AC Circuit Virtual Lab

What is AC Circuit Virtual Lab?

Pop the case off a guitar pedal or a cheap radio and you'll find the same three components staring back at you: resistors, capacitors, and inductors wired into an AC signal path. They are the building blocks of every analog filter, tuner, and power supply in the room. The catch is that AC current and AC voltage do not always peak at the same instant — capacitors make current lead voltage, inductors make it lag, and a real circuit blends both effects. This virtual lab gives you an oscilloscope, real component sliders, and a phasor diagram so you can see the phase shift as a number, a waveform, and a rotating arrow at the same time. Build a series RLC circuit, sweep the source frequency, and watch the impedance triangle rotate as the circuit slides through resonance.

Parameters explained

Frequency(Hz)

Frequency sets how many AC cycles the source completes each second. In this lab it is the fastest way to move the same RLC circuit from capacitive behavior to inductive behavior. At low frequency, the capacitor's reactance is large and can dominate the phase shift; at high frequency, the inductor's reactance grows and can dominate instead. Somewhere between those extremes, X_L and X_C become equal, so the circuit is at resonance and the current reaches its maximum for the fixed 10 V source. Move only Frequency to see current, impedance, and phase change together.

Resistance(Ω)

Resistance controls the real, energy-dissipating part of the circuit. Unlike inductive or capacitive reactance, resistance does not depend on frequency, so it sets the floor for total impedance at resonance. Lower resistance gives a larger current peak and a higher Q value, making the resonance curve narrower and more selective. Higher resistance damps the circuit, lowers the peak current, and makes the transition from capacitive to inductive behavior less sharp. Adjust only Resistance to isolate how damping changes the phasor diagram and the frequency-response graph.

Inductance(mH)

Inductance measures how strongly the coil opposes changes in current. The slider is shown in millihenries, while the calculation converts it to henries. As Inductance increases, inductive reactance X_L = 2πfL increases at the same frequency, so current tends to lag the source voltage more strongly. Larger L also lowers the resonant frequency because f_0 depends on 1/sqrt(LC). This makes the slider useful for showing that resonance is set by the pair of energy-storage components, not by resistance alone. Hold Capacitance fixed and raise Inductance to watch the resonance marker shift left on the graph.

Capacitance(μF)

Capacitance measures how much charge the capacitor stores for a given voltage. The slider is shown in microfarads, while the calculation converts it to farads. Increasing Capacitance lowers capacitive reactance X_C = 1/(2πfC), so the capacitor offers less opposition to AC current at the same frequency. It also lowers the resonant frequency when Inductance is unchanged, because a larger energy-storage capacity makes the LC exchange slower. Change only Capacitance to see how a larger capacitor changes current, phase angle, and the apparent filter cutoff.

Common misconceptions

• MisconceptionIn an AC circuit, voltage and current always peak at the same time, just like in a battery circuit.

  CorrectOnly in a pure resistor are V and I in phase. A capacitor makes the current peak before the voltage does (current leads), and an inductor makes the current peak after (current lags). The oscilloscope shows this as a horizontal shift between the two waves.

• MisconceptionImpedance is just the sum R + X_L + X_C — you add reactances like resistors in series.

  CorrectReactances are 90° out of phase with R, so you cannot just add them. Impedance is Z = √(R² + (X_L − X_C)²), which is the Pythagorean combination of the resistive and net reactive parts. That is why the phasor diagram is a right triangle.

• MisconceptionAt resonance the capacitor and inductor stop doing anything because they cancel out.

  CorrectThey are still active — energy sloshes back and forth between them every cycle. What cancels is their net opposition to current, because X_L equals X_C. Each component still has a real, often very large, voltage across it; it just adds to zero around the LC pair.

• MisconceptionThe voltage across an individual component can never be larger than the source voltage.

  CorrectIn a series RLC circuit at or near resonance, V_L and V_C can each be many times the fixed source amplitude. They are equal in magnitude but 180° out of phase, so they cancel in the loop equation while individually being large. This is the same selectivity principle used in tuned circuits such as radio receivers.

How teachers use this lab

1. Phase shift discovery: students keep Resistance, Inductance, and Capacitance fixed, then sweep Frequency across the resonance point. They record whether the circuit behaves more capacitive, resistive, or inductive and compare the sign of the phase angle to the phasor diagram.
2. Resonance experiment: students use the sliders to set a series RLC circuit, then sweep Frequency below and above the computed peak in 10 to 20 steps. Students record V_R, V_L, V_C, and I at each step, plot the curves, and identify the resonant frequency.
3. Misconception probe: at resonance, ask students to predict whether V_L will be smaller, equal, or larger than the source voltage. Many will say smaller; the simulation will show that V_L can be much larger when X_L >> R.
4. Q-factor lab: students hold Frequency, Inductance, and Capacitance fixed, then change only Resistance. They should connect lower resistance with a sharper current-vs-frequency peak and stronger frequency selectivity.
5. Filter comparison challenge: students vary Frequency and Capacitance one at a time, then explain how the response differs across settings using impedance and phase evidence.