Another situation is a bit, less misleading: think of the word "charge" as applied to gunpowder. A charge is placed in an old cannon, followed by a cannonball. It would be silly to assume that, because we've "charged" the cannon, the cannon now has an electrical charge. Yet whenever we state that we've "charged" a capacitor, we do assume that an electrical charge has been stored inside. This is just as silly as mistaking gunpowder for electrostatic charges. Charging a capacitor (or a battery) is like charging a cannon; in both situations we are inserting energy, not electrical charge.

Here's yet another way to visualize it. Whenever we "charge" a capacitor, the path for current is through the capacitor and back out again. The extra electrons on one plate force electrons to leave the other plate, and vice versa. Visualize a capacitor as being like a belt-driven wind-up motor. If we shove the rubber belt along, the spring-motor inside the capacitor winds up. If next we let the rubber belt go free, the wound-up spring inside the motor drives the belt backwards, and the spring becomes "discharged." But no quantity of "belt" is stored inside this motor. The belt flows through the device, and we wouldn't want to label this motor as a "machine which accumulates rubber." Yet this is exactly what we're saying whenever we state that capacitors "store charge."

One more try. Capacitors store charge in the same way that resistors store charge, and inductors store charge. Inductors are full of mobile electrons, inductors are devices for storing charge!!!! Nope. An inductor isn't a bucket for accumulating electrons, and neither is a capacitor. Instead, both types of component behave like piece of wire But capaictors are magical wires which, whenever we run a current along it, the total charge inside the wire stays constant, but a voltage (and a charge imbalance) appears at the two ends.

My favorite capacitor analogy is a heavy hollow sphere which is completely full of water and is divided in half with a flexible rubber plate through its middle. Hoses are connected to the two halves of the thick irong sphere, and they act as connecting wires. The rubber plate is an analogy for the dielectric. The two regions of water symbolize the capacitor plates.

Imagine that the rubber plate is flat and undistorted at the start. If I connect a pump to the two hoses and turn it on for a moment, the pump will pull water from one half of the iron sphere and simultaneously force it into the other. This will bend the rubber divider plate more and more. The more the plate bends, the higher the back-pressure the plate exerts, and finally the pressure-diff will grow strong enough that the pump will stall. Next I seal off the hose connections and remove the pump. I now have created a "charged" hydraulic capacitor.





Now think: in this analogy, water corresponds to electric charge. How much water have I put into my iron sphere? None! The sphere started out full, and for every bit of water that I took out of one side, I put an equal amount into the other at the same time. Same as when running a current through a conductor. When the pump pushed water into one side, this extra water also forced some water out of the other side. No water passed through the rubber, instead there was some rubber-current in the divide. Even so, essentially I drove a water current through my hydraulic capacitor, and this current pushed on the rubber plate and bent it sideways. Where is the energy stored? Not in the water, but in the potential energy of the stretched rubber plate. The rubber plate is an analogy to the electrostatic field in the dielectric of a real capacitor.

It would be misleading to say "this iron sphere is a device for accumulating water." And neither say "this sphere can be charged with water, and the stored water can be retrieved during discharge." Both statements are wrong. No water was injected into the sphere while it was being "charged." (And when I wind up an old watch, am I "storing steel" inside it, putting more iron into its spring? Lol.

Imagine that I now connect a single length of pre-filled hose between the two halves of my "energized" water sphere. As soon as the last connection is complete, the bent rubber plate will drive a sudden immense spurt of water through this already-full hose. Water from one side will be pushed into the other side, and the rubber plate will relax. I've discharged my hydraulic capacitor. How much water has been removed from the sphere? None! A momentary current has flowed through the sphere device, and the rubber plate is back to the middle again, and the water has become a bit warm through friction against the surfaces of the hose. The stored energy has been "discharged," but no water has escaped. The hydraulic capacitor has lost its energy, but still contains the same amount of water.





I never really understood capacitors until I started trying to construct proper water-analogies for them. Then I discovered that my electronics and physics classes had sent me down a dead-end path with their garbage about "capacitors store electric charge." Since my discovery, I've gained significantly more expertise in circuit design, which leads me to a sad thought. Maybe the more skilled of electrical engineers and scientists gain their extreme expertise not through classroom learning. Instead they gain expertise in spite of our K-12 classroom learning. Maybe the experts are experts only because they have fought free of the wrong parts of grade school science, while the rest of us are still living under the yoke of the many physics misconceptions we were carefully taught in early grades.

