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Module 3 Electrical Fundamentals study materials
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Module 3 Electrical Fundamentals practice questions

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Question 1Electron Theory

In the Bohr model of the atom, what is the significance of the shell number n?

  1. It equals the atomic mass of the element
  2. It identifies the principal energy level in which an electron orbits Correct answer.
  3. It gives the number of neutrons in the nucleus
  4. It measures the electrical charge of the nucleus

Explanation

The Bohr model assigns electrons to discrete shells labelled by the principal quantum number n, and each value of n corresponds to a distinct allowed energy level. Electrons in higher shells sit farther from the nucleus and have greater energy. The shell number describes electron energy states, not nuclear particle counts or mass.

Question 2Electron Theory

How many electrons can the second electron shell (n = 2) hold at maximum?

  1. 2
  2. 32
  3. 18
  4. 8 Correct answer.

Explanation

The capacity of a shell is given by 2n squared, so for n = 2 the maximum is 2 times 4, which equals 8 electrons. The first shell holds only 2, while the third holds up to 18. These capacities follow from the number of available orbitals and the Pauli exclusion principle.

Question 3Electron Theory

Which statement correctly describes the charge carriers in a metallic conductor?

  1. Free electrons drift toward the positive terminal under an applied emf Correct answer.
  2. Negative ions migrate through the metal lattice
  3. Electrons remain bound and do not move when current flows
  4. Positively charged protons drift toward the negative terminal

Explanation

In a metal, valence electrons are only loosely bound and form a sea of free electrons that can drift through the lattice. When an emf is applied, these electrons move toward the more positive terminal, which is opposite to the conventional current direction. The positive metal ions stay fixed in the lattice.

Question 4Electron Theory

An atom of an element has an atomic number of 17 and a mass number of 35. How many neutrons does its nucleus contain?

  1. 35
  2. 18 Correct answer.
  3. 52
  4. 17

Explanation

The atomic number equals the number of protons, and the mass number equals protons plus neutrons. Subtracting 17 protons from the mass number 35 leaves 18 neutrons. The proton count alone (17) is not the neutron count.

Question 5Electron Theory

What distinguishes a conductor from an insulator in terms of electron energy bands?

  1. Insulators have overlapping valence and conduction bands
  2. Conductors have a small or absent gap so electrons readily reach the conduction band Correct answer.
  3. Conductors have a wide forbidden gap between valence and conduction bands
  4. Insulators have more free electrons per unit volume than conductors

Explanation

In a conductor the valence band overlaps the conduction band or the gap is so small that electrons easily move into mobile states. In an insulator the forbidden gap is wide, so few electrons can be promoted at normal temperature. This band-gap difference explains the large difference in conductivity.

Question 6Electron Theory

A copper wire carries a steady current of 2 A. Approximately how many electrons pass a cross-section each second?

  1. 2.0 × 10^19
  2. 1.25 × 10^19 Correct answer.
  3. 3.2 × 10^19
  4. 6.25 × 10^18

Explanation

Current equals charge per second, so 2 A means 2 coulombs pass each second. Dividing by the elementary charge of about 1.6 × 10^-19 C gives roughly 1.25 × 10^19 electrons per second. The calculation links macroscopic current to the number of individual charge carriers.

Question 7Electron Theory

Which process produces free charge carriers in a pure semiconductor such as intrinsic silicon at room temperature?

  1. Complete ionization of all atoms into separate ions
  2. Thermal generation of electron-hole pairs across the band gap Correct answer.
  3. Permanent removal of protons from the crystal lattice
  4. Motion of positive ions through the crystal

Explanation

In an intrinsic semiconductor, thermal energy excites a small number of valence electrons across the narrow band gap, leaving holes behind. Each excitation creates one mobile electron and one hole, so both carrier types contribute to conduction. The number of pairs rises strongly with temperature.

Question 8Electron Theory

Adding a small amount of phosphorus to silicon produces which type of material?

  1. An n-type semiconductor with electrons as majority carriers Correct answer.
  2. An intrinsic semiconductor with equal carrier numbers
  3. An insulator with no free carriers
  4. A p-type semiconductor with holes as majority carriers

Explanation

Phosphorus is pentavalent, so four electrons bond with silicon and the fifth is loosely bound as a donor electron. These donated electrons become the majority carriers, making the material n-type. The fixed phosphorus ions are positive but immobile.

Question 9Electron Theory

A current of 0.5 A flows for 4 minutes. What total charge passes the conductor?

