Simple description of how a capacitor works

[jwnagy]

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

 

[GoldDigger]

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

The analogy is not bad. You can improve on the description somewhat by also explaining that the greater the pressure difference between the two sides of the diaphragm the more water can move as the diaphragm is displaced. Very good analogy to the charge on the capacitor (number of electrons moved / integral of the current) being proportional to the applied voltage (pressure differenece) times a measure of the size and flexibility of the diaphragm (the capacitance).
If you want to get really deep (and I don't think you do) you can note that the capacitance is proportional to the size of the plates and the ease with which electrons can be moved from one side to the other (inverse to gap distance, proportional to dielectric constant)


The words that you are using in the first part are not nearly as good though. The are simple, but they are not correct.

1. AC does not cycle on and off. The voltage (usually) rises to positive peak for 1/4 cycle, falls back toward zero for next 1/4 cycle, then goes to negative peak over 1/4 cycle and returns to zero in the last 1/4 cycle.

2. There are no electrons in the electric field between the plates!!! The dielectric is an insulator.
There is an electric field because electrons are being moved through the external circuit from one plate to the other in response to the external applied voltage. Result is a net negative charge on one plate (extra electrons) and a matching positive charge on the other (missing electrons)

3. The current does not turn off. It declines smoothly to zero then starts to build up in the opposite direction. The maximum current is at the voltage zero crossings because that is where the voltage is changing most rapidly.

During the first 1/4 current flows one way in capacitor.
During second and third quarter cycles it flows in the opposite direction, in the fourth quarter cycle it goes back in the original direction of flow.

At either voltage peak the current is zero.

I think that you have made a good first try at a simple but accurate explanation and with a little more work you should be OK.

 

[Smart $]

...
2. There are no electrons in the electric field between the plates!!! The dielectric is an insulator.
There is an electric field because electrons are being moved through the external circuit from one plate to the other in response to the external applied voltage. Result is a net negative charge on one plate (extra electrons) and a matching positive charge on the other (missing electrons)
...

I know what you are attempting to say, but the highlighted sentence is technically inaccurate. The dielectric material does contain electrons, but they are stable-element electrons, which is why the material is dielectric. Metaphorically, think of it like a horse ranch. The little horsies are all pent up in the stable. No horsies out running open range, the metaphor equivalent of a good conductor.

(I know you didn't need the metaphor GD... it's for the newbs)

 

[junkhound]

Metaphors:

a. DC is not 'blocked' by a capacitor, it just stops flowing when "the bucket is filled", aka capacitor charged.

fill bucket from one pipe, dump it to another, then fill from the opposite side, water flows.

b. assume the 'blocked' is part of the metaphor

1000 refugees run up to the south side of a double fence, they all run up to the fence until the fence is packed with folks who see they cannot get over it
suddenly many run to boats and try to sail around the end of the fence, and the folks on the south side now see folks on the north side where there is food and also run for the boats
all of a sudden, the food on the north side runs out, and the crowd there runs back to the boats,
and on and on......

etc ad infinitum....

 

[jwnagy]

Capacitors

Capacitors

The analogy is not bad. You can improve on the description somewhat by also explaining that the greater the pressure difference between the two sides of the diaphragm the more water can move as the diaphragm is displaced. Very good analogy to the charge on the capacitor (number of electrons moved / integral of the current) being proportional to the applied voltage (pressure differenece) times a measure of the size and flexibility of the diaphragm (the capacitance).
If you want to get really deep (and I don't think you do) you can note that the capacitance is proportional to the size of the plates and the ease with which electrons can be moved from one side to the other (inverse to gap distance, proportional to dielectric constant)


The words that you are using in the first part are not nearly as good though. The are simple, but they are not correct.

1. AC does not cycle on and off. The voltage (usually) rises to positive peak for 1/4 cycle, falls back toward zero for next 1/4 cycle, then goes to negative peak over 1/4 cycle and returns to zero in the last 1/4 cycle.

2. There are no electrons in the electric field between the plates!!! The dielectric is an insulator.
There is an electric field because electrons are being moved through the external circuit from one plate to the other in response to the external applied voltage. Result is a net negative charge on one plate (extra electrons) and a matching positive charge on the other (missing electrons)

3. The current does not turn off. It declines smoothly to zero then starts to build up in the opposite direction. The maximum current is at the voltage zero crossings because that is where the voltage is changing most rapidly.

During the first 1/4 current flows one way in capacitor.
During second and third quarter cycles it flows in the opposite direction, in the fourth quarter cycle it goes back in the original direction of flow.

