# Fundamental questions about capacitors? [closed]

I have read several chapters in several textbooks, watched several in-depth tutorials, etc. There are still somethings that were either not explained clearly enough for me, or not explained at all.

What I Think I Know About Capacitors:

1) Capacitors are, at their core, two plates separated by a dielectric. The capacitance of the capacitor is affected by the surface area of the plates, the material that composes the plates, the material that composes the dielectric, and the thickness of the dielectric.

2) How fast a capacitor charges is dictated by RC time constants. There are 5 time constants. Each time constant increases by approx. 63% of the previous constant. The formula for the first constant is RC = T where R is resistance in Ohms, C is capacitance in farads, and T is the time constant in seconds.

3) Capacitors charge when applying positive charge to one plate and negative charge to the other. These charges on each respective plate are attracted to each other, yet cannot cancel each other out because they're separated by an insulator: the dielectric. Because of this, the charges "hold" each other in place until there is an avenue to travel and cancel each other out.

4) Capacitors resist sudden changes in voltage.

5) When the capacity of the capacitor is reached, if the capacitor is connected to the circuit in series, current will no longer be able to flow. However current will flow until the capacitor is charged. If a capacitor is connected in parallel, current will flow, and the capacitor will also charge.

6) Capacitors charge to the voltage of the supply voltage.

Questions:

1) In reference to "rule" number 5, when a capacitor is in parallel will current flow from the time current is applied to the circuit, to when it is removed, or will the capacitor "leech" current from the rails until it is charged, then current can flow normally through the circuit?

2) In reference to "rule" number 2, if there is no resistor, how fast does the capacitor charge? Instantly?

3) How does one know how much CURRENT can be supplied by a capacitor at a given voltage?

4) If one plate of a capacitor is suddenly given a path to ground which is not connected to the other plate, does it retain its charge? If this is not true and the charge on the plate connected to ground is not retained, when it is "swept away" to ground, what happens to the charge of the other plate?

5) Is rule number 4 only applicable to capacitors in parallel? If not, why would a capacitor in series resist a change in voltage?

If I've misunderstood any concepts related to the capacitor, I would be happy to hear about how and why I did.

## closed as too broad by Leon Heller, PeterJ, nidhin, Daniel Grillo, RicardoMay 28 '15 at 11:51

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Single design questions are required. – Leon Heller May 27 '15 at 21:45
• @LeonHeller I actually have a question about this in meta currently. It can be found here...meta.electronics.stackexchange.com/questions/5254/… Looking at the question guidelines found here... electronics.stackexchange.com/help/dont-ask it doesn't strictly prohibit multiple questions. It also stands to reason that multiple closely-related sub-questions would be a benefit, not a downfall. If my meta question comes up with a similar conclusion to yous, I would be happy to separate the questions. – Allenph May 27 '15 at 21:53
• 1: The former; 2: instantly (in reality, zero resistance doesn't exist); 3: infinite, depends on the resistance looking out from the capacitor; 4: no current flows to "ground" unless there's a path to the other plate of the cap to ground. Electricity only flows in loops; 5: no, it applies to all capacitors, because the charge can't teleport (i.e. move instantaneously somewhere else), hence the voltage must be continuous, since it is determined directly by the capacitance between the plates (constant) and the amount of charge on the plates (a continuously variable quantity). – Shamtam May 27 '15 at 22:02
• No time for a full answer, hence the quick comment. – Shamtam May 27 '15 at 22:02

I won't try to answer every one of your questions, but there's one key equation that can help to answer several of them:

$$Q=CV$$

This is the fundamental relationship of a capacitor: The charge "stored" or separated between the two plates is equal to the capacitance of the device times the voltage between the plates.

when a capacitor is in parallel will current flow from the time current is applied to the circuit,

From the equation we see that current must flow through the capacitor (in order to increase or decrease Q) whenever the voltage across it is changing.

In reference to "rule" number 2, if there is no resistor, how fast does the capacitor charge? Instantly?

It charges as fast as the external circuit can provide charge (or move charge between the plates) to charge it.

How does one know how much CURRENT can be supplied by a capacitor at a given voltage?

It can either supply a large current for a short time or a small current for a long time, until the "stored" charge is exhausted.

Real capacitors (as opposed to ideal ones) also have parasitic resistance and inductance that limits their ability to deliver very large currents or to change the current flow very quickly.

why would a capacitor in series resist a change in voltage?

Because in order for the voltage to change, charge must be moved from one plate to the other. For the voltage across the capacitor to change instantly, an infinite current must flow through the attached circuit to transfer the charge. No physical circuit is able to carry an infinite current.