Simple description of how a capacitor works

[ggunn]

I thought that was a slang term we used locally.

I think the term came from a physical pot of viscous fluid with a plate in it connected to a rod. The viscosity of the fluid slows down the movement of the rod.

 

[junkhound]

Interestingly enough, in RLC circuits, you come across some of the same forms of equations that you find in mass-spring-damper systems. And the capactance indeed takes the place of the spring, in such equations.

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For reference, to not mislead others with incorrect statements, refer to following or similar. Capacitance is similar to mass in equations.

http://lpsa.swarthmore.edu/Analogs/ElectricalMechanicalAnalogs.html

 

[Sahib]

I like the membrane tank analogy of a capacitor as well. It also brings up a common misconception of capacitors, when we say that they "store charge".

When the membrane is in the unstressed neutral position, is the tank completely "diswatered", just as a capacitor would be "discharged"? It contains no water at all...right?
When the membrane tank is in an extreme position, does this "heavily watered" membrane tank contain more water than before?

Interestingly enough, the total amount of water in the membrane tank is the same, no matter what the position of the membrane. So it isn't storing any additional water by pumping it from one side to the other. It stores the same amount of water no matter what, and what it really stores, is energy in the elastic deformation of the membrane.

Similarly for a capacitor, it always has the same net charge (i.e. zero), and it always has the same population of electrons. It isn't storing any charge in the unit as a whole, when one plate has an electron surplus and the other an electron deficiency. So it is really storing energy in the electric field between the plates, due to the non-neutral distribution of charge, when the capacitor is energized ("charged").

I was going to use this to explain how a capacitor works with alternating current:

Q: Why is direct current blocked in a capacitor and alternating current is allowed to flow?

A: "Because AC is cycling on and off. This means that the electrons in the electrical field between the plates is constantly having voltaic pressure applied to them, first in one direction, then in the other. Every half-cycle the current turns off, the capacitor discharges, and the field begins to charge again, with current flow in the opposite direction.

We can liken this to a pipe filled with water, having a membrane (Dielectric) separating the interior into two halves (plates). Both sides of the pipe have the same initial pressure (potential) applied against both sides of the membrane. Applying more pressure one side forces the membrane to flex and exert pressure on the water on the other side of the pipe. Release the extra pressure and the pressure against the membrane equalizes (field discharges). Applying extra pressure in the opposite direction now moves the membrane in the opposite direction and water on that side of the membrane is moved in that direction. Water is moved on each side of the pipe without the water in the pipe ever crossing the membrane or coming into direct contact."

Does this strike as a simple explanation? Is it correct? :?
Jeff

For DC, such analogy works. For AC it does not work because of presence of phase difference between voltage and current.It is not clear how to take into account the phase difference in the anology.

 

[GoldDigger]

For DC, such analogy works. For AC it does not work because of presence of phase difference between voltage and current.It is not clear how to take into account the phase difference in the anology.

It actually works perfectly. The displacement of the water is in phase with the pressure (voltage). But the current (flow of water) is 90 degrees out of phase, assuming that the applied pressure is sinusoidal.

 

[Sahib]

But the current (flow of water) is 90 degrees out of phase, assuming that the applied pressure is sinusoidal.

That point is not clear because the water flow would lag and not lead pressure variation due to inertia.

 

[GoldDigger]

That point is not clear because the water flow would lag and not lead pressure variation due to inertia.

Which is why a long pipe full of water corresponds to an inductor. (With some resistance too, but that can be minimized.)

 

[Carultch]

For reference, to not mislead others with incorrect statements, refer to following or similar. Capacitance is similar to mass in equations.

http://lpsa.swarthmore.edu/Analogs/ElectricalMechanicalAnalogs.html

Your source indicates that there are two methods of making these analogies. My explanation follows "Mechanical Analog II (Force Voltage)".

In my opinion, it intuitively makes more sense for the capacitor to be analogous to the spring. Because an energized capacitor is a static concept, and so is a compressed or extended spring. While energy of mass in motion and energy stored in an inductor depend on either motion in progress, or current actively flowing. The spring constant needs to be inverted to make them analogous, because we talk about stiffness instead of flexibility for springs.

 

[Carultch]

In my first semester AC circuits class (back when Hector was a pup) LC filters were first introduced as analogous to a spring and dashpot system. Anyone else remember what a dashpot is?

I would think that an LC filter would be analogous to an ideal frictionless mass on spring system. And an RC filter would be the spring and dashpot with negligible mass. And RL would be a mass on a dashpot. The whole RLC system would be a combination of all three mechanical components

We learned the word damper instead of dashpot in my school, and in concept it would be an unsealed piston inside of a linearly viscous fluid.

 

[GoldDigger]

The fluid in a dashpot could just as well be a gas if compression effects can be neglected.

 

[Carultch]

The fluid in a dashpot could just as well be a gas if compression effects can be neglected.

It is very rare that you'd find a gas, whose resistance to an object's motion is linear. Most air drag situations involve turbulent flow, where inertial pressure matters more than viscous effects. As a result, the air drag is proportional to the square of speed, instead of directly to speed. I'd expect other types of gas to be a similar story.

The fluid in the dashpot is usually understood to be something like oil, where viscosity is high, the flow is laminar, and the resistance to motion is proportional to speed.

 

[GoldDigger]

It may be rare, and it may not have the same characteristics, but there is a specific product category of "air dashpot". Among other things they are lighter and have a lower inertial component.
http://airpot.com/product-category/applications/

 

[Besoeker]

To keep them from shutting down too fast, yep.

SU Carbs - for constant pressure operation.

 

[Sahib]

Dash pot-just to introduce time lag to closing of contacts.