← Electrical Systems

1 Reading the notation

Electricity reuses several letters for different quantities, so context matters. Each symbol below is used on this page.

📖 Symbols used in electricity

10 terms

Cover the right column and check you can say what each one means.

V, I, R
V = voltage (volts), I = current (amps), R = resistance (ohms, Ω). V = IR. I is also used for rotational inertia in mechanics: different topic, different meaning.
Q
Charge, in coulombs (C). For a capacitor, Q = CV.
C
Capacitance, in farads (F). Also the symbol for the coulomb, read the context.
L
Inductance, in henries (H).
ε (epsilon)
EMF, the total energy per coulomb a source supplies. Different from terminal voltage, which is ε − Ir.
τ (tau)
The time constant, τ = RC. Time to reach ~63% of the final voltage when charging. τ also means torque in mechanics.
XC and XL
Reactance, AC resistance of a capacitor and an inductor, in ohms. XC falls with frequency; XL rises.
Z
Impedance, total AC opposition, combining R, XC and XL.
rms (root mean square)
The effective AC value: the DC value that would deliver the same power. Vrms = Vpeak/√2. NZ mains is 230 V rms, so its peak is about 325 V.
Ω (omega, capital)
The ohm, unit of resistance. Lower-case ω is angular frequency: a different quantity.
✓

Same letter, different meaning

I = current here but rotational inertia in mechanics; C = capacitance or coulombs; τ = time constant here but torque in mechanics. Always read the topic first.

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