# B Question on how capacitors function...

1. Oct 31, 2016

### cmkluza

I've looked at a few videos on how capacitors work, and have a fairly good, if basic, idea of how they charge (current causes collection of opposite charge in plates, creating electric field between two plates), but what I'm not seeing is how they discharge. No videos I've seen have said anything about this, they've just stated that they discharge. How/why does this discharge occur? Am I correct in assuming that the opposite charges at the plates push each other, causing current? If so, would this mean that discharging current goes in the opposite direction of charging current?

Is my current interpretation of how capacitors work correct? If no, where am I going wrong? If yes, is there anything you guys could add to it?

Thanks for any input!

2. Oct 31, 2016

### Bystander

Yes and yes. Electrolytics are a whole another breed of "cat."

3. Nov 1, 2016

### cmkluza

Sorry to revive an old post, but I just had another question come to mind. Can capacitors simultaneously charge and discharge? From what I've heard, and seen in practice, capacitors add stability to the voltage in a circuit (make it "smoother"). But, I'm struggling to connect this to my current understanding of how capacitors work. Surely, that would require simultaneous charge and discharge, no? But, given that charge and discharge currents have opposite flowing directions, can this really happen? I can't imagine that it would. Perhaps I'm looking at the situation incorrectly. Could you (or anyone else who sees this) help me understand how capacitors provide "smoother" power to a circuit?

4. Nov 2, 2016

### CWatters

Not exactly. A smoothing capacitor is usually connected in parallel with a voltage source that is non-ideal (eg a voltage source that varies so needs "smoothing"). For example consider the extreme (but common) case where the voltage source switches ON and OFF. When it's ON the voltage source supplies power to the load and charges the capacitor. When the voltage source is OFF the capacitor discharges delivering energy stored in the capacitor to the load. The capacitor acts a bit like a backup battery so the load has a power source all the time. The capacitor charges when the voltage source is ON and discharges when it's OFF. Not both at the same time.

There is much more to it than that simple view. A capacitor is not an ideal voltage source. The voltage on a capacitor falls as it's discharged so it can't maintain a constant voltage to the load.

Capacitor equations..

Q=VC (handy tip to remember this is "Queen=Victoria Cross")
so
V=Q/C
differentiate
dV/dt = (dQ/dt)/C
dQ/dt = I
so
dV/dt = I/C

so the voltage changes at a rate that depends on the current I drawn by the load and the capacitance C. To maximise voltage stability you want small I and big C. Unfortunately the capacitance C determines the physical size and cost of a capacitor, so usually some sort of compromise has to be achieved. Usually the voltage doesn't have to be 100% constant because the load can tolerate a "ripple" (small variation in the voltage).

5. Nov 2, 2016