why capacitor can not consider as active device as it has also ability to control the flow of electron (as charging and discharging)?

 

Capacitors and inductors are energy storage elements. They can only ever return the same stored energy with which they are supplied from an external source - and that's an ideal situation. Real capacitors and inductors have losses and cyclically varying energy in a capacitor or inductor will eventually decay to zero without further stimulus from an external energy source.

Nor can capacitors or inductors really control anything - they simply respond to an input stimulus. They can of course be used to control energy flow in a circuit which is energized by an external source. But the same is true of a resistor which is not an active device.

 

why capacitor can not consider as active device as it has also ability to control the flow of electron (as charging and discharging)?

The flow of electrons into a capacitor is a constant due to the characteristics of a capacitor.

In order for a capacitor to "control" that flow (be an active device), the rate would need to be a variable instead of a constant.

 

Capacitors and inductors are energy storage elements. They can only ever return the same stored energy with which they are supplied from an external source - and that's an ideal situation. Real capacitors and inductors have losses and cyclically varying energy in a capacitor or inductor will eventually decay to zero without further stimulus from an external energy source.

Nor can capacitors or inductors really control anything - they simply respond to an input stimulus. They can of course be used to control energy flow in a circuit which is energized by an external source. But the same is true of a resistor which is not an active device.

would you please explain the first paragraph in more simple words.
thanks

 

The flow of electrons into a capacitor is a constant due to the characteristics of a capacitor.

In order for a capacitor to "control" that flow (be an active device), the rate would need to be a variable instead of a constant.

i think you are indicating the rate as of rate of charging and discharging of capacitor.
but the rate of charging and discharging keeps on varying as time passes due to accumulation of charges in capacitor.then how it can be constant?
thanks.

 

i think you are indicating the rate as of rate of charging and discharging of capacitor.
but the rate of charging and discharging keeps on varying as time passes due to accumulation of charges in capacitor.then how it can be constant?
thanks.

The result of the charging formula is constant for any given value of a capacitor at any given time. It my not respond linearly, but it cannot change it's behavior, either.

 

The result of the charging formula is constant for any given value of a capacitor at any given time. It my not respond linearly, but it cannot change it's behavior, either.

i hope you would not irritated' but i am not getting your point.

 

This type of question begs for a definition of "Active Device." So, I searched on it.

Here is the definition by an agency of the US government:
http://www.its.bldrdoc.gov/fs-1037/dir-001/_0139.htm

Active Device: A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Note: Examples of active devices include controlled power supplies, transistors, LEDs, amplifiers, and transmitters.

Doesn't sound bad, but I was surprised to see LED's included.

So here is another definition:
http://www.answers.com/topic/active-device

(electronics) A component, such as an electron tube or transistor, that is capable of amplifying the current or voltage in a circuit.

And finally, from the mother source itself:
http://www.allaboutcircuits.com/vol_3/chpt_1/2.html

An active device is any type of circuit component with the ability to electrically control electron flow (electricity controlling electricity). In order for a circuit to be properly called electronic, it must contain at least one active device. Components incapable of controlling current by means of another electrical signal are called passive devices. Resistors, capacitors, inductors, transformers, and even diodes are all considered passive devices.

We can see that capacitors do not control current by means of another electrical signal.

Now I ask, how does one reconcile the US governmental agency and the AAC definitions? Must the controlling signal be electrical? Can it be mechanical, heat, light, etc.?

John

 

why capacitor can not consider as active device as it has also ability to control the flow of electron (as charging and discharging)?

Naming a capacitor an active device due to its abilitiy to affect the electro flow is rather superficial. From the same perspective, you could name a resistance an active device, as it regulates the current trough it according to the voltage applied to it.

However, a real active device can regulate a single quantity (voltage or current) related to it, independently of all the other elements the circuit it is in has.