active device

Discussion in 'Homework Help' started by amaresh92, Nov 6, 2010.

  1. amaresh92

    Thread Starter New Member

    Oct 31, 2010
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    why capacitor can not consider as active device as it has also ability to control the flow of electron (as charging and discharging)?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    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.
     
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  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    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.
     
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  4. amaresh92

    Thread Starter New Member

    Oct 31, 2010
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    would you please explain the first paragraph in more simple words.
    thanks
     
  5. amaresh92

    Thread Starter New Member

    Oct 31, 2010
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    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.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    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.
     
  7. amaresh92

    Thread Starter New Member

    Oct 31, 2010
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    i hope you would not irritated' but i am not getting your point.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    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

    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

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

    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
     
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  9. Georacer

    Moderator

    Nov 25, 2009
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    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.
     
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