[Hey, M. Steinberg's C.A.S.T.L.E. electricity curriculum uses the same analogy! In all my search of textbooks, this is the only one I've found. In section 3.9, students construct a 2-chambered air capacitor with a balloon membrane stretched between the two chambers. To "charge" it we take air from one side and pump it into the other. ]

[Hey^2!!! I just found that Oliver Lodge was building mechanical analogies for Maxwell's descriptions of EM fields and circuits... and for an 1880 lecture he built a capacitor hydraulic analogy consisting of a water-filled sphere with rubber! His was a glass sphere containing a smaller balloon, both full of water with no bubbles.]

LINKS About that word 'charge'

U. Waterloo, Water analogy to circuits

Water tank analogy

CASTLE capacitor curriculum Falstad's circuit sim w/visible charges The Maxwellians: Lodge's hydraulic analogies

Extra notes: Capacitors store just as much charge as coils do! In both devices the total amount of charge stays constant. Both capacitors and inductors are components for storing electromagnetic energy. They're two sides of the same EM phenomenon: a coil stores energy in a volume containing a magnetic field, while a capacitor does something similar with electric fields. Coils are "discharged" by interrupting a large current and collapsing the b-field, while capactors are discharged by shorting-out a large voltage and collapsing the e-field. Neither stores any "electricity" (unless by the word 'electricity' you mean magnetic field?) Of course you can place a coil atop an insulating platform, then use a VandeGraaff generator to give your coil a large net-charge! You can do the same with a big electrolytic capacitor too. :)



Bill Beaty here again. Two points: First, the heated topic about currents being impossible between capacitor plates ...seems to be about Vacuum Capacitors, not capacitors in general. Modern capacitors are quite different than vacuum capacitors, and inside a modern dielectric is a large electron-current. Relative Permittivity can be seen as a ratio between the small Maxwell's displacement current inside a vacuum, versus the larger polarization current (electron flow: the shifting of electrons) in the ferroelectric PZT material. The dielectric constant in modern ceramic capacitors is above 2,000, so the vast majority of the current is a charge-flow, a real current from shifting the electrons in the ferroelectric ceramic. The Maxwell displacement current of the vacuum is insignificant: well below 1%.

Second: it might help to ask whether, down at the micro level inside a metal, in the space BETWEEN the electrons, is there any electric current? If there is, then there's certainly a current in the space between the metal plates of a vacuum capacitor. Or said differently: if we have a current-sensor, and a charged particle approaches and passes it, does our sensor indicate an extremely brief pulse, where the pulse-width is associated with the diameter of the charged particle? Or instead does our sensor see each moving particle as being very large and "fuzzy," so that the measured current is found in the fields surrounding each particle, and the current extends forwards and back from the particle location? A clamp-on current meter (Rogowski coil) doesn't detect the actual charges or their motions Instead it detects changing flux-linkage. A clamp-on sensor would report that each charged particle is indeed large and fuzzy, and the current exists in the empty spaces between the flowing charges. Current is not only found on the flowing particles' surfaces where the charge actually resides. So, a clamp-on sensor would 'see' currents in a vacuum-capacitor's empty gap. The changing fields are in that gap, so that's where the current appears.

Only two points? Third point, suppose we construct a capacitor where the dielectric takes the form of a long rod: much thicker than the diameter of capacitor plates. Use a long narrow PZT lead-zirconate- titanate rod with tiny capacitor plates attached to its circular ends. Now apply some 27MHz amperes. Do you still insist that the current within this rod is zero? Really? It's not. Next, suppose we obtain a coil-shaped spiral rod of PZT. A ceramic "ferroelectric coil." If we apply some RF amperes through this spiralled dielectric, we'll certainly detect a strong magnetic field surrounding the device. If the current is supposedly zero within all capacitor dielectrics (as some people angrily insist,) how can we explain this? Simple: capacitor dielectrics do contain current after all. The current in the dielectric is exactly the same as the amperes in the capacitor leads.