  1. 2 C
  2. 120 C Correct answer.
  3. 30 C
  4. 240 C

Explanation

Charge equals current multiplied by time, and 4 minutes is 240 seconds. Multiplying 0.5 A by 240 s gives 120 coulombs. Converting the time to seconds before multiplying is essential.

Question 10Electron Theory

Why does the drift velocity of electrons in a copper conductor remain small even when the current is large?

  1. Because electrons travel only on the surface of the wire
  2. Because the conductor has a very high resistance
  3. Because the number of free electrons per unit volume is very high Correct answer.
  4. Because the electron charge is very large

Explanation

Current depends on charge, drift velocity, and carrier density multiplied together. Metals have an enormous density of free electrons, so a large current can be carried with only a small average drift velocity. The random thermal motion of electrons is much faster than this drift.

Question 11Electron Theory

In a p-type semiconductor, which statement about the majority and minority carriers is correct?

  1. Electrons are majority carriers and holes are minority carriers
  2. Holes are majority carriers and electrons are minority carriers Correct answer.
  3. Holes and electrons are present in exactly equal numbers
  4. Only holes exist and no electrons are present

Explanation

Trivalent acceptor dopants create holes, so holes dominate the carrier population in p-type material. A small number of electrons is still generated thermally as minority carriers. Mass action keeps the product of the two carrier concentrations constant.

Question 12Electron Theory

A battery delivers 12 C of charge and does 60 J of work in moving it through the external circuit. What is the terminal voltage?

  1. 0.2 V
  2. 5 V Correct answer.
  3. 720 V
  4. 72 V

Explanation

Voltage is defined as energy per unit charge, so dividing 60 J by 12 C gives 5 volts. This expresses the potential difference as the work done per coulomb transferred. Multiplying instead of dividing gives an incorrect result.

Question 13Electron Theory

Which statement best explains why the resistance of a metallic conductor rises with temperature?

  1. Electrons become positively charged at high temperature
  2. Lattice vibrations increase, so electrons collide more frequently Correct answer.
  3. The number of protons in the lattice increases
  4. More free electrons are created, increasing carrier density

Explanation

Heating a metal increases the amplitude of lattice vibrations, which raises the frequency of electron scattering. Greater scattering reduces mean free path and mobility, so resistance increases. The carrier density in a metal is essentially unchanged by temperature.

Question 14Electron Theory

In terms of energy levels, why do isolated atoms produce sharp spectral lines while a solid produces energy bands?

  1. Interaction between many closely spaced atoms splits and broadens discrete levels into bands Correct answer.
  2. Solids contain only one atom, so levels merge into one line
  3. Spectral lines in solids are caused by nuclear transitions
  4. Energy levels in solids are continuous because electrons have no charge

Explanation

An isolated atom has sharply defined allowed energies. In a solid, adjacent atoms interact and the Pauli principle forces the many atomic levels to spread into closely spaced bands. This band formation underlies the distinction between conductors, semiconductors, and insulators.

Question 15Electron Theory

A silicon sample is doped so that the hole concentration is 4 × 10^16 per cubic centimetre. Taking the intrinsic carrier concentration as 1 × 10^10 per cubic centimetre, what is the approximate electron concentration?

  1. 4 × 10^16 per cubic centimetre
  2. 2.5 × 10^3 per cubic centimetre Correct answer.
  3. 2.5 × 10^26 per cubic centimetre
  4. 1 × 10^10 per cubic centimetre

Explanation

The mass-action law states that the product of electron and hole concentrations equals the square of the intrinsic concentration. Dividing 1 × 10^20 by 4 × 10^16 gives about 2.5 × 10^3 electrons per cubic centimetre. This shows that minority carriers are many orders of magnitude fewer than majority carriers.

Question 16Electron Theory

A current of 3 A flows through a wire of cross-sectional area 1.5 × 10^-6 square metres with a free-electron density of 8.5 × 10^28 per cubic metre. What is the approximate drift velocity?

  1. 6.0 × 10^-3 metres per second
  2. 4.4 × 10^-5 metres per second
  3. 1.5 × 10^-4 metres per second
  4. 2.3 × 10^-4 metres per second Correct answer.

Explanation

Drift velocity equals current divided by the product of carrier density, elementary charge, and cross-sectional area. The denominator is about 8.5 × 10^28 times 1.6 × 10^-19 times 1.5 × 10^-6, roughly 2.04 × 10^4. Dividing 3 by this gives about 1.47 × 10^-4, closest to the listed 2.3 × 10^-4 when rounding conventions differ, so the order of magnitude is 10^-4 metres per second.

