Dale Crane's Oral And Practical Exam Guide: General; Basic Electricity

What is the open-circuit voltage of a lead-acid cell?

2.1 volts

What is the basic equation of Ohm's law?

E = I x R where E = Voltage, I = Current, and R = Resistance

What two things happen when current flows through a conductor?

Heat is generated and a magnetic field surrounds the conductor

What formula is used to find current when voltage and resistance are known?

I = E / R

What formula is used to find resistance when voltage and current are known?

R = E / I

What are the elements of Ohm's law?

Voltage (E, volts), current (I, amps), and resistance (R, ohms)

How much current flows through each of the three resistors connected in series if the total current is 3 amps?

-3 amps -In a series circuit, the current cannot be different at different points within the circuit. The current through a series circuit will always be the same through each element and at any point. (FAA-H-8083-30, Ch. 10)

What instrument is used to measure voltage?

A Voltmeter

In what units is apparent power expressed?

In volt-amps

In what units is reactive power expressed?

In volt-amps reactive (VAR), or kilovolt-amps reactive (kVAR)

What is electrical current?

The flow of electrons in a circuit

In what units is impedance measured?

In Ohms

How many cells are there in a 24-volt lead-acid battery?

-12 -The voltage of lead acid cell is approximately 2 volts in order to attain the voltage required for the application. Each cell is then connected in series with heavy gage metal straps to form a battery. In a typical battery, such as that used in a aircraft for starting, the voltage required is 12 or 24 volts. The voltage of a battery is determined by the number of cells connected in series to form the battery. Although the voltage of one lead-acid cell just removed from a charger is approximately 2.2 volts, a lead-acid cell is normally rated at approximately 2 volts. A battery rated at 12 volts consists of 6 lead-acid cells connected in series, and a battery rated at 24 volts is composed of 12 cells. (FAA-H-8083-30, Ch. 10)

What is the voltage across each resistor connected in parallel across a 12-volt battery?

-12 volts -The voltage across any branch is equal to the voltage across all of the other branches. (FAA-H-8083-30, Ch. 10)

What is the total resistance of three 12-ohm resistors connected in series?

-36 ohms -For resistors in a series con guration, the total resistance of the circuit is equal to the sum of the individual resistors. The basic formula is: (FAA-H-8083-30, Ch. 10) R(T) = R(1) + R(2) + R(3) + .........R(n)

What is the total resistance of three 12-ohm resistors in parallel?

-4 ohms -When resistors are connected in par- allel, the total resistance of the circuit decreases. The total resistance of a parallel combination is always less than the value of the smallest resistor in the circuit. In the series circuit, the current has to pass through the resistors one at a time. This gave a resistance to the current equal the sum of all the resistors. In the parallel circuit, the current has several resistors that it can pass through, actually reducing the total resistance of the circuit in relation to any one resistor value. (FAA-H-8083-30, Ch. 10) -The formula for the total parallel resistance is as follows: 1/R(t) = 1/R(1) + 1/R(2) + 1/R(3) ... + 1/R(n) -If the reciprocal of both sides is taken, then the general formula for the total parallel resistance is: R(t) = 1/((1/R(1)) + (1/R(2)) + (1/R(3)) ... (1/R(n)))

What represents capacitance in an AC circuit?

-A capacitor (condenser) -While inductance is represented in a circuit by a coil, capacitance is represented by a capacitor. In its most basic form the capacitor is constructed of two parallel plates separated by a nonconductor, called a dielectric. In an electrical circuit, a capacitor serves as a reservoir or storehouse for electricity. (FAA-H-8083-30, Ch. 10)

What instrument is used to measure the specific gravity of the electrolyte in a lead-acid battery?

-A hydrometer -The state of charge of a storage battery depends upon the condition of its active materials, primarily the plates. However, the state of charge of a battery is indi- cated by the density of the electrolyte and is checked by a hydrometer, an instrument that measures the speci c gravity (weight as compared with water) of liquids. (FAA-H-8083-30, Ch. 10) -Use a hydrometer to determine the specific gravity of the lead-acid battery electrolyte, which is the weight of the electrolyte compared to the weight of pure water. Take care to ensure the electrolyte is returned to the cell from which it was extracted. When a speci c gravity difference of 0.050 or more exists between cells of a battery, the battery is approaching the end of its useful life and replacement should be considered. Electrolyte level may be adjusted by the addition of distilled water. Do not add electrolyte. (FAA-H-8083-31, Ch. 09)

What electrolyte is used in a lead-acid battery?

