used to explore some more of Edexcel’s interactive demos. Well worth a look…

Finally, years ago, when I was a boy, there was radio. We had a gizmo on the front called a tuner, a twiddly thing which changed the plate overlap and hence the capacitance of a variable capacitor.  Looked like this….cap1

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