RC circuits — does the total current always lead the source voltage?
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ATrue
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BFalse
Answer
Correct Answer: True
Explanation
Introduction / Context:This item checks phase intuition: in networks containing only resistors and capacitors (no inductors), is the net current ahead (leading) or behind (lagging) the source voltage?
Given Data / Assumptions:
- RC network (series or parallel) with sinusoidal excitation.
- No inductive elements present.
Concept / Approach:Capacitive current leads voltage by 90 degrees for an ideal capacitor. The presence of resistance reduces the lead, but cannot reverse it to a lag because there is no inductive reactance to pull the phase negative. Therefore, the total current in an RC-only network leads the source voltage by some angle between 0 and +90 degrees.
Step-by-Step Solution:
Series RC: current I is common and leads source voltage by angle θ = arctan(1 / (ω * R * C)).Parallel RC: total current Itot = IR + IC; IR is in phase with V, IC leads V by 90 degrees; vector sum leads V by 0 < θ < 90 degrees.Thus, in both topologies, without L the net effect is a leading current.Verification / Alternative check:Compute the input admittance Y = G + jB for an RC network; susceptance B = +ωC > 0, so the current phasor leads the voltage phasor by angle φ = arctan(B/G) ≥ 0.
Why Other Options Are Wrong:
- “False” would suggest zero or lagging phase for some RC arrangement, which would require inductive behavior not present in R–C-only circuits.
Common Pitfalls:Confusing “output current in a branch” with “total line current.” Individual branch currents may have different phases, but the total seen by the source still leads for RC-only networks.
Final Answer:True