The living nature of capacitance
If you have not already done so I suggest you read my blog on electricity as a living energy flow before reading this one. In many ways a capacitor in a circuit is like the open light switch described there, but its stopping of electron movements is slower because its plates and the dielectric material between them, that can be air, allow for some further movement of electrons in them.
Capacitors are energy storage devices. The many cylindrical capacitors we see contain a pair of closely coiled plates with a dielectric material between them. The capacitance shown is an electrolytic one (it has + and – connections). It has a voltage rating of 25 volts and a capacitance of 4,700 ยต F.
One volt applied across it would give it a charge of 47/1000,000 coulombs. Now a coulomb of charge is equal to an amp flowing for a second; its the movement into the capacitor of 6.24 million trillion electrons. So one volt applied to that capacitor would add 29,328 trillion electrons to it. The protons in that capacitor resist the electrons and they in return resist the voltage supply pressure of 1 volt. Their resistance that we measure as voltage is their desire for change.
The attraction of light objects to amber that had been rubbed by fir was known to Greeks as far back as six centuries B.C. In fact elektron is the greek word for amber. A William Gilbert in his De Magente work of 1600 used that term. His work extensively investigated magnetic and electric attractions, concluding that there was no connection between the two, that electric effects were weaker than magnetic effects and did not involve repulsion. Electrons were of course not known to him and not discovered until 1897.
The earliest capacitor was a Leyden jar, invented in 1745. It had foil on the inside and outside of a glass bottle and a metal rod attached to the inner foil. Some early versions used water as the inner conductive material instead of the foil. The rod was charged up by friction rubbing using an electrostatic generator. Experimenters would receive shocks by touching the outer foil and the rod. They of course did not know that electrons added to the inner foil were discouraging electrons and attracting protons in the outer foil and glass and storing energy doing so.
In the simple circuit shown the capacitor’s air gap acts like an open circuit switch blocking electron movements across it. Its plates however, continue for a short time to accumulate (negative plate) and release (positive plate) electrons so that energy is active both across and within the plate gap.
The electrons may not be jumping the air gap but their accumulating numbers are serving photon energy to the increasing energy desiring proton surpluses on the opposite plate. The build ups are also influencing the dielectric material between the plates, as I below describe, Those build ups diminish and stop because, as science will say, like charges repel one another. The light bulb initially bright, dims and goes out as the resisting voltage across the capacitor becomes the same as that across the battery.
Remove the capacitor from the circuit and it still has that voltage. It has energy stored in its plate and in its dielectric material. Their electron to proton energy links have been distorted from their otherwise desired state by the energy demands of the supply they were connected to. We call those distortions polarisations.
We might think of the electron motions as along the plates from or to the points of connection but most of their vibrating motions are in the direction of the desired energy flow across the plates. Connect the capacitor into a circuit and it behaves like a battery with a very tiny energy store that is rapidly depleted as the plate and dielectric materials return to their preferred state
Capacitors are used extensively in electronic circuit boards. Their slow discharge can be used in timers and their storage ability used to provide smoothing where steady d.c.
supplies are required. Millions of miniscule metal oxide semi (MOS) conductor capacitors are used to store light related charges on digital camera sensors. When you view a camera image each point on the image is the result of what one such capacitor collected.
Not so well known is the use of larger banks of capacitors in power factor correction. In another blog I explain inductance and inductive loads. Such electrical loads happen in industry where many machines use coiled copper wirings. Their inductance can add to the electron current flow and delay it relative to voltage so that the apparent rate of energy use is higher than actual. The power factor is the ratio of the real to apparent.
Electricity companies penalise low ratios because their supply equipment’s capability is being used to handle currents that are in excess of those required to deliver the energy. The installation of capacitor banks at industrial premises can improve the power factor and reduce the current taken from the supply making it more representative of the energy taken and thereby reducing any penalty.