About the reactance of capacitor

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    Capacitor Reactance
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Discussion Overview

The discussion centers around the concept of capacitive reactance and how capacitors resist current flow in electrical circuits. Participants explore the mechanisms behind capacitive behavior, particularly in relation to direct current (DC) and alternating current (AC), as well as the implications of terminology used in describing these phenomena.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express confusion about how capacitors resist current flow and the meaning of capacitive reactance.
  • One participant argues that the phrase "a built-up electric field resists the change of voltage" is misleading, suggesting that it implies a mechanism for energy loss, which does not accurately describe capacitor behavior.
  • Another participant explains that a capacitor behaves like an open circuit after being charged, allowing initial current flow but eventually opposing further current as the potential difference across it equals the supply voltage.
  • It is noted that capacitors block DC but allow AC to pass, with the degree of reactance decreasing at higher frequencies due to the rapid change in voltage direction.
  • A later reply introduces the idea of series electrical resistance in the voltage source, suggesting that practical circuits always have some resistance that affects capacitor charging behavior.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the terminology used to describe capacitive behavior, with some expressing differing views on the implications of the term "resists." The discussion remains unresolved regarding the best way to articulate the concepts involved.

Contextual Notes

There is an acknowledgment of the complexity involved in describing capacitor behavior, particularly in relation to the definitions of terms like "resist" and "opposes." The discussion also highlights the practical considerations of series resistance in circuits, which may influence the understanding of capacitive reactance.

lindacheung66
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For inductors, it is the induced e.m.f. which resists the current flow.
However, I don't quite understand how capacitors resist the current flow.
Why is there capacitive reactance?
What does 'A built-up electric field resists the change of voltage on the element' mean?
 
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lindacheung66 said:
For inductors, it is the induced e.m.f. which resists the current flow.
However, I don't quite understand how capacitors resist the current flow.
Why is there capacitive reactance?
What does 'A built-up electric field resists the change of voltage on the element' mean?

That last quote doesn't really make strict sense. Using the word "resists", in this context, implies resistance (a mechanism for losing energy as current is conducted).
A capacitor, being basically an open circuit, will not carry on conducting forever, when connected to a battery. Initially, however, some charge can flow into it. As the charge increases, an increasing potential difference will form across the terminals and, once this PD is equal to the supply voltage, no more current will flow (there is an exponential change). The PD across the capacitor 'opposes' the supply voltage, rather than "resisting" it. (Here, I am talking in terms of the usage of those words in the context of electrical circuits and not in general English language use).
That's why we say that a capacitor 'blocks' DC. For an alternating current , particularly at a high enough frequency, the direction of the applied AC voltage changes rapidly enough for the capacitor never to get fully charged and so the capacitor 'lets through' an alternating current. The higher the frequency, the less the opposing PD becomes so the Reactance decreases proportionally.
 
Thank you very much.
You have difinitely dispersed my misunderstanding.
 
Good good. Keep 'em coming. :smile:

btw, I should have included, in my hand waving model of charging a capacitor, some amount of Series Electrical Resistance in the source of the Voltage. There will always be finite resistance, in practice, so it is a reasonable thing to do. Without the added resistance, you get other awkward things happening and more headaches!
 

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