[islandlabs] Electrostatics: What is a 1 Farad Capacitor?

Jim Robert jim.mixtake at gmail.com
Tue Sep 22 08:32:14 EDT 2009

damn... really well done! you should post it on a blog for the rest of the
world :)

On Mon, Sep 21, 2009 at 10:29 PM, Burns, William <burns at cshl.edu> wrote:

> Hey:
> Another question came up at the last meeting:
> "What is a Farad?"
> Also, there seemed to be a question about what a capacitor actually was,
> so the second question could be:
> "What is a capacitor?"
> As per usual, I gave the best answers I could at the time, but without
> the feeling of confidence/conviction that the subject deserves.
> After spending some time w/ Google:
> A capacitor is a tiny battery for storing electrical charges.
> It will conduct electricity for only a brief period until it is filled
> then (as a battery does) can apply it's small charge to other parts of a
> circuit.
> note 1: I say "tiny battery" because most of the capacitors we see are
> very small.
> note 2: Originally, the term "battery" referred to collections of
> capacitors, and was later applied to the electro-chemical devices that
> we call batteries today.
> Capacitors work by storing opposing (positive and negative) charges.
> Conductive elements (like wire, or tin-foil) have free electrons that
> can move about "freely".
> If a voltage is applied to a pair of parallel wires: (where the wires do
> not touch each other, one wire gets a positive voltage applied to it,
> and the other wire gets a negative voltage)
> Some negative electrons will be drawn (borrowed/donated) out of the
> "positive" wire, and an equal number of electrons should be pushed into
> the "negative" wire.
> The wires are now said to be the charged elements of a "charged"
> capacitor. This is slightly misleading. Even though one wire has "too
> many" electrons, and the other wire has "too few", the *net* charge on
> the *pair* of wires remains the same.
> Capacitance measures how "easy" it is for a capacitor to store large
> numbers of electrons in the negative element, and "donate" from the
> positive element.
> The electron "capacity" of these pairs of elements varies
> proportionately w/ the "area" of the element, and inversely w/ the
> distance between them.
> So, widely spaced wires would have a much lower capacity for
> storing/donating electrons than closely spaced sheets of conductive
> foil.
> HYDRAULIC ANALOGY (for how capacitors work)
> I like the "balloon" analogy for capacitors.
> Imagine that wiring is like plumbing, a battery is like a pump, and
> voltage is like the pressure created by the pump.
> A capacitor would be like a wide section of pipe that had a balloon
> stretched across it's opening before being fitted together w/ another
> section of pipe.
> Water can be pumped through the wide pipe until the pump pressure is
> matched by the balloon. The balloon stores that water until the pressure
> is relieved, and then it pushes the water back the way it came.
> If the wide capacitor_pipe were in parallel w/ the pump, the
> capacitor_pipe could maintain water pressure in the event of a (very)
> brief pump failure.
> When asked what a farad was, I answered that it's a measure of charge.
> Really, it's the coulomb that measures charge.  (roughly 6.24*10^18th
> electrons worth)
> The Farad is the measure of capacitance.
> To be more correct, I should have said that:
> The farad is a unit for measuring the charge required to raise the
> potential-difference across a capacitor by 1 volt.
> A 1 Farad capacitor would read 1 volt, when it was holding a 1 coulomb
> charge.
> A 1000 microfarad (MFD a.k.a uF) capacitor would read 1 volt when
> holding a .001 coulomb charge, would hold .012 coulombs when connected
> to a 12-volt battery, or read 1000 volts when holding a 1 coulomb
> charge. (if it didn't explode first)
> The Amp is the unit for measuring of the rate of current flow. (1 Amp =
> 1 coulomb per second)
> So...
> If we could maintain a constant 1-amp current flowing through a 1 Farad
> capacitor (which would be very difficult) the potential difference
> across that capacitor would increase by 1 volt per second, until the
> capacitor reached it's rated voltage. Charging a capacitor beyond it's
> rated (breakdown) voltage is an invitation for catastrophic capacitor
> failure.
> If we apply a 1-volt potential-difference to a 1 Farad capacitor (which
> is much easier) the charge on the capacitor will asymptotically approach
> 1 Coulomb.
> Figuring out the charge rate of a capacitor in an R.C. timing circuit is
> crucial to predicting the oscillation frequency of that circuit. (or you
> could just try random guesses 'cause I don't know how to do that yet)
> I hope that helps someone.
> If anyone can figure out how to fit that onto a 3x5 card, please let me
> know.
> -Bill
> Charging rate of a capacitor:
> www.algebralab.org/practice/practice.aspx?file=Reading_ChargingCapacitor
> .xml
> Leyden Jar:
> http://en.wikipedia.org/wiki/Leyden_jar
> Electrostatic demonstrations:
> http://www.lhup.edu/~dsimanek/scenario/e-stat.htm
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