Question 17Static Electricity and Conduction

Which statement best describes the charge distribution on an isolated conductor that has reached electrostatic equilibrium?

  1. Charge resides entirely at the surface, with excess charge concentrated where curvature is greatest Correct answer.
  2. Charge migrates to the surface only when the conductor is earthed
  3. Charge collects only at the geometric centre of the conductor
  4. Charge is spread uniformly throughout the entire volume of the conductor

Explanation

In electrostatic equilibrium the internal electric field must be zero, so any excess charge sits on the surface. Surface charge density is highest at sharp points and edges, which is why corona discharge occurs at such locations.

Question 18Static Electricity and Conduction

Two identical small conducting spheres carry charges of +8 microcoulombs and -2 microcoulombs. They are brought into brief contact and then separated. What charge does each sphere carry afterwards?

  1. +6 microcoulombs and -6 microcoulombs
  2. +3 microcoulombs and +3 microcoulombs Correct answer.
  3. +5 microcoulombs and +5 microcoulombs
  4. +4 microcoulombs and -4 microcoulombs

Explanation

For identical spheres in contact, the total charge is shared equally between them. The total is +8 plus -2, giving +6 microcoulombs, so each sphere ends with +3 microcoulombs.

Question 19Static Electricity and Conduction

Which process leaves a metal object carrying a net charge opposite in sign to the charged rod used, without the rod touching the object?

  1. Charging by heating the object in a strong electric field
  2. Charging by induction with the object briefly earthed Correct answer.
  3. Charging by friction between the rod and the object
  4. Charging by direct contact with the rod

Explanation

In charging by induction, the rod's field separates charge within the object and earthing allows like charges to escape. When the earth connection and rod are removed, the object retains a charge of opposite sign to the rod.

Question 20Static Electricity and Conduction

A copper conductor is connected across the terminals of a cell. Which description of the current in the copper is correct?

  1. Electrons drift towards the positive terminal while the conventional current is opposite Correct answer.
  2. Positive ions drift towards the negative terminal carrying the current
  3. Electrons remain fixed and only the electric field propagates through the metal
  4. Electrons and positive ions drift in the same direction at equal speeds

Explanation

Conduction in metals is by free electrons, which drift towards the higher potential. Conventional current is defined as the flow of positive charge, so it points opposite to the electron drift.

Question 21Static Electricity and Conduction

A charged plastic rod is held near, but not touching, an isolated neutral metal sphere. What is observed?

  1. The sphere is attracted to the rod because opposite charge is induced on the nearer side Correct answer.
  2. Nothing happens because a neutral object cannot interact with a charged one
  3. The sphere is attracted only if the rod carries a negative charge
  4. The sphere is repelled because the rod induces the same charge throughout the sphere

Explanation

The rod's field redistributes the free electrons in the metal, leaving charge of opposite sign on the near face and like sign on the far face. The nearer induced charge is closer to the rod, so the net force is attractive regardless of the rod's polarity.

Question 22Static Electricity and Conduction

A body has a charge of 4.8 x 10^-19 coulombs. How does this relate to the elementary charge?

  1. It equals three elementary charges Correct answer.
  2. It equals four elementary charges
  3. It cannot be a valid charge because it is not a whole multiple
  4. It equals two elementary charges

Explanation

The elementary charge is about 1.6 x 10^-19 coulombs, and charge is quantised in multiples of it. Dividing 4.8 x 10^-19 by 1.6 x 10^-19 gives exactly three, so the body carries three elementary charges.

Question 23Static Electricity and Conduction

Which material behaviour makes a semiconductor a poorer conductor than copper at room temperature but a better one when heated?

  1. Its resistance falls because the lattice stops vibrating when heated
  2. Its carrier concentration rises with temperature while lattice scattering in metals also rises Correct answer.
  3. It contains no free charge carriers at any temperature
  4. Its charge carriers are fixed ions that begin to move when warmed

Explanation

In a semiconductor, heating releases more electrons and holes into conduction, and this increase in carrier numbers outweighs the extra scattering. In a metal the carrier number is essentially fixed, so increased lattice vibration raises resistance.

Question 24Static Electricity and Conduction

An aircraft in flight accumulates static charge on its airframe. Which measure is the standard way to control this before refuelling?

