# What is a capacitor? [video]

[Video Link] Collin Cunningham makes excellent educational videos about electronics. Here's his latest one, about capacitors. (Via Make)

## Notable Replies

1. I'm not watching unless he explains the flux capacitor...

2. I think I had the hardest time figuring out transistors initially — one of the biggest things that helped me was this site, which includes important clarifications like:

Current does not flow. Electric current never flows, since an electric current is not a stuff.

3. I find that to be a very misleading explanation of how a BJT works. A BJT is certainly a current controlled device, accordingly current is simply created by voltage and resistance, but it is a important distinction. You can not simply wire a battery directly into the base of a transistor and not have it cause problems (at least not usually) - this is something that can be done with the gate on a MOSFET (since it is a voltage controlled device).

For the simple ON/OFF explanation it works well enough (simplifying it to the level of a two step diode), but even they say:

Just by coincidence, the tiny base/emitter current is proportional to the large collector/emitter current. The transistor ACTS as if it is amplifying current. But it's really using a small change in voltage to create a large change in current.

Ironically that small change in voltage causes a small change in the input current, which in the vast majority of circuits will show a resistor (or some type of input resistance) that causes this voltage/current relationship. When trying to pin point input bias currents for a BJT output stage of an audio amplifier one does not look to the spec sheets for voltages, they would be presented in terms of current. You would then use that, along with the voltage that your amplifier is running off of to calculate the amount of input resistance to give you the needed bias current.

4. This video has the same problem that I have with nearly every "introductory" electoronics explanation that covers capacitors - how are they used?

Great, I can charge it. But either 1) it has to discharge in bursts, or 2) my input power is stronger than the rate that I'm discharging the capacitor. So what good is it? When would I use it?

Someone who doesn't really understand electronics well can sort of understand how a diode is used, or how a resistor is used, how electricity flows conceptually, etc. But never seem to get what the applications for a capacitor are versus, well, a constant power source.

I'm not asking the question, just wish these things would take that extra step for beginners.

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