Dale Crane's Oral and Practical Exam Guide: General; Material and Processes Questions And Answers What constitutes a capacitor? - ANSWER - -Two conductors separated by an insulator -In its most basic form the capacitor is constructed of two parallel plates separated by a nonconductor, ca lled a dielectric. (FAA -H-8083 -30, Ch. 10) What is the purpose of a capacitor? - ANSWER - -It Stores electrical energy in a electrostatic field. -In an electrical circuit, a capacitor serves as a reservoir or storehouse for electricity. (FAA -H-8083 -30, Ch. 10) Does a capacitor in an AC circuit cause the current to lead or lag the voltage? - ANSWER - -It causes the current to lead the voltage. -In a circuit in which there is only capacitance, current leads the applied voltage as contrasted with a circuit in which there is inductance, where the current lags the voltage. (FAA -H-8083 -30, Ch. 10) What is basic unit of capacitance? - ANSWER - -The farad The basic unit of capacitance is the farad and is given by the letter f. (FAA -H-8083 -30, Ch. 10) Why should Electrolytic capacitors not be used in an AC circuit? - ANSWER - -They are polarized. An Electrolytic capacitor will pass current of polarity, but will block current of the opposite polarity. -Unlike the other types, electrolytic capacitors are generally polarized, with the positive lead marked with a "+" and the negative lead marked with a "−" and should only be subjected to direct voltage or pulsating direct voltage only. (FAA -H-8083 -30, Ch. 10) What is meant by inductance? - ANSWER - -The abil ity to store electrical energy in electromagnetic fields. -The primary aspect of the operation of a coil is its property to oppose any change in current through it. This property is called inductance. When current flows through any conductor, a magnetic f ield starts to expand from the center of the wire. As the lines of magnetic force grow outward through the conductor, they induce an emf in the conductor itself. The induced voltage is always in the direction opposite to the direction of the current flow. (FAA -H-8083 -30, Ch. 10) What is the basic unit of inductance? - ANSWER - -The henry -The henry is the basic unit of inductance and is symbolized with the letter H. An electric circuit has an inductance of one henry when current changing at the rate of one ampere per second induces a voltage of one volt into the circuit. (FAA -H-8083 -30, Ch. 10) What is an example of an inductor used in a magneto? - ANSWER - The coil Does an inductor in an AC circuit cause the current to lead or lag the voltage? - ANSWER - It causes the electromagnet to lag behind the voltage. How can you find the polarity of an electromagnet? - ANSWER - -Hold the electromagnet in your left hand with your fingers encircling the coil in the direction electron flow (from negative to positive). Y our thumb will point to the north pole of the electromagnet. -The polarity of the electromagnet is determined by the left -hand rule in the same manner as the polarity of the coil without the core was determined. If the coil is grasped in the left hand in such a manner that the fingers curve around the coil in the direction of electron flow (minus to plus), the thumb will point in the direction of the North Pole . (FAA -H-8083 -30, Ch. 10) What is meant by impedance? - ANSWER - -The total opposition to the flo w of alternating current. If is the vector sum of resistance, capacitive reactance, and inductive reactance. -The combined effects of resistance, inductive reactance, and capacitive reactance make up the total opposition to current flow in an AC circuit. T his total opposition is called impedance and is represented by the letter Z. (FAA -H-8083 -30, Ch. 10) In what units is impedance measured? - ANSWER - In Ohms What are the five sources of electrical energy? - ANSWER - -Magnetism, chemical energy, light, heat , and pressure. -Electrical energy can be produced in a number of methods. The four most common are pressure, chemical, thermal, and light. (FAA -H-8083 -30, Ch. 10) -Mechanical (generator), chemical (battery), photoelectric (light), and thermal heat. What is the basic unit of power in a DC circuit? - ANSWER - The watt What is meant by a kilowatt? - ANSWER - 1,000 watts What is meant by a megawatt? - ANSWER - 1,000,000 watts What is the formula for power in a DC circuit? - ANSWER - -Power = Voltage times current (P = E x I) -For a simple DC electrical system, power dissipation can then be given by the equation: P= I(E) (FAA -H-8083 -30, Ch. 10) What is the relationship between mechanical and electrical power? - ANSWER - 1 horsepower = 746 