At either voltage peak the current is zero.

I think that you have made a good first try at a simple but accurate explanation and with a little more work you should be OK.

Great! I may have tried to simplify it too much. I will re-work this and clean it up a bit.

Jeff

 

[jwnagy]

Metaphors:

a. DC is not 'blocked' by a capacitor, it just stops flowing when "the bucket is filled", aka capacitor charged.

fill bucket from one pipe, dump it to another, then fill from the opposite side, water flows.

b. assume the 'blocked' is part of the metaphor

1000 refugees run up to the south side of a double fence, they all run up to the fence until the fence is packed with folks who see they cannot get over it
suddenly many run to boats and try to sail around the end of the fence, and the folks on the south side now see folks on the north side where there is food and also run for the boats
all of a sudden, the food on the north side runs out, and the crowd there runs back to the boats,
and on and on......

etc ad infinitum....

DC blocked....yeah. Ya got to empty the bucket before you fill it again.

 

[rt66electric]

up-down up-down bumpy road of life

up-down up-down bumpy road of life

I always thought of it as like a spring and a Shock-absorber.

the a/c wave is like a axle hitting bumps(spring) and the capacitor(shock-absorber) levels it out .:?

 

[GoldDigger]

I always thought of it as like a spring and a Shock-absorber.

the a/c wave is like a axle hitting bumps(spring) and the capacitor(shock-absorber) levels it out .:?

That is a really bad analogy, since the main purpose of a shock absorber is to waste energy while the capacitor stores it.

 

[mgookin]

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

I'm going to say I think you are going to confuse them (I don't know who your audience is). I suggest you use the PHY101 method to explain in the simplest terms possible what's happening with electricity, not water.

But if you're hard set on using the water analogy I'll suggest you mention a constant source (as you'd (hopefully) have with electricity). On first read it sounds like the system runs out of supply and comes to equilibrium (stops). Also you can change your word pipe to the word vessel. You could do it with a tank with an influent and an effluent.

 

[Carultch]

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

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").

 

[Carultch]

That is a really bad analogy, since the main purpose of a shock absorber is to waste energy while the capacitor stores it.

A shock absorber consists of two kinds of responses for mechanical vibrations. Both a spring and a damper.

The ideal spring does not waste any energy, because what is applied as work done on the spring is absorbed as elastic energy. It is the damper behavior of the shock absorber that does the wasting of energy, which is more analogous to a resistor than a capacitor.

 

[Carultch]

I always thought of it as like a spring and a Shock-absorber.

the a/c wave is like a axle hitting bumps(spring) and the capacitor(shock-absorber) levels it out .:?

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.


A capacitor and a spring are both components that store energy of position. A spring stores energy when it is deformed from its unstressed position. A capacitor stores energy when charge is redistributed from the neutral-on-both plates configuration.

A resistor and a friction-based component both dissipate the mechanical/electrical energy of the system, and convert it into thermal energy/heat.

An inductor and a mass's inertia are both components that store energy of motion. In the case of a mass's inertia, it stores energy because of its mass and speed. In the case of inductance, it is storing energy because of the magnetic field, and the magnetic field exists because of the current flow. Inductors oppose a change of of the state of current flow that would disrupt the magnetic field, and induce a back-emf to oppose the change. A water analogy for an inductor would be a high inertia free spinning paddlewheel, that has blades within the flow of the water.

 

[GoldDigger]

Ideally the analogy for the inductor should be a high inertia positive displacement pump, since the flow cannot change instantly and then there is no frictional interaction as there could be with a paddle wheel.
It also has to work equally well in both directions.
The "discharged" state of the capacitor would have to be with the membrane centered in the tank.

 

[Strathead]

I know what you are attempting to say, but the highlighted sentence is technically inaccurate. The dielectric material does contain electrons, but they are stable-element electrons, which is why the material is dielectric. Metaphorically, think of it like a horse ranch. The little horsies are all pent up in the stable. No horsies out running open range, the metaphor equivalent of a good conductor.

(I know you didn't need the metaphor GD... it's for the newbs)

Please Lord, don't let me imply that the sky is blue if smart $ is around!:angel:

 

[ggunn]

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.

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?

 

[Besoeker]

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?

Used to have them in motor starters. George Ellison Ltd was a popular manufacturer in UK.

 

[Strathead]

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 do.

 

[K8MHZ]

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?

The thing on old carburetors?

 

[ggunn]

The thing on old carburetors?

To keep them from shutting down too fast, yep.

 

[K8MHZ]

To keep them from shutting down too fast, yep.

I thought that was a slang term we used locally.