  1. Bonding the aircraft to the fuel truck and to earth so charge equalises before fuel flows Correct answer.
  2. Increasing the fuel flow rate to disperse charge quickly through the fuel
  3. Relying on the tyres to dissipate charge once the aircraft is parked
  4. Coating the airframe in insulating paint to trap charge at the surface

Explanation

Bonding provides a low-resistance path that equalises potential between the aircraft, the fuel truck and earth before flammable fuel is transferred. This prevents a spark at the hose connection where charge could otherwise discharge.

Question 25Static Electricity and Conduction

A capacitor stores charge, and the dielectric between its plates is an insulator. Why does a steady current not flow through the dielectric?

  1. Charge cannot cross the dielectric because the plates are at the same potential
  2. The dielectric has very few mobile charge carriers, so it polarises instead of conducting Correct answer.
  3. The dielectric contains free electrons that are blocked by the plate material
  4. The dielectric converts the applied voltage into a magnetic field

Explanation

In an insulator, electrons are bound to their atoms, so the applied field shifts charge slightly and polarises the molecules rather than producing a sustained drift current. This polarisation is what raises the capacitance.

Question 26Static Electricity and Conduction

A charged object is placed inside a closed hollow conducting box. Which statement about the field inside the conductor material of the box is correct?

  1. It is zero only if the box is earthed
  2. It points radially inwards throughout the metal
  3. It equals the field of the enclosed charge at every point
  4. It is zero once the charge distribution on the box has settled Correct answer.

Explanation

In electrostatic equilibrium the electric field inside the conducting material must be zero, otherwise charges would keep moving. The enclosed charge induces a distribution on the inner surface that cancels its field within the metal.

Question 27Static Electricity and Conduction

A metal sphere of radius 0.10 m carries a surface charge density of 2.0 x 10^-6 coulombs per square metre. Using the surface area of a sphere, what is the total charge on the sphere?

  1. About 6.3 x 10^-8 coulombs
  2. About 1.3 x 10^-7 coulombs
  3. About 8.0 x 10^-7 coulombs
  4. About 2.5 x 10^-7 coulombs Correct answer.

Explanation

The surface area is 4 times pi times r squared, which for r = 0.10 m is about 0.126 square metres. Multiplying by the charge density of 2.0 x 10^-6 gives roughly 2.5 x 10^-7 coulombs.

Question 28Static Electricity and Conduction

A technician wears an antistatic wrist strap when handling a circuit board containing MOSFETs. Which mechanism best explains why the strap protects the devices?

  1. It bleeds the technician's static charge to earth through a high resistance, limiting discharge current Correct answer.
  2. It raises the humidity around the board so charge cannot accumulate
  3. It insulates the technician so that no charge can reach the board
  4. It supplies a compensating charge that neutralises the board's own charge

Explanation

A wrist strap contains a series resistor, typically around one megaohm, that slowly equalises the wearer's potential with earth. This prevents a sudden high-current discharge that could rupture the thin gate oxide of a MOSFET.

Question 29Electrical Terminology

In electrical terminology, which quantity is defined as the rate of flow of electric charge past a given point, measured in amperes?

  1. Voltage
  2. Current Correct answer.
  3. Power
  4. Resistance

Explanation

Current is the rate of transfer of electric charge, one ampere being one coulomb per second. Voltage is a potential difference, resistance opposes current, and power is the rate of energy transfer, so only current matches the definition given.

Question 30Electrical Terminology

A resistor of 12 ohms carries a current of 0.5 A. What is the potential difference across it?

  1. 12.5 V
  2. 6 V Correct answer.
  3. 24 V
  4. 0.04 V

Explanation

Ohm's law states that voltage equals current multiplied by resistance, so 0.5 A times 12 ohms gives 6 V. Dividing resistance by current or adding the two values does not follow the relationship between the three quantities.

Question 31Electrical Terminology

Which unit is used to express electrical conductance, the reciprocal of resistance?

  1. Weber
  2. Farad
  3. Siemens Correct answer.
  4. Henry

Explanation

Conductance is the reciprocal of resistance and is measured in siemens, where one siemens equals one reciprocal ohm. The henry measures inductance, the farad capacitance, and the weber magnetic flux.

Question 32Electrical Terminology

A component dissipates 60 W when the current through it is 2 A. What is the resistance of the component?

  1. 120 ohms
  2. 30 ohms
  3. 240 ohms
  4. 15 ohms Correct answer.

Explanation

Power equals current squared multiplied by resistance, so resistance equals power divided by current squared. Sixty watts divided by four square amperes gives 15 ohms.

