Capacitive Reactance: Calculate Xc & V=I*Xc

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Discussion Overview

The discussion revolves around the concept of capacitive reactance (Xc) in alternating current (a.c.) circuits, specifically addressing the relationship between voltage (V), current (I), and Xc. Participants explore the mathematical derivation of these relationships and the role of complex numbers in the analysis.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant derives the current (I) in terms of voltage (Vc) and capacitive reactance (Xc), suggesting that V = I * Xc should hold true.
  • Another participant asserts that the derivative of the sine function is cosine, implying that complex numbers are unnecessary for this relationship.
  • A later reply corrects the expression for current, indicating that I = I0 cos(wt) and proposing a different relationship involving the tangent function.
  • One participant clarifies that the relationship V = I * Xc applies to amplitudes or rms values, not instantaneous values, and suggests that complex numbers are needed to account for phase differences.
  • Another participant questions the introduction of imaginary numbers, asking about their role in describing phase using the exponential form e^(i*theta).

Areas of Agreement / Disagreement

Participants express differing views on the necessity of complex numbers in the analysis of capacitive reactance and the interpretation of the relationship between voltage and current. The discussion remains unresolved regarding the role of complex numbers and the correct formulation of the relationships.

Contextual Notes

Participants have not reached consensus on whether complex numbers are essential for understanding the phase relationships in a.c. circuits, and there are unresolved aspects regarding the definitions and interpretations of the mathematical relationships discussed.

Devil Moo
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Hi,

A pure capacitor C is in an a.c. circuit.

Vc = V0 * sin(w*t)
Q = C * Vc
= C * V0 * sin(w*t)
I = dQ/dt
= wCV0 sin(wt)
then I0 = wCV0
Xc = V0 / I0 = 1/(wC)

So why people would say V = I * Xc?

Is it a must to include complex number?
 
Last edited:
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The derivative of sin is cos.

You don't need complex numbers.
 
Whoops!
I = I0 cos(wt)
Then V = I * Xc * tan(wt)
not V = I * Xc
 
The relationship V=I*Xc (with real numbers) is between the amplitudes or rms values and not between instantaneous values.
Otherwise you need to consider Xc as a complex number, to take into account the phase difference between v and i.
 
But why do we introduce imaginary root in it?
Is it used to describe the phase based on the e^(i*theta)?
 

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