watts What is mea nt by true power in an AC circuit? - ANSWER - -The product of the circuit voltage and the current that is in phase with this voltage -The power dissipated in the resistance of a circuit, or the power actually used in the circuit (FAA -H-8083 -30, Ch. 10) In what units is true power expressed? - ANSWER - In watts What is meant by apparent power in an AC circuit? - ANSWER - -The product of the circuit voltage and the circuit current -That power apparently available for use in an AC circuit containing a reactive component. It is the product of effective voltage times the effective current, expressed in volt-amperes. Only when the AC circuit is made up of pure resistance is the apparent power equal to the true power. (FAA -H-8083 -30, Ch. 10) In what units is appare nt power expressed? - ANSWER - In volt -amps What is meant by reactive power in an AC circuit? - ANSWER - -The power consumed in the inductive and capacitive reactances in an AC circuit. Relative power is also called wattless power. -Does no work and is ret urned to the line (FAA -H-8083 -30, Ch. 10) In what units is reactive power expressed? - ANSWER - In volt -amps reactive (VAR), or kilovolt -amps reactive (kVAR) What is meant by power factor in an AC circuit? - ANSWER - -The ratio of true power to apparent po wer. It is also the ratio of circuit resistance to circuit impedance. -The ratio of the true power to the apparent power is called the power factor and is usually expressed in percent. In equation form, the relationship is: Power Factor (PF) = (100 x Watts (True Power))/(Volts x Amperes (Apparent Power)) (FAA -H-8083 -30, Ch. 10) Which law of electricity is the most important for an aircraft mechanic to know? - ANSWER - -Ohm's law -The two fundamental properties of current and voltage are related by a third property known as resistance. In any electrical circuit, when voltage is applied to it, a current will result. The resistance of the conductor will determine the amount of current that flows under the given voltage. In most c ases, the greater the circuit resistance, the less the current. If the resistance is reduced, then the current will increase. This relation is linear in nature and is known as Ohm's law. (FAA -H-8083 -30, Ch. 10) What are the elements of Ohm's law? - ANSWER - Voltage (E, volts), current (I, amps), and resistance (R, ohms) What is voltage? - ANSWER - -Electrical voltage -Unlike current, which is easy to visualize as a flow, voltage is a variable that is determined between two points. Often we refer to voltage as a value across two points. It is the electromotive force (emf) or the push or pressure felt in a conductor that ultimately moves the electrons in a flow. The symbol for emf is the capital letter "E." (FAA -H-8083 -30, Ch. 10) What is the basic unit of vo ltage? - ANSWER - The volt What instrument is used to measure voltage? - ANSWER - A Voltmeter To measure voltage, is a voltmeter placed in series or in parallel with the source of voltage? - ANSWER - -In parallel -When voltmeters are used, they are connecte d in parallel with a circuit. What is electrical current? - ANSWER - The flow of electrons in a circuit What is the basic unit of current flow? - ANSWER - The ampere (amp) What part of an amp is a milliamp? - ANSWER - One thousandth (.001) of an amp What instrument is used to measure current flow? - ANSWER - An ammeter What two things happen when current flows through a conductor? - ANSWER - Heat is generated and a magnetic field surrounds the conductor To measure current through a component, is an ammeter placed parallel or in series with the component? - ANSWER - -In series -Always connect an ammeter in series with the element through which the current flow is to be measured. (FAA -H-8083 -30, Ch. 10) What is meant by resistance in an electrical circuit? - ANSWER - -The opposition to the flow of electrons -The resistance of the conductor will determine the amount of current that flows under the given voltage. In most cases, the greater the circuit resist ance, the less the current. If the resistance is reduced, then the current will increase. This relation is linear in nature and is known as Ohm's law. (FAA -H-8083 -30, Ch. 10) What is the basic unit of electrical resistance? - ANSWER - The Ohm What is a me gohm? - ANSWER - One million (1,000,000) ohms What four things affect the resistance of an electrical conductor? - ANSWER - -The material, the cross -sectional area, the length, or the temperature The resistance of a material is determined by four properties : material, length, area, and temperature (FAA -H-8083 -30, Ch. 10);
