EASA Module 3 key terms

A stable subatomic particle carrying a single negative charge, found orbiting the nucleus of an atom. Its drift through a conductor constitutes electric current, and its charge magnitude equals that of a proton.

An electron in the outermost shell of an atom that is only weakly bound to its nucleus, so it can drift from atom to atom when a voltage is applied, allowing the material to conduct.

Electric charge that accumulates on an insulated body and remains at rest there. It builds up by friction, contact or induction, and discharges suddenly when a conducting path to earth appears.

The force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance between them, acting along the line joining the charges.

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

The energy converted, or work done, per unit charge as charge moves between two points in a circuit, measured in volts. It is the voltage measured across a component.

The energy per unit charge that a source such as a battery or generator gives to charge to drive it around a circuit, measured in volts. It equals the open-circuit terminal voltage.

The generation of an electromotive force in a conductor whenever the magnetic flux linking it changes, whether by moving the conductor, moving the field, or varying the current producing the field.

The property of certain crystals, such as quartz, that develop a voltage across their faces when mechanically stressed, and that change shape when a voltage is applied to them.

An electrochemical cell that converts chemical energy into electrical energy and cannot usefully be recharged, because the reactions at its electrodes are not readily reversible once discharged.

A cell such as a lead-acid battery whose electrode reactions are reversible, so it can be recharged. Electrical energy is stored as chemical energy and returned during discharge.

The resistance present inside a source of EMF, within its electrodes and electrolyte. It causes the terminal voltage to fall below the EMF when the source delivers current, and to rise when charging.

A circuit in which components are connected end to end so the same current flows through each. The 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 nodes, so each has the same voltage across it. Total current is the sum of branch currents and total resistance is less than the smallest branch.

Around any closed loop of a circuit, the algebraic sum of the electromotive forces equals the algebraic sum of the potential drops, so the net voltage around the loop is zero.

For a conductor at constant temperature, the current through it is directly proportional to the voltage across it and inversely proportional to its resistance, giving V = I x R.

A property of a material, symbol rho, equal to the resistance between opposite faces of a cube of unit side. Resistance also depends on length and cross-sectional area: R = rho x l / A.

The fractional change in resistance of a material for each degree Celsius rise in temperature. It is positive for metals and negative for semiconductors such as thermistors.

The rate at which electrical energy is converted into another form, measured in watts. In a DC circuit it equals voltage multiplied by current, and may also be calculated as I squared R.

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

An insulating material placed between the plates of a capacitor. Its molecules polarise in the electric field, which increases the capacitance compared with an air gap of the same dimensions.

The time, equal to resistance multiplied by capacitance, for the voltage across a capacitor in a resistor-capacitor circuit to change by about 63 percent of the difference towards its final value.

The amount of magnetic flux passing through unit area perpendicular to the field, measured in teslas. One tesla is one weber per square metre, and it indicates the strength of the field.

The property of a material describing how readily magnetic flux is established within it. Ferromagnetic materials such as iron have a much higher permeability than air or non-magnetic materials.

The property of a circuit, measured in henries, whereby a changing current induces an electromotive force that opposes the change. One henry gives one volt per ampere per second of change.

The direction of an induced electromotive force is always such that it opposes the change producing it, so an induced current sets up a magnetic field that opposes the motion or flux change.

A split ring on the armature shaft of a DC machine that reverses the connection of each armature coil to the external circuit every half revolution, so current in the external circuit stays unidirectional.

The electromotive force induced in the armature of a motor by its own rotation, which opposes the applied voltage and limits armature current. It is proportional to speed and field flux.

Current that periodically reverses direction, normally following a sine wave and described by its frequency, peak value and phase relative to the applied voltage.

The value of an alternating current or voltage that produces the same heating effect in a resistor as the equivalent direct value. For a sine wave it is 0.707 times the peak value.

The opposition to alternating current offered by a capacitor or an inductor, measured in ohms. It varies with frequency and causes a 90 degree phase shift between voltage and current.

The total opposition to alternating current from resistance and reactance combined, measured in ohms. It equals the vector sum of resistance and net reactance in the circuit.

The condition in a circuit containing inductance and capacitance where the two reactances are equal and cancel. Impedance is then minimum in a series circuit and maximum in a parallel circuit.

The ratio of true power in watts to apparent power in volt-amperes in an alternating current circuit, equal to the cosine of the phase angle between voltage and current.

The ratio of the number of turns on the secondary winding of a transformer to the number on the primary. It sets the ratio of secondary voltage to primary voltage.

Circulating currents induced in the bulk iron core of a transformer or machine by a changing magnetic flux. They cause heating losses and are reduced by laminating the core.

A network of resistors, capacitors or inductors that passes signal frequencies below a chosen value with little attenuation while progressively reducing frequencies above that value.

The speed of the rotating magnetic field produced by a polyphase stator winding, equal to 120 times the supply frequency divided by the number of poles of the machine.

The difference between the synchronous speed of the rotating field and the actual rotor speed of an induction motor, usually expressed as a percentage of synchronous speed. Some slip is needed to induce rotor current.

The field produced by three-phase currents in stator windings displaced 120 degrees apart. It rotates at synchronous speed and drags the rotor around with it.