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Capacitor Question

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Jim Repass

New Member
I have a question about capacitors. I have read two different descriptions of how they work and wish to know which is correct. The first was an analogy to a water pipe supplying water to fill a container. The container will hold a supply of water and after the valve is shut off, the container will still supply water until it is empty (see figure 1). The second description was that it is like a bucket. When it is full of water (fully charged), it will pour its contents into the circuit (see figure 2). Each of these descriptions will have a drastically different effect on a circuit. Which is correct? Are both correct, if so how? Is there a place I can learn the basic functions and descriptions of different types of capacitors? Thank you. –Jim Repass
 

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JimB

Super Moderator
Most Helpful Member
Water analogies of electrical systems can be misleading when a correct understanding is required.

Your first description is not completely correct, but far better than the second one which is just plain wrong.

JimB
 

crutschow

Well-Known Member
Most Helpful Member
As part of the analogy for the first figure, you can note that the water pressure (voltage) gets higher as the bucket (capacitor) is filled.

Also unlike a bucket there's no hard limit as to how much you can fill the capacitor (except if you exceed its voltage rating by too much it will short).
 

Externet

Well-Known Member
Jim, you may prefer to replace your bucket analogy with an elastic membrane inserted in a pipe.
Pushing water in with pressure (voltage) deforms the elastic membrane.
 

OutToLunch

New Member
i think mechanical analogies are better than fluid ones. A capacitor can be thought of as a spring (energy storage), an inductor can be thought of as a mass (inertia), and a resistor would be a dashpot.
 

colin55

Well-Known Member
Capacitors have different effects in a circuit, depending on the surrounding components and how it is placed.
You need to provide a circuit using a capacitor, before the actual operation of the capacitor can be described.
 

Sceadwian

Banned
One of the more common uses for a capacitor is power supply decoupling where the mebrane description works prety well, you can think of it as a section of pipe made out of rubber. When the pressure goes up relative to the outside of the pipe the pipe deforms and stores energy, when the pressure drops the membrane relaxes and provides a buffering effect on pulsations in the water pressure. Devices like these actually exist in the real world, they're used to prevent standing waves from building up in pulsating pump system, you hit the wrong harmonic with a complex plumbing setup and the entire system would resonant.

Another way to look at capacitors is as AC decoupling caps where it would be a flexible membrane cutting a pipe in half to block water flow, it'll still let pressure waves pass but it won't let water flow.
 

smanches

New Member
Hate to use a fluid analogy here, but a coupling capacitor works very much like a water hammer prevention device (or whatever they are called.)

You could think of a big power capacitor, like in power supplies, like an expansion tank in a heating system.

Again, as was mentioned, the application of the capacitor denotes how it behaves.
 

Sceadwian

Banned
That's what I just said smanches =)
 

Jim Repass

New Member
Thank you all for the great responses. I wonder if plumbers use electrical analogies for their projects? I would like to find a website that will teach me about capacitors, if anyone knows of any pleas let me know. –Jim Repass
 
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