In a parallel circuit, the same voltage drives both branches, and the total current is the vector sum of the branch currents. The resistor draws current in phase with the voltage: I_R = V / R. The capacitor draws current that leads the voltage by 90 degrees: I_C = j ω C V. Adding these currents as phasors gives the total current as I_total = V × (1/R + j ω C). This quantity (1/R + j ω C) is the admittance Y of the parallel network, so I_total = V × Y. To get the magnitude, take the magnitude of the admittance: |Y| = sqrt((1/R)^2 + (ω C)^2). Therefore, the magnitude of the total current is |I_total| = V_rms × |Y| = V_rms × sqrt((1/R)^2 + (ω C)^2). This approach directly reflects how the resistor and capacitor currents combine in parallel. While you could also use the equivalent impedance Z_eq and write |I| = V_rms / |Z_eq| (since |Z_eq| = 1/|Y|), the admittance form is the most straightforward for parallel circuits because it sums the branch currents directly.

A wave is the actual activity that rises and falls periodically. It refers to the repeating oscillation or disturbance itself, like how voltage or current in AC can rise to a peak and fall back down in a regular pattern. A period, on the other hand, is the amount of time one complete cycle takes. A waveform is the graphical shape of that variation over time, not the activity itself. An AC waveform just specifies the kind of waveform associated with alternating current. So, the word that best describes the physical activity of rising and falling in a repeating way is a wave.

Subharmonics are frequencies that are fractions of the fundamental, such as f0/2, f0/3, and so on. This means a subharmonic is a division (a submultiple) of the fundamental frequency. The other options describe frequencies that are multiples of the fundamental (harmonics), the same as the fundamental, or independent of it, none of which are subharmonics.

Reactive power is the part of electrical power that moves back and forth between the source and the reactive elements (inductors and capacitors) without doing net work over a cycle. It is represented by the symbol Q in standard AC theory, as the imaginary component of the complex power S = P + jQ. The active (real) power P relates to voltage and current by P = VI cos φ, while reactive power relates by Q = VI sin φ, where φ is the phase angle between voltage and current. The apparent power magnitude is S = sqrt(P^2 + Q^2), and the sign of Q indicates whether the load is inductive (current lags voltage, positive Q) or capacitive (current leads voltage, negative Q). If your course uses a different letter for reactive power, that’s a labeling choice, but the concept remains tied to the sin φ relationship and the separation of power into real and reactive components.