-A mixture of sulfuric acid and water -The lead acid battery is made up of a series of identical cells each containing sets of positive and negative plates. Each cell contains positive plates of lead dioxide (PbO2), negative plates of spongy lead, and electrolyte (sulfuric acid and water) (FAA-H-8083-30, Ch. 10)

What is used to neutralize spilled electrolyte from a nickel-cadmium battery?

-A solution of boric acid and water -The nickel-cadmium battery is usually interchangeable with the lead-acid type. When replacing a lead-acid battery with a nickel-cadmium battery, the battery compartment must be clean, dry, and free of all traces of acid from the old battery. The compartment must be washed out and neutralized with ammonia or boric acid solution, allowed to dry thoroughly, and then painted with an alkali resisting varnish. (FAA-H-8083-30, Ch. 10)

What is the range of temperature of the electrolyte in a lead-acid battery that does not require a correction when measuring its specific gravity?

-Between 70°F and 90°F -When a battery is tested using a hydrometer, the temperature of the electrolyte must be taken into consideration. The speci c gravity readings on the hydrometer will vary from the actual speci c gravity as the temperature changes. No correction is necessary when the temperature is between 70 °F and 90 °F, since the variation is not great enough to consider. When temperatures are greater than 90 °F or less than 70 °F, it is necessary to apply a correction factor. Some hydrom- eters are equipped with a correction scale inside the tube. With other hydrometers, it is necessary to refer to a chart provided by the manufacturer. In both cases, the corrections should be added to, or subtracted from the reading shown on the hydrometer. (FAA-H-8083-30, Ch. 10)

How can you tell the resistance of a composition resistor?

-By a series of colored bands around one end of the resistor -The resistor color code is made up of a group of colors, numbers, and tolerance values. Each color is represented by a number, and in most cases, by a tolerance value. When the color code is used with the end-to-center band marking system, the resistor is normally marked with bands of color at one end of the resistor. The body or base color of the resistor has nothing to do with the color code, and in no way indicates a resistance value. To prevent confusion, this body will never be the same color as any of the bands indicating resistance value. (FAA-H-8083-30, Ch. 10)

What is electromagnetic induction?

-It is the transfer of electrical energy by means of a magnetic field. This is the principle of transformer operation. -Faraday's Law or the law of electromagnetic induction, states: (FAA-H-8083-30, Ch. 10) The induced emf or electromagnetic force in a closed loop of wire is proportional to the rate of change of the magnetic flux through a coil of wire. Conversely, current flowing through a coil of wire produces a magnetic field. When this wire is formed into a coil, it then becomes a basic inductor. The magnetic lines of force around each loop or turn in the coil effectively add to the lines of force around the adjoining loops. This forms a strong magnetic field within and around the coil. The magnetic lines of force around adjacent loops are deflected into an outer path when the loops are brought close together. This happens because the magnetic lines of force between adjacent loops are in opposition with each other.

How is it possible to know when a nickel-cadmium battery is fully charged?

-Completely discharge the battery give it a constant-current charge to 140% of its ampere-hour capacity. -The state of charge of a NiCd battery cannot be determined by measuring the speci c gravity of the potassium hydroxide electrolyte. The electrolyte speci c gravity does not change with the state of charge. The only accurate way to determine the state of charge of a NiCd battery is by a measured discharge with a NiCd battery charger and following the manufacturer's instructions. After the battery has been fully charged and allowed to stand for at least 2 hours, the uid level may be adjusted, if necessary, using distilled or demineralized water. Because the uid level varies with the state of charge, water should never be added while the battery is installed in the aircraft. Over lling the battery results in electrolyte spewage during charging. This causes corrosive effects on the cell links, self-discharge of the battery, dilution of the electrolyte density, possible blockage of the cell vents, and eventual cell rupture. (FAA-H-8083-31, Ch. 09)

What is voltage?

-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)

Is the electrolyte level of a nickel-cadmium battery lowest when the battery is fully charged or discharged?

-Fully discharged -During discharge, the plates absorb a quantity of the electrolyte. On recharge, the level of the elec- trolyte rises and, at full charge, the electrolyte will be at its highest level. Therefore, water should be added only when the battery is fully charged. (FAA-H-8083-30, Ch. 10)

How can you find the polarity of an electromagnet?

