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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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If voltage drop across capcitor is 100V and current is 5A, what is value of capacitive reactance?
10Ω
20Ω
30Ω
40Ω
20Ω
Quick Summary: Capacitive reactance ($X_C$) is the opposition offered by a capacitor to the flow of alternating current. It is determined by the ratio of the RMS voltage drop across the capacitor ($V_C$) to the RMS current flowing through it ($I_C$), expressed as $X_C = \frac{V_C}{I_C}$.
Capacitive reactance (XC) is the opposition offered by a capacitor to the flow of alternating current. It is determined by the ratio of the RMS voltage drop across the capacitor (VC) to the RMS current flowing through it (IC), expressed as XC=ICVC.
XC=IV — Formula to calculate capacitive reactance when voltage and current are known.
XC=2πfC1 — Fundamental formula relating reactance to frequency and capacitance.
In an AC circuit, a capacitor stores and releases charge continuously as the voltage changes. This charging and discharging process results in a current that leads the voltage by 90 degrees. The opposition to this current flow, known as capacitive reactance, is inversely proportional to both the frequency of the supply and the capacitance of the component.
Capacitive reactance is measured in Ohms (Ω).
It causes a phase shift where current leads voltage by 90°.
Reactance decreases as frequency increases.
For the given values, XC=5A100V=20Ω.
Used in AC filters to block DC.
Essential for phase shifting in motors.
Reactance is frequency dependent, making it unstable in wide-band circuits.
Pure capacitors do not dissipate real power.
Coupling capacitors in amplifiers.
Power factor correction units.
The calculation is XC=5100=20Ω.
Options A, C, and D are incorrect calculations based on the provided parameters.
B is correct — Using Ohm's law for AC circuits, XC=IV=5100=20Ω.
Always remember that in an AC circuit with a capacitor, current leads the voltage. The reactance value XC must always be treated as a negative imaginary component in complex impedance calculations (−jXC).