I volunteer at a club to teach teens Arduino and basic electronics: LEDs, resistors, voltage, current, etc. I also describe what they do:
Batteries, chargers and other power supplies:
- These supply plenty of electrons to make the circuits work.
- The higher the number of electrons it generates at once, the higher the voltage it supplies. Too many can burn out a device not expecting it: explosion!
- The higher the number of electrons it can supply per time period, the higher its rated current. If the circuit demands more than the supply can actually deliver, the supply either overheats and explodes, or gives up by stopping producing electrons (lowers the voltage).
LEDs get "excited" when electrons pass through them, and they glow when there are enough:
- It takes a certain number of electrons to get things started - but once that happens LEDs will quickly let so many through that they overheat and explode!
Resistors don't let all the electrons through at once - the higher their value, the fewer electrons they let pass:
- They make up the difference in heat;
- If there are lots of electrons (high voltage) but little resistance (value), there will be a high number of electrons per time period (current). The resistor will get hot, and maybe overheat and explode!
- Putting a resistor inline with an LED will limit the number of electrons per time period (current) passing through the LED, stopping the LED from receiving too many at once, and preventing overheating and explosions.
(I've found that emphasising explosions keeps them interested and amused: see LEDs no longer bang - or whimper!)
But when I get to capacitors, I've never found a convincing explanation - and the Wikipedia Capacitor "Hydraulic analogy" just makes me cringe: it hand-waves a lot of the effects.
So I describe it thusly:
A capacitor is two large plates of thin metal, separated by a very thin layer of "stuff" (called a dielectric) that doesn't let electrons through. They can be manufactured by rolling that triple "sandwich" into a tight cylinder or other compact form (with suitable insulation); they still work the same. The larger the plates involved or the closer the two plates are to each other, the higher rating the capacitor has. Of course, the dielectric has limitations: for example, too high a voltage between the plates will burn straight through the dielectric. This will "short" the two plates together into a very low value resistance: explosion!
If you put a capacitor across a battery, interesting things happen. Think about when the capacitor was manufactured: both plates of metal have (about) the same number of electrons in each of them. When you put the combination across a battery, then one side gets extra electrons supplied. Those electrons can't pass through the thin dielectric; but since they're so close, they actually electrically "repel" the other plate's electrons, forcing them out the other side of the capacitor. As more electrons go in one side, more are repelled and forced out the other. Note none are passing through the capacitor: the ones leaving just don't want to be there anymore!
However that effect is limited. Pretty soon the second side starts to run out of electrons to repel - the ones still there are bound too strongly to their atoms to be so easily pushed out - and the 'apparent' current leaving the capacitor dwindles to nothing. The capacitor is considered "saturated", or fully charged. The electrons in the over-supply side still kind of want to stick around: they're still being pushed by the incoming voltage, and are even attracted to the positive charge of the opposite side that they helped to create! If the voltage reduces though, then some electrons will come back out: the capacitor is discharging.
If you disconnect the capacitor from the battery completely, each side still holds an unequal number of electrons. In fact, if you were to touch both ends you'd provide a path that would allow all of the excess electrons on one side to immediately make up the deficiency of electrons on the other - and there'd be nothing other than your inate resistance to limit the passage of those electrons! In other words, if the resistance was low enough, there'd be an explosion - in YOU! (Ouch!) Note that since there's no passage of electrons through the capacitor - they're all there just waiting to move - it can discharge extraordinarily quickly.
So, given that a capacitor can store excess electrons but also release them quickly, that means that they're good at "smoothing" peaks and troughs in either voltage or current - until it saturates or runs out of charge.
So, I fully agree that the above does not follow the standard description of capacitors. But that's not the question - is the above wrong?