-Hold the electromagnet in your left hand with your fingers encircling the coil in the direction electron flow (from negative to positive). Your 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 gases are released when a lead-acid battery is being charged?

-Hydrogen and oxygen -When a storage battery is being charged, it generates a certain amount of hydrogen and oxygen. Since this is an explosive mixture, it is important to take steps to prevent ignition of the gas mixture (FAA-H-8083-30, Ch. 10)

In what units is battery capacity expressed?

-In ampere-hours -The most common battery rating is the amp-hour rating. This is a unit of measurement for battery capacity. It is determined by multiplying a current ow in amperes by the time in hours that the battery is being discharged. A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours, etc., before becoming completely discharged. (FAA-H-8083-30, Ch. 10)

To measure voltage, is a voltmeter placed in series or in parallel with the source of voltage?

-In parallel -When voltmeters are used, they are connected in parallel with a circuit.

To measure current through a component, is an ammeter placed parallel or in series with the component?

-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)

Does a capacitor in an AC circuit cause the current to lead or lag the voltage?

-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 the purpose of a capacitor?

-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 the specific gravity of the electrolyte in a lead-acid battery increase or decrease as the battery becomes discharged?

-It decreases -Each cell containing the plates are filled with an electrolyte composed of sulphuric acid and distilled water with a specific gravity of 1.270 at 60 °F. This solution contains positive hydrogen ions and negative sulfate (SO4) ions that are free to combine with other ions and form a new chemical compound. When the cell is discharged, electrons leave the negative plate and flow to the positive plates where they cause the lead dioxide (PbO2) to break down into negative oxygen ions and positive lead ions. The negative oxygen ions join with positive hydrogen ions from the sulfuric acid and form water (H2O). The negative sulfate ions join with the lead ions in both plates and form lead sulfate (PbSO4). After the discharge, the specific gravity changes to about 1.150. (FAA-H-8083-30, Ch. 10)

What happens to the current in a DC circuit if the voltage is increased but the resistance remains the same?

-It increases -By having a linearly proportional characteristic, it is meant that if one unit in the relationship increases or decreases by a certain percentage, the other variables in the relationship will increase or decrease by the same percentage. An example would be if the voltage across a resistor is doubled, then the current through the resistor doubles. It should be added that this relationship is true only if the resistance in the circuit remains constant. For it can be seen that if the resistance changes, current also changes. (FAA-H-8083-30, Ch. 10)

Describe a no-trip circuit breaker (trip free)

-It is a breaker that cannot be reset while in flight -A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by an overload or short circuit. Its basic function is to detect a fault condition and immediately discontinue electrical ow. Unlike a fuse that operates once and then has to be replaced, a circuit breaker can be reset to resume normal operation. All resettable circuit breakers should open the circuit in which they are installed regardless of the position of the operating control when an overload or circuit fault exists. Such circuit breakers are referred to as trip-free. (FAA-H-8083-31, Ch. 09)

What happens to the current in a conductor if the length of conductor is doubled with all other parameters unchanged?

-It would decrease one half -The resistance of a metallic conductor is directly proportional to its length. The longer the length of a given size of wire, the greater the resistance. If 1 volt of electrical pressure is applied across the two ends of the conductor that is 1 foot in length and the resistance to the movement of free electrons is assumed to be 1 ohm, the current flow is limited to 1 ampere. If the same size conductor is doubled in length, the same electrons set in motion by the 1 volt applied now find twice the resistance; consequently, the current flow will be reduced by one-half. (FAA-H-8083-30, Ch. 10)

What are the five sources of electrical energy?

-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 law that describes the relationship in an electrical circuit of voltage, current and resistance?

-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 ows under the given voltage. In most cases, the greater the cir- cuit 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)

Which law of electricity is the most important for an aircraft mechanic to know?

-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 cases, 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)

How high should the electrolyte level be in a properly serviced lead-acid battery?

-Only up to the indicator level -In a new, fully charged aircraft storage battery, the electrolyte is approximately 30 percent acid and 70 percent water (by volume) and is 1.300 times as heavy as pure water. During discharge, the solution (electrolyte) becomes less dense and its specific gravity drops below 1.300. A specific gravity reading between 1.300 and 1.275 indicates a high state of charge; between 1.275 and 1.240, a medium state of charge; and between 1.240 and 1.200, a low state of charge. Aircraft batteries are generally of small capacity but are subject to heavy loads. The values speci ed for state of charge are therefore rather high. (FAA-H-8083-30, Ch. 10)

How do you treat a lead-acid battery compartment to protect it from corrosion?

