Electrical Machines and Electronic Devices Practice Quiz (68 questions)

1. A resistor carries a current of $2\,\mathrm{A}$ at constant temperature and has a resistance of $6\,\Omega$. What voltage appears across it?

A. $18\,\mathrm{V}$

B. $8\,\mathrm{V}$

C. $12\,\mathrm{V}$

D. $3\,\mathrm{V}$

Answer: (C)

Explanation

Ohm’s law gives $V=IR$, so the voltage is $2\times6=12\,\mathrm{V}$. A value of $3\,\mathrm{V}$ would result from dividing voltage by resistance rather than applying the stated relationship.

2. At a circuit node, currents of $5\,\mathrm{A}$ and $2\,\mathrm{A}$ enter while one current leaves. What is the leaving current according to Kirchhoff’s current law?

A. $2.5\,\mathrm{A}$

B. $10\,\mathrm{A}$

C. $7\,\mathrm{A}$

D. $3\,\mathrm{A}$

Answer: (C)

Explanation

KCL requires the algebraic sum of currents at a node to be zero, so the leaving current must equal $5+2=7\,\mathrm{A}$. The $3\,\mathrm{A}$ value would represent the difference between the entering currents rather than their total.

3. A closed loop contains a $24\,\mathrm{V}$ source and two voltage drops of $9\,\mathrm{V}$ and $15\,\mathrm{V}$. Which law is satisfied by this voltage balance?

A. Kirchhoff’s voltage law

B. The maximum-power theorem

C. Ohm’s law

D. Kirchhoff’s current law

Answer: (A)

Explanation

Kirchhoff’s voltage law states that the algebraic sum of voltages around a closed path is zero; here the $24\,\mathrm{V}$ rise equals the two drops. Kirchhoff’s current law instead applies to current balance at a node.

4. A charge of $12\,\mathrm{C}$ passes a point in $3\,\mathrm{s}$. What average current flows?

A. $4\,\mathrm{A}$

B. $0.25\,\mathrm{A}$

C. $36\,\mathrm{A}$

D. $9\,\mathrm{A}$

Answer: (A)

Explanation

Electric current is the rate of charge flow, $i=\frac{dq}{dt}$, so $i=12/3=4\,\mathrm{A}$. Multiplying charge by time would not represent a rate.

5. Which procedure is characteristic of nodal analysis?

A. Replace the network with a source and series impedance

B. Choose a reference node, apply KCL, and solve for node voltages

C. Set the load impedance equal to the source impedance

D. Assign mesh currents, apply KVL, and solve for loop currents

Answer: (B)

Explanation

Nodal analysis chooses a reference node, applies KCL at unknown-voltage nodes, and solves for their voltages. Assigning mesh currents and applying KVL describes mesh analysis instead.

6. An analyst assigns circulating currents to independent meshes and writes voltage equations around each mesh. Which method is being used?

A. Mesh analysis

B. Nodal analysis

C. Maximum-power analysis

D. Norton reduction

Answer: (A)

Explanation

Mesh analysis assigns mesh currents and applies KVL to each mesh before solving the resulting equations. Nodal analysis would assign node voltages and apply KCL.

7. How does Thevenin’s theorem represent a linear bilateral network at its terminals?

A. A voltage source in series with an impedance

B. A voltage source in parallel with an impedance

C. A current source in series with an impedance

D. A current source in parallel with an open circuit

Answer: (A)

Explanation

Thevenin’s theorem replaces the network with a single voltage source $V_{TH}$ in series with a single impedance. A current source in parallel with an impedance is the Norton form.

8. A source has impedance $Z_S=4+j3\,\Omega$. Which load impedance receives maximum power?

A. $-4+j3\,\Omega$

B. $4+j3\,\Omega$

C. $4-j3\,\Omega$

D. $3-j4\,\Omega$

Answer: (C)

Explanation

Maximum power transfer occurs when the load impedance is the complex conjugate of the source impedance, so the load is $4-j3\,\Omega$. Using the same sign for the reactive term would not provide the required conjugate.

9. What quantity represents a circuit’s opposition to alternating-current flow and is measured in ohms?

A. Electric charge

B. Impedance

C. Power factor

D. Conductance

Answer: (B)

Explanation

Impedance, denoted by $Z$, is the opposition offered to alternating current and is measured in ohms. Resistance describes a resistor’s opposition but does not include the full AC opposition represented by impedance.

10. At resonance in an AC circuit, what relationship exists between the applied voltage and resulting current?

A. They differ in phase by $90^\circ$ and the power factor is zero

B. They are in phase and the power factor is unity

C. The voltage leads the current and the power factor is inductive

D. The current leads the voltage and the power factor is capacitive

Answer: (B)

Explanation

Resonance occurs when voltage and current are in phase, giving a unity power factor. A $90^\circ$ phase difference describes purely reactive behavior rather than resonance.