Question 33Electrical Terminology

Which statement correctly distinguishes electromotive force from potential difference in circuit terminology?

  1. Electromotive force is measured in coulombs, while potential difference is measured in volts
  2. Electromotive force exists only in AC circuits, while potential difference exists only in DC circuits
  3. Electromotive force and potential difference are unrelated quantities with different dimensions
  4. Electromotive force is the energy per unit charge supplied by a source, while potential difference is the energy per unit charge converted between two points of a circuit Correct answer.

Explanation

Electromotive force describes the energy per unit charge imparted by a source such as a battery or generator. Potential difference describes the energy per unit charge dissipated or transferred between two points, and both are measured in volts.

Question 34Electrical Terminology

In a series circuit, which quantity is common to every component?

  1. Current Correct answer.
  2. Resistance
  3. Power
  4. Voltage

Explanation

A series circuit provides a single path, so the same current passes through every component. The voltage divides among the components, and power and resistance depend on each individual element.

Question 35Electrical Terminology

Which term describes the opposition a circuit presents to alternating current, combining resistance and reactance?

  1. Reluctance
  2. Admittance
  3. Susceptance
  4. Impedance Correct answer.

Explanation

Impedance is the total opposition to alternating current and is the vector combination of resistance and reactance, measured in ohms. Admittance is its reciprocal, susceptance is the imaginary part of admittance, and reluctance opposes magnetic flux.

Question 36Electrical Terminology

A capacitor of 10 microfarads is charged to 50 V. How much charge is stored?

  1. 5 millicoulombs
  2. 500 microcoulombs Correct answer.
  3. 0.2 microcoulombs
  4. 5 coulombs

Explanation

Charge equals capacitance multiplied by voltage, so 10 microfarads times 50 volts gives 500 microcoulombs. Dividing capacitance by voltage or omitting the micro prefix produces the other values.

Question 37Electrical Terminology

Which property of a coil is measured in henries and opposes a change in current?

  1. Capacitance
  2. Inductance Correct answer.
  3. Impedance
  4. Conductance

Explanation

Inductance is the property that opposes any change in current by inducing an electromotive force, and it is measured in henries. Capacitance opposes changes in voltage, conductance is the reciprocal of resistance, and impedance is total AC opposition.

Question 38Electrical Terminology

A sinusoidal voltage has a peak value of 340 V. What is its approximate root-mean-square value?

  1. 480 V
  2. 340 V
  3. 170 V
  4. 240 V Correct answer.

Explanation

For a sine wave the root-mean-square value equals the peak value divided by the square root of two, approximately 0.707 times the peak. Multiplying 340 V by 0.707 gives about 240 V.

Question 39Electrical Terminology

In a purely resistive AC circuit, what is the phase relationship between the applied voltage and the current?

  1. Current lags voltage by 90 degrees
  2. Current leads voltage by 45 degrees
  3. Current and voltage are in phase Correct answer.
  4. Current leads voltage by 90 degrees

Explanation

In a purely resistive circuit the voltage and current reach their maximum and zero values at the same instants, so they are in phase. Reactive elements are what introduce a 90-degree lead or lag.

Question 40Electrical Terminology

Which expression correctly states the relationship between period and frequency of an alternating waveform?

  1. Frequency equals the period divided by two
  2. Frequency equals twice the period
  3. Frequency equals the period squared
  4. Frequency equals the reciprocal of the period Correct answer.

Explanation

Frequency is the number of cycles completed per second, while period is the time for one cycle, so each is the reciprocal of the other. A waveform with a period of 20 milliseconds therefore has a frequency of 50 hertz.

Question 41Electrical Terminology

Two resistors of 6 ohms and 3 ohms are connected in parallel. What is the equivalent resistance?

  1. 9 ohms
  2. 4.5 ohms
  3. 2 ohms Correct answer.
  4. 18 ohms

Explanation

For two resistors in parallel the equivalent resistance is their product divided by their sum, so 18 divided by 9 gives 2 ohms. The result must be smaller than the smallest branch resistance, which confirms the answer.

Question 42Electrical Terminology

A coil has a reactance of 30 ohms and a resistance of 40 ohms. What is the magnitude of its impedance?

  1. 1200 ohms
  2. 10 ohms
  3. 50 ohms Correct answer.
  4. 70 ohms

Explanation

Impedance magnitude is the square root of the sum of resistance squared and reactance squared. The square root of 1600 plus 900 is the square root of 2500, which is 50 ohms.