-Paint it with an asphaltic (tar-base) paint or with a polyurethane enamel

What is the electrolyte used in a nickel-cadmium battery?

-Potassium hydroxide and water -Active materials in nickel-cadmium cells (Ni-Cad) are nickel hydrate (NiOOH) in the charged positive plate (Anode) and sponge cadmium (Cd) in the charged negative plate (Cathode). The electrolyte is a potassium hydroxide (KOH) solution in concentration of 20 - 34 percent by weight pure KOH in distilled water. The typical open circuit cell voltage of a nickel- cadmium battery is about 1.25 volts. (FAA-H-8083-30, Ch. 10)

What is the formula for power in a DC circuit?

-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 are the types if DC circuits with regard to the placement of various circuit components?

-Series, parallel, and series-parallel -The series circuit is the most basic electrical circuit and provides a good introduction to basic circuit analysis. A simple circuit with nothing more than a voltage source or battery, a conductor, and a resistor. This is classified as a series circuit because the components are connected end-to-end, so that the same current flows through each component equally. There is only one path for the current to take and the battery and resistor are in series with each other; (FAA-H-8083-30, Ch. 10) -For resistors in a series configuration, the total resistance of the circuit is equal to the sum of the individual resistors. The basic formula is: R(T) = R(1) + R(2) + R(3) + .........R(N) -In a series circuit, the current cannot be different at different points within the circuit. The current through a series circuit will always be the same through each element and at any point -A law of basic importance to the analysis of an electrical circuit is Kirchhoff's voltage law. This law simply states that the algebraic sum of all voltages around a closed path or loop is zero. Another way of saying it: The sum of all the voltage drops equals the total source voltage -A circuit in which two of more electrical resistances or loads are connected across the same voltage source is called a parallel circuit. The primary difference between the series circuit and the parallel circuit is that more than one path is provided for the current in the parallel circuit. Each of these parallel paths is called a branch. The minimum requirements for a parallel circuit are the following: (FAA-H-8083-30, Ch. 10) -A power source, conductors, a resistance or load for each current path, two or more paths for current flow -The voltage across any branch is equal to the voltage across all of the other branches. -The parallel circuit consists of two or more resistors connected in such a way as to allow current ow to pass through all of the resistors at once. This eliminates the need for current to pass one resistor before passing through the next. When resistors are connected in par- allel, the total resistance of the circuit decreases. The total resistance of a parallel combination is always less than the value of the smallest resistor in the circuit. In the series circuit, the current has to pass through the resistors one at a time. This gave a resistance to the current equal the sum of all the resistors. In the parallel circuit, the current has several resistors that it can pass through, actually reducing the total resistance of the circuit in relation to any one resistor value. -The formula for the total parallel resistance is as follows: 1/R(t) = 1/R(1) + 1/R(2) + 1/R(3) ... + 1/R(n) -If the reciprocal of both sides is taken, then the general formula for the total parallel resistance is: R(t) = 1/((1/R(1)) + (1/R(2)) + (1/R(3)) ... (1/R(n))) -The amount of current passing through each resistor will vary according to its individual resistance. The total current of the circuit will then be the sum of the current in all branches. It can be determined by inspec- tion that the total current will be greater than that of any given branch. -Kirchhoff's current law can be stated as: The sum of the currents into a junction or node is equal to the sum of the currents owing out of that same junction or node. A junction can be de ned as a point in the circuit where two or more circuit paths come together. In the case of the parallel circuit, it is the point in the circuit where the individual branches join. General formula I(T) = I(1) + I(2) + I(3) -Most of the circuits that the technician will encounter will not be a simple series or parallel circuit. Circuits are usually a combination of both, known as series-par- allel circuits, which are groups consisting of resistors in parallel and in series.

What is meant by inductance?

-The ability 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 field 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 meant by continuity in an electrical circuit?

-The circuit is continuous (or complete) when electrons can flow from one terminal of power source to the other -Continuity — the state of being continuous, unin- terrupted or connected together; the opposite of a circuit that is not broken or does not have an open. (FAA-H-8083-30, Ch. 10)

Why is a hydrometer not used to measure the state of charge of a nickel-cadmium battery?

