ETR 113

Flux

The total group of magnetic lines of force in a magnetic field. Measured in webers (Wb). 1 Wb = 10⁸ lines.

Flux density

Flux per unit cross-sectional area: B = Φ / A. Measured in teslas (T). 1 T = 1 Wb/m².

Retentivity

A material's ability to retain magnetism after the magnetizing force is removed. High-retentivity materials make good permanent magnets.

Ferromagnetic

Materials (iron, nickel, cobalt, their alloys) that are strongly attracted to magnets and can be magnetized. They have high permeability.

Hysteresis

The lag between the magnetizing force (H) and the resulting flux density (B) when a ferromagnetic material is magnetized/demagnetized. Shown by the B-H curve (hysteresis loop).

Reluctance

Opposition to the establishment of magnetic flux in a material — the magnetic analog of electrical resistance. ℛ = l / (μ·A). Units: At/Wb.

Hall effect

A small voltage (Hall voltage) generated across a conductor or semiconductor when it carries current in a magnetic field perpendicular to the current. Used in Hall-effect sensors to detect position, speed, or magnetic field strength.

Permeability (μ)

How easily a magnetic field can be established in a material. High permeability → easy to magnetize. Permeability of vacuum μ₀ = 4π × 10⁻⁷ Wb/(At·m).

Magnetomotive force (Fm)

The "driving force" that establishes magnetic flux. Fm = N·I (ampere-turns). Analogous to voltage in an electric circuit.

Electromagnetic induction

The process by which a voltage is induced in a conductor by a changing magnetic field — the principle behind generators and transformers.

cycle

One complete repetition of a waveform — one positive alternation plus one negative alternation.

Alternation

One half of a cycle — either the positive half or the negative half.

Period (T)

The time required to complete one cycle. Measured in seconds (s).

Frequency

The number of cycles per second. Measured in hertz (Hz). f = 1/T.

RMS

Root-mean-square. The DC equivalent that produces the same heating. VRMS = 0.707 × Vpeak. Your DMM reads in RMS.

Peak value

Maximum value reached during one alternation (one half-cycle). Vp.

Peak-to-peak

Voltage from positive peak to negative peak. Vpp = 2 × Vp.

Average value

Full cycle: average = 0. Half cycle: Vavg = 0.637 × Vpeak.

Alternator

An AC generator. Uses slip rings (not a commutator) so the output is AC. Rotor = rotating part, stator = stationary windings.