Module 3 Electrical Fundamentals key terms

A negatively charged subatomic particle that orbits the nucleus of an atom. Its movement between atoms, or drift through a conductor, is what constitutes electric current in most practical circuits.

An electron in the outer shell of an atom that is only weakly bound to the nucleus and can move easily from atom to atom. Materials with many free electrons are good conductors.

The outermost shell of electrons around an atom's nucleus. The number of electrons in this shell largely determines whether a material behaves as a conductor, semiconductor or insulator.

An atom or molecule that has gained or lost one or more electrons, giving it a net electric charge. Positive ions have lost electrons; negative ions have gained them.

An accumulation of electric charge on a surface or within an insulating material, where the charge cannot readily flow away. It produces forces, sparks and shocks, and is a recognised ignition hazard around aircraft fuelling.

The electrical connection of two or more conductive parts so they are held at the same potential, preventing sparks from static charge building up between them. It is distinct from earthing, which connects to the airframe or ground.

A material, usually a metal such as copper or aluminium, that contains many free electrons and therefore allows electric current to flow through it with little opposition.

A material in which electrons are tightly bound to their atoms, so almost no current flows even when a voltage is applied. Rubber, glass and most plastics are common examples.

The rate of flow of electric charge past a point in a circuit, measured in amperes. One ampere corresponds to one coulomb of charge passing a point each second.

The difference in electric potential between two points, measured in volts. It is the electrical pressure that drives current through a circuit; one volt is one joule per coulomb.

The voltage measured between two specified points in a circuit, representing the work done per unit charge in moving between them. It is measured with a voltmeter connected in parallel.

The energy per unit charge supplied by a source such as a battery or generator to drive current around a circuit, measured in volts. It is the open-circuit voltage of the source.

The SI unit of electric current, defined as one coulomb of charge passing a point per second. It is commonly abbreviated to amp and given the symbol A.

The SI unit of electric charge, equal to the charge carried by approximately 6.24 x 10^18 electrons. One coulomb flowing past a point in one second is one ampere.

The relationship stating that current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, provided temperature stays constant. Expressed as V = I x R.

The generation of an electromotive force in a conductor when it experiences a changing magnetic field, or moves through one. This principle underlies generators, transformers and inductors.

A law stating that the induced electromotive force is proportional to the rate of change of magnetic flux linkage through a coil. Faster flux change or more turns gives a larger induced voltage.

The generation of a voltage across certain crystals, such as quartz, when they are mechanically stressed. It is used in vibration sensors, pressure transducers and some ignition systems.

A temperature sensor made from two dissimilar metals joined at a hot junction, which generates a small voltage proportional to the temperature difference between junctions. It exploits the thermoelectric effect.

A semiconductor device that converts light energy directly into electrical energy. Light falling on the cell releases electrons, producing a small voltage across its terminals.

An electrochemical battery cell that cannot be recharged because its chemical reaction is not readily reversible. Once discharged, it must be replaced.

A rechargeable battery cell in which the chemical reaction is reversible, so it can be restored to a charged state by passing current through it in reverse. Lead-acid aircraft batteries are of this type.

The resistance within a voltage source itself, which causes its terminal voltage to fall as the current drawn from it increases. It dissipates energy as heat inside the source.

A circuit in which components are connected end to end so the same current flows through each. Total resistance is the sum of the individual resistances, and the supply voltage divides between them.

A circuit in which components are connected across the same two points, so each has the full supply voltage across it and the current divides between branches. Total resistance is always less than the smallest branch resistance.

A law stating that the total current entering a junction equals the total current leaving it, because charge cannot accumulate at a point. It is the basis of current-divider calculations.

A law stating that the algebraic sum of all voltages around any closed loop in a circuit is zero. In practice, the sum of voltage drops equals the applied voltage.

The opposition a material offers to the flow of electric current, measured in ohms. It converts electrical energy into heat and depends on material, length, cross-section and temperature.

A property of a material describing how strongly it opposes current flow, independent of the shape of the sample. It is quoted in ohm-metres and used to compare conductor materials.

A system of coloured bands on a resistor that encodes its resistance value and tolerance. Each colour represents a digit, multiplier or tolerance figure, read from the band nearest one end.

The permissible variation of a component's actual value from its marked or nominal value, usually expressed as a percentage. A 100 ohm resistor with 5 percent tolerance may lie between 95 and 105 ohms.

The rate at which electrical energy is transferred or converted, measured in watts. In a DC circuit it equals voltage multiplied by current, and can also be found from current squared times resistance.

The SI unit of power, equal to one joule of energy transferred per second. One watt is also produced when one ampere flows under a potential difference of one volt.

The ability of a component to store electric charge for a given applied voltage, measured in farads. One farad stores one coulomb when one volt is applied across it.

A component of two conducting plates separated by an insulator that stores energy in an electric field. It blocks steady direct current but passes changing current, and opposes voltage change.

The insulating material between the plates of a capacitor. Its permittivity determines how much charge the capacitor can store for a given voltage and plate area.

The product of resistance and capacitance (R x C) in a resistor-capacitor circuit. It is the time taken for the capacitor to charge to about 63 percent of the supply voltage, or discharge by the same fraction.

The total number of magnetic field lines passing through a given area, measured in webers. Its rate of change through a coil determines the voltage induced in that coil.

The strength of a magnetic field expressed as flux per unit area, measured in teslas. One tesla is one weber of flux passing through one square metre.

A measure of how readily a material supports the formation of a magnetic field within it. Ferromagnetic materials such as iron have high permeability and concentrate flux lines.

The opposition a magnetic circuit offers to magnetic flux, analogous to resistance in an electric circuit. It depends on material permeability, path length and cross-sectional area.

The property of a coil that opposes any change in current through it by inducing a voltage, measured in henries. It arises from the magnetic field linking the coil's own turns.

A component, usually a coil of wire, that stores energy in a magnetic field and opposes changes in current. It passes steady direct current freely but opposes alternating or changing current.

A law stating that an induced electromotive force always acts in a direction that opposes the change producing it. It explains why an inductor resists increases and decreases in current.

A current that periodically reverses direction, typically following a sinusoidal waveform. Aircraft systems commonly use 115 volts at 400 hertz to save weight in transformers and motors.

The number of complete cycles of an alternating waveform occurring each second, measured in hertz. It equals the reciprocal of the period of one cycle.

The root-mean-square value of an alternating quantity, equal to the steady direct value that would produce the same heating effect. For a sine wave it is peak value divided by the square root of two.

The total opposition to alternating current flow from resistance and reactance combined, measured in ohms. It accounts for both the magnitude and phase relationship between voltage and current.

The opposition to alternating current caused by capacitance or inductance, measured in ohms. Capacitive reactance falls as frequency rises, while inductive reactance rises with frequency.

The ratio of true power to apparent power in an alternating current circuit, equal to the cosine of the phase angle between voltage and current. It is one for a purely resistive circuit.