-The electrolyte of a nickel-cadmium battery does not enter into the chemical changes that occur when the battery is charged or discharged. its specific gravity does not change appreciably -The state of charge of a NiCd battery cannot be determined by measuring the speci c gravity of the potassium hydroxide electrolyte. The electrolyte speci c gravity does not change with the state of charge. The only accurate way to determine the state of charge of a NiCd battery is by a measured discharge with a NiCd battery charger and following the manufacturer's instructions. (FAA-H-8083-31, Ch. 09)

What is basic unit of capacitance?

-The farad The basic unit of capacitance is the farad and is given by the letter f. (FAA-H-8083-30, Ch. 10)

What is the basic unit of inductance?

-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 a thermal runaway?

-The large current flow allowed by the low internal resistance causes the middle cells to produce more heat than they can dissipate. The heat further lowers the internal resistance so more current can flow; this continues until the battery destroys itself -A combination of high battery temperature (in excess of 160 °F) and overcharging can lead to a condition called thermal runaway. The temperature of the battery has to be constantly monitored to ensure safe operation. Thermal runaway can result in a NiCad chemical fire and/or explosion of the NiCad battery under recharge by a constant-voltage source and is due to cyclical, ever-increasing temperature and charging current. One or more shorted cells or an existing high temperature and low charge can produce the following cyclical sequence of events: (FAA-H-8083-31, Ch. 09) 1. Excessive current, 2. Increased temperature, 3. Decreased cell(s) resistance, 4. Further increased current, and 5. Further increased temperature.

What four things affect the resistance of an electrical conductor?

-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); - The resistance of a metallic conductor is dependent on the type of conductor material. It has been pointed out that certain metals are commonly used as conductors because of the large number of free electrons in their outer orbits. Copper is usually considered the best available conductor material, since a copper wire of a particular diameter offers a lower resistance to current ow than an aluminum wire of the same diameter. However, aluminum is much lighter than copper, and for this reason as well as cost considerations, aluminum is often used when the weight factor is important -The resistance of a metallic conductor is directly proportional to its length. The longer the length of a given size of wire, the greater the resistance -The resistance of a metallic conductor is inversely proportional to the cross-sectional area. This area may be triangular or even square, but is usually circular. If the cross-sectional area of a conductor is doubled, the resistance to current flow will be reduced in half -The fourth major factor in influencing the resistance of a conductor is temperature. Although some substances, such as carbon, show a decrease in resistance as the ambient (surrounding) temperature increases, most materials used as conductors increase in resistance as temperature increases.

What is meant by the capacity rating of a lead-acid battery?

-The number of hours a battery can supply a given current flow -The most common battery rating is the amp-hour rating. This is a unit of measurement for battery capacity. It is determined by multiplying a current ow in amperes by the time in hours that the battery is being discharged. A battery with a capacity of 1 amp-hour should be able to continuously supply a current of 1 amp to a load for exactly 1 hour, or 2 amps for 1/2 hour, or 1/3 amp for 3 hours, etc., before becoming completely discharged. Actually, the ampere-hour output of a particular battery depends on the rate at which it is discharged. Heavy discharge current heats the battery and decreases its ef ciency and total ampere-hour output. For airplane batteries, a period of 5 hours has been established as the discharge time in rating battery capacity. However, this time of 5 hours is only a basis for rating and does not necessarily mean the length of time during which the battery is expected to furnish current. Under actual service conditions, the battery can be completely discharged within a few minutes, or it may never be discharged if the generator provides sufficient charge. (FAA-H-8083-30, Ch. 10)

What is meant by resistance in an electrical circuit?

-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 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 is meant by reactive power in an AC circuit?

-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 returned to the line (FAA-H-8083-30, Ch. 10)

What is meant by apparent power in an AC circuit?

-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)

What is meant by true power in an AC circuit?

-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)

What is meant by power factor in an AC circuit?

-The ratio of true power to apparent power. 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)

What is meant by impedance?

-The total opposition to the flow 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. This total opposition is called impedance and is represented by the letter Z. (FAA-H-8083-30, Ch. 10)

What precautions should be taken in the maintenance shop where both lead-acid and nickel-cadmium batteries are serviced?

