Alternating Current - Key Concepts

Vocabulary flashcards covering key AC circuit concepts, components, and devices from the notes (AC voltage/current, rms, phasors, reactance, impedance, resonance, transformers, and power).

Alternating Voltage (AC Voltage)

A voltage that varies with time in a sinusoidal manner (such as the mains supply) and can be transformed to different values using transformers.

Alternating Current (AC)

Current that changes direction with time; in AC circuits, voltage and current can be out of phase depending on circuit elements.

RMS (Root Mean Square)

A measure of AC quantity representing the effective DC value that would produce the same heating effect; for a sine wave, Irms = Ipeak/√2 and Vrms = Vpeak/√2.

Resistance (R)

Opposition to current flow in a conductor; in a pure resistor, voltage and current are in phase and P = Irms^2 R = Vrms^2 / R.

Inductive Reactance (X_L)

XL = ωL; opposition to changes in current due to an inductor; current lags voltage by 90°; XL increases with frequency.

Capacitive Reactance (X_C)

XC = 1/(ωC); opposition to charging of a capacitor; current leads voltage by 90°; XC decreases with frequency.

Impedance (Z)

Total opposition to current in an AC circuit combining resistance and reactance; for a series circuit Z^2 = R^2 + (XL − XC)^2; units are ohms.

Phase Angle (φ)

Angle between voltage and current; positive when current lags voltage (inductive), negative when current leads (capacitive).

Power Factor (cos φ)

Cosine of the phase angle; P = V I cos φ; cos φ = 1 for a pure resistor; cos φ = 0 for a purely inductive or capacitive circuit.

Phasor

A rotating vector representing a sinusoidally varying quantity; magnitude corresponds to the amplitude and the angle to the phase.

Phasor Diagram

Graphical representation of voltage and current phasors and their phase relationships in an AC circuit.

Inductor

An element with inductive reactance; current lags voltage by 90° in a purely inductive circuit; in such a circuit, average power is zero.

Capacitor

An element with capacitive reactance; current leads voltage by 90° in a purely capacitive circuit; in such a circuit, average power is zero.

Series LCR Circuit

A circuit with a resistor, inductor, and capacitor in series; its impedance is Z = √(R^2 + (XL − XC)^2) and can exhibit resonance.

Resonance

Condition in a series LCR circuit where XL = XC, giving minimum impedance and maximum current; ω0 = 1/√(LC).

Resonant Frequency (ω0)

The natural frequency at which a series LCR circuit resonates; ω0 = 1/√(LC) (f0 = ω0/2π).

Transformer

Device using mutual induction with a primary (Np turns) and a secondary (Ns turns); Vs/Vp = Ns/Np and Is/ Ip ≈ Np/Ns in an ideal transformer.

Step-Up Transformer

Ns > Np; Vs > Vp; Is < Ip; voltage is increased while current is decreased.

Step-Down Transformer

Ns < Np; Vs < Vp; Is > Ip; voltage is decreased while current is increased.

Mutual Induction

Induction of emf in the secondary coil due to a changing magnetic flux produced by the primary coil.

Joule Heating

Power dissipated as heat in a resistive element; P = I^2 R; in AC, the average power uses I_rms.

Wattless Current

Current that dissipates no average power (reactive power); occurs in pure inductive or pure capacitive circuits when cos φ = 0.

Peak Value and Amplitude

The maximum instantaneous value: Vm is the peak voltage, im is the peak current; rms values relate by Vrms = Vm/√2 and Irms = Im/√2.

Household RMS Voltage

A common rms value for mains voltage, typically about 220–240 V; a corresponding peak voltage is Vm ≈ 311–340 V (Vm = √2 V_rms).