EASA Module 3 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Valence electrons

Electrons in the outermost shell of an atom. They determine whether a material is a conductor, semiconductor or insulator, and they are the electrons that move to produce current.

Conventional current direction

The direction of current drawn from positive to negative. It is opposite to the actual direction of electron movement in a metal conductor.

Coulomb's law

The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation: F = kQ1Q2/r^2.

Static dischargers

Sharp-pointed wicks fitted to aircraft wing and tail trailing edges. They bleed accumulated static charge into the air to reduce radio interference and lightning-strike risk.

Ohm's law

For a metallic conductor at constant temperature, current is directly proportional to applied voltage and inversely proportional to resistance: I = V/R.

Electrical power equation

Power in a DC circuit equals voltage multiplied by current: P = VI, which is also equal to I^2R and to V^2/R.

Faraday's law of induction

The EMF induced in a coil equals the rate of change of flux linkage: e = -N dPhi/dt. More turns or faster flux change give a larger EMF.

Piezoelectric effect

Applying mechanical pressure to certain crystals, such as quartz, produces a voltage across them; conversely an applied voltage makes the crystal change shape.

Lead-acid cell voltage

A lead-acid cell produces about 2.0 to 2.1 volts on open circuit. A 24-volt battery therefore contains 12 cells connected in series.

Battery terminal voltage

The voltage at the terminals equals the internal EMF minus the internal volt drop: V = E - Ir, where r is the internal resistance.

Kirchhoff's voltage law

Around any closed loop, the algebraic sum of the EMFs equals the algebraic sum of the potential differences; equivalently the sum of all voltages in the loop is zero.

Thevenin's theorem

Any linear two-terminal network can be replaced by one EMF in series with one resistance, obtained from the open-circuit voltage and the resistance with sources replaced internally.

Resistivity formula

Resistance equals resistivity times length divided by cross-sectional area: R = rho L/A. Resistivity is a material property measured in ohm-metres.

Temperature coefficient of resistance

The fractional change in resistance per kelvin: R = R0(1 + alpha x deltaT). Metals have positive coefficients, so their resistance rises when heated.

Capacitor energy storage

The energy stored in a charged capacitor is W = 0.5 x C x V^2, where C is capacitance in farads and V is the voltage across it.

RC time constant

The product of resistance and capacitance, tau = RC, in seconds. After one time constant a capacitor charges to 63.2 per cent of the supply voltage.

Magnetic flux density

Flux density B is magnetic flux per unit area, B = Phi/A, measured in tesla (webers per square metre). It describes the strength of the field.

Reluctance

The opposition of a magnetic circuit to flux: S = l/(mu0 mur A), in ampere-turns per weber. Longer paths and smaller areas increase reluctance.

Inductor EMF equation

A changing current induces a back EMF in an inductor of e = -L di/dt. Large inductance or a fast current change produces a large opposing voltage.

Inductive reactance

The opposition an inductor offers to alternating current: XL = 2 x pi x f x L, in ohms. It rises in direct proportion to frequency.

Back EMF

The EMF generated by a spinning DC motor armature that opposes the supply voltage. It limits armature current to (V - Eb)/Ra.

Fleming's left-hand rule

With thumb, first finger and second finger mutually at right angles: first finger along the field, second along the current, thumb gives the force on a motor conductor.

RMS value

The value of a sine wave that produces the same heating effect as an equal DC value: RMS = peak/1.414 = 0.707 x peak.

Power factor

The cosine of the phase angle between voltage and current, cos(phi) = R/Z. It also equals real power divided by apparent power.

Series resonance

In a series RLC circuit resonance occurs when XL equals XC. Impedance is then minimum and equal to R, and the current is maximum.

Impedance of an RLC circuit

Total opposition in a series RLC circuit: Z = square root of (R^2 + (XL - XC)^2) in ohms, with tan(phi) = (XL - XC)/R.

Transformer turns ratio

The secondary-to-primary voltage ratio equals the turns ratio, while currents change inversely: V1/V2 = N1/N2 = I2/I1.

Current transformer rule

A current transformer secondary must never be open-circuited while the primary carries current, because an extremely high and lethal voltage would be induced.

Low-pass filter cut-off

The frequency at which a simple RC low-pass filter output falls to 0.707 of its input: fc = 1/(2 x pi x R x C).

Decibel voltage ratio

A logarithmic measure of gain or attenuation: dB = 20 log10(Vout/Vin) for voltages. Halving the voltage is about minus 6 dB.

Synchronous speed

The speed of the rotating field in an AC machine: Ns = 120f/p, where f is the supply frequency and p the number of poles.

Induction motor slip

The fractional difference between synchronous speed and rotor speed: s = (Ns - N)/Ns. Slip is typically a few per cent at full load.