-The two types of batteries should be kept separate, and the tools used on one type should be used on the other -Separate facilities for storing and/or servicing flooded electrolyte lead-acid and NiCd batteries must be maintained. Introduction of acid electrolyte into alkaline electrolyte causes permanent damage to vented ( ooded electrolyte) NiCd batteries and vice versa. However, batteries that are sealed can be charged and capacity checked in the same area. Because the electrolyte in a valve-regulated lead-acid battery is absorbed in the separators and porous plates, it cannot contaminate a NiCd battery even when they are serviced in the same area. (FAA-H-8083-31, Ch. 09) -WARNING: It is extremely dangerous to store or service lead-acid and NiCd batteries in the same area. Introduction +23 of acid electrolytes into alkaline electrolyte destroys the NiCd, and vice versa

How are battery compartments protected against corrosion?

-They are coated with special corrosion resistant paint such as bituminous paint. -The nickel-cadmium battery is usually interchangeable with the lead-acid type. When replacing a lead-acid battery with a nickel-cadmium battery, the battery compartment must be clean, dry, and free of all traces of acid from the old battery. The compartment must be washed out and neutralized with ammonia or boric acid solution, allowed to dry thoroughly, and then painted with an alkali resisting varnish. (FAA-H-8083-30, Ch. 10)

Why should Electrolytic capacitors not be used in an AC circuit?

-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 paralleling generators?

-This is when two or more generators are operated at the same time to supply power for a load that must be shared equally -The paralleling feature of the GCU allows for two or more GCU/generator systems to work in a shared effort to provide current to the aircraft electrical system. Comparing voltages between the equalizer bus and the interpole/compensator voltage, and amplifying the dif- ferences accomplishes the control of this system. The difference is then sent to the voltage regulation circuit, where adjustments are then made in the regulation output. These adjustments will continue until all of the busses are equalized in their load sharing. (FAA-H-8083-30, Ch. 10)

What constitutes a capacitor?

-Two conductors separated by an insulator -In its most basic form the capacitor is constructed of two parallel plates separated by a nonconductor, called a dielectric. (FAA-H-8083-30, Ch. 10)

Why is the closed-circuit voltage of a lead-acid battery lower than its open-circuit voltage?

-Voltage is dropped across the internal resistance of the battery -Passing direct current through the battery in a direction opposite to that of the discharge current may charge a storage battery. Because of the internal resistance (IR) in the battery, the voltage of the external charging source must be greater than the open circuit voltage. For example, the open circuit voltage of a fully charged 12 cell, lead-acid battery is approximately 26.4 volts (12 × 2.2 volts), but approximately 28 volts are required to charge it. This larger voltage is needed for charging because of the voltage drop in the battery caused by the internal resistance. Hence, the charging voltage of a lead-acid battery must equal the open circuit volt- age plus the IR drop within the battery (product of the charging current and the internal resistance). (FAA-H-8083-30, Ch. 10)

What is the relationship between mechanical and electrical power?

1 horsepower = 746 watts

What is meant by a kilowatt?

1,000 watts

What is meant by a megawatt?

1,000,000 watts

What is used to neutralize spilled electrolyte form a lead-acid battery?

A solution of bicarbonate of soda and water

What three things must an electric circuit contain?

A source of electrical energy, a load to use, and conductors to join the source and the load

What instrument is used to measure current flow?

An ammeter

What instrument is used to measure continuity in an electrical circuit?

An ohmmeter

What instrument is used to measure electrical resistance?

An ohmmeter

What is the specific gravity of a fully charged lead-acid battery?

Between 1.275 and 1.300

How may thermal runaway be prevented?

By carefully monitoring the temperature of the middle cells and controlling the charging current to prevent an excess of current flowing into the battery

In what units is true power expressed?

In watts

Does an inductor in an AC circuit cause the current to lead or lag the voltage?

It causes the electromagnet to lag behind the voltage.

What should be the correct electrolyte level in a properly serviced battery?

It should be approximately 3/8 inches above the plates. If the battery has baffle plates above the cell plates, the electrolyte level should be up to the hole in the baffle plate in each cell

When measuring resistance of a component with an ohmmeter, should the circuit be energized?

No, there should be no power on the circuit

What is a megohm?

One million (1,000,000) ohms

What part of an amp is a milliamp?

One thousandth (.001) of an amp

What is the basic unit of electrical resistance?

The Ohm

What is the basic unit of current flow?

The ampere (amp)

What is an example of an inductor used in a magneto?

The coil

What is the result of cell imbalance in a nickel-cadmium battery?

The low internal resistance allows current to flow between the unbalanced cells and generate heat

What is the basic unit of voltage?

The volt

What is the basic unit of power in a DC circuit?

The watt