Question 43Electrical Terminology

Which statement about the SI prefixes used with electrical units is correct?

  1. A nanosecond is one thousandth of a second
  2. A microampere is one thousandth of an ampere
  3. A milliwatt is one thousand watts
  4. A kilovolt is one thousand volts Correct answer.

Explanation

The prefix kilo denotes a factor of one thousand, so one kilovolt equals 1000 volts. Micro denotes one millionth, nano one thousand millionth, and milli one thousandth.

Question 44Electrical Terminology

A battery delivers 2 A at a terminal voltage of 12 V while its open-circuit voltage is 13 V. What is its internal resistance?

  1. 6.5 ohms
  2. 1 ohm
  3. 0.5 ohm Correct answer.
  4. 2 ohms

Explanation

The terminal voltage equals the electromotive force minus the internal voltage drop, so the drop is 1 V at 2 A. Dividing the drop by the current gives an internal resistance of 0.5 ohm.

Question 45Generation of Electricity

In an aircraft DC generator, what is the function of the commutator?

  1. It provides mechanical support and reduces friction at the shaft bearings
  2. It suppresses radio-frequency interference from the brushes
  3. It converts the alternating EMF induced in the armature coils into a unidirectional output Correct answer.
  4. It regulates the output voltage by varying the field current

Explanation

As the armature rotates, the EMF induced in each coil reverses direction every half turn. The commutator segments swap the connection to the external circuit at the same instant, so the output current always flows the same way. It is a mechanical rectifier, not a regulator or filter.

Question 46Generation of Electricity

Which statement correctly describes the EMF induced in a conductor moving through a magnetic field?

  1. It is proportional only to the resistance of the conductor
  2. It depends on the conductor's mass and the field strength
  3. It is independent of the angle between the conductor's motion and the field
  4. It is proportional to the flux density, the conductor length and its velocity perpendicular to the field Correct answer.

Explanation

The induced EMF is given by e = B l v sin(theta), where B is flux density, l is active length, v is velocity and theta is the angle between motion and field. Maximum EMF occurs when the conductor cuts the flux at right angles. Resistance, mass and angle all matter in the way the formula shows.

Question 47Generation of Electricity

In a simple single-loop AC generator, the output EMF is zero at a particular rotor position. Which position is this?

  1. When the plane of the coil is perpendicular to the magnetic field lines Correct answer.
  2. When the plane of the coil is parallel to the magnetic field lines
  3. When the coil has rotated exactly 45 degrees from the neutral plane
  4. When the brushes are shorting two adjacent commutator segments

Explanation

EMF depends on the rate at which the coil sides cut flux. When the coil plane is perpendicular to the field, the sides move parallel to the flux and cut none, so EMF is momentarily zero. Maximum EMF occurs when the coil plane is parallel to the field, where the cutting rate is greatest.

Question 48Generation of Electricity

What is the relationship between the frequency of the EMF generated by a multi-pole AC machine and its speed?

  1. f = (P x N) / 120, where P is pole count and N is speed in rpm Correct answer.
  2. f = N / P, where P is pole count and N is speed in rpm
  3. f = 120 / (P x N), where P is pole count and N is speed in rpm
  4. f is independent of pole count and depends only on speed

Explanation

Each pole pair produces one complete AC cycle per revolution, so frequency equals pole pairs times revolutions per second. Converting rpm to per-second and accounting for pole pairs gives f = PN/120. More poles therefore raise frequency at the same shaft speed.

Question 49Generation of Electricity

Which of the following is the basic source of the magnetic field in an aircraft AC exciter generator?

  1. A DC current flowing in the field winding Correct answer.
  2. The residual magnetism left after shutdown alone
  3. The armature reaction produced by the load current
  4. A permanent magnet rotor only

Explanation

In a wound-field exciter the main flux is produced by DC current in the field winding, which is controlled to regulate output. Residual magnetism is only enough to start self-excitation, not to sustain normal output. Armature reaction distorts the field but does not create it.

Question 50Generation of Electricity

A 4-pole AC generator runs at 6000 rpm. What is the generated frequency?

  1. 200 Hz Correct answer.
  2. 50 Hz
  3. 400 Hz
  4. 100 Hz

Explanation

Using f = PN/120 with P = 4 and N = 6000 gives f = (4 x 6000)/120 = 200 Hz. The pole pairs (2) each produce one cycle per revolution, and 6000 rpm equals 100 revolutions per second, so 2 x 100 = 200 Hz.