11. Which condition identifies series resonance?

A. The circuit has a power factor below unity at resonance

B. The total admittance has a nonzero imaginary part

C. The inductive reactance is twice the capacitive reactance

D. The inductive and capacitive reactances are equal and opposite

Answer: (D)

Explanation

Series resonance occurs when $X_L=X_C$, so the inductive and capacitive reactances cancel. A nonzero imaginary admittance or a non-unity power factor indicates that the circuit is not at the stated resonant condition.

12. Before wiring an electrical installation, which action best follows safe installation practice?

A. Keep the supply energized and use conductive tools carefully

B. Remove protective equipment after identifying the live parts

C. Place conductive tools near exposed parts for convenient access

D. Disconnect the supply and use correctly rated non-conductive tools

Answer: (D)

Explanation

Safe installation requires disconnecting the supply, identifying exposed live parts, and using protective equipment and correctly rated non-conductive tools. Working on an energized supply with conductive tools increases the risk of contact with live parts.

13. What operating principle allows a transformer to induce an EMF in its secondary coil?

A. Mutual induction between two inductively coupled coils

B. Mechanical rotation of a conductor in a magnetic field

C. Electrostatic charging between two insulated plates

D. Resistance heating within the secondary winding

Answer: (A)

Explanation

An alternating source creates changing core flux that links both coupled coils and induces secondary EMF through mutual induction. Mechanical rotation is the operating principle of rotating machines such as generators, not transformers.

14. Which expression gives the primary EMF of a transformer?

A. $E_1=4.44N_1I_1R_1$

B. $E_1=4.44N_1fB_mA$

C. $E_1=4.44B_mA/N_1f$

D. $E_1=4.44N_2fB_mA$

Answer: (B)

Explanation

The primary EMF is determined by the primary turns, frequency, maximum flux density, and core area according to $E_1=4.44N_1fB_mA$. The expression using $N_2$ applies to the secondary winding, while the other forms do not represent the transformer EMF equation.

15. How does the speed of a synchronous motor differ from that of an induction motor when the load increases?

A. The synchronous motor maintains synchronous speed, while the induction motor slows below synchronous speed

B. The synchronous motor slows below synchronous speed, while the induction motor maintains its speed

C. Both motors maintain synchronous speed because load does not affect their operation

D. Both motors slow by the same amount because their rotors depend on induced current

Answer: (A)

Explanation

A synchronous motor runs at synchronous speed independently of load, whereas an induction motor operates below synchronous speed and its speed decreases as load rises. The induction motor requires slip, so it does not maintain synchronous speed.

16. A conductor of length $l$ carries current $I$ in a magnetic flux density $B$. What electromagnetic force acts on it under the stated relationship?

A. $F=BIl$

B. $F=Bl/I$

C. $F=BI/l$

D. $F=B/I l$

Answer: (A)

Explanation

The electromagnetic force is calculated by multiplying magnetic flux density, current, and conductor length: $F=BIl$. Dividing by current or length gives relationships that do not match the stated force law.

17. What energy conversion takes place in a DC motor?

A. Mechanical energy is converted into direct-current electrical energy

B. Alternating-current electrical energy is converted into chemical energy

C. Direct-current electrical energy is converted into mechanical energy

D. Thermal energy is converted into alternating-current electrical energy

Answer: (C)

Explanation

A DC motor uses the force on current-carrying conductors in a magnetic field to convert electrical input into mechanical output. Conversion from mechanical energy to electrical energy describes a DC generator instead.

18. Which rule determines the direction of force on a current-carrying conductor in a DC motor?

A. Fleming’s left-hand rule

B. Fleming’s right-hand rule

C. Lenz’s law

D. The right-hand grip rule

Answer: (A)

Explanation

Fleming’s left-hand rule gives the direction of motor force on a current-carrying conductor in a magnetic field. Fleming’s right-hand rule is associated with the direction of induced current in generator action.

19. A conductor has $B=2\,\text{Wb/m}^2$, $I=3\,\text{A}$, and $l=0.5\,\text{m}$. What force acts on it?

A. $6\,\text{N}$

B. $12\,\text{N}$

C. $1\,\text{N}$

D. $3\,\text{N}$

Answer: (D)

Explanation

Using $F=BIl$ gives $F=2\times3\times0.5=3\,\text{N}$. The other values result from omitting a factor or using an incorrect multiplication.

20. What is the correct current path and action in a basic DC motor?

A. Current enters the loop without brushes or a commutator, and the loop produces rotation by electrostatic attraction

B. Current passes directly from the permanent magnets into the loop, whose field cancels the source field and stops rotation

C. Current flows from the loop into the source, while the permanent magnets induce current through mechanical resistance

D. Current passes through the brushes and commutator into the loop, whose field interacts with the permanent-magnet field to produce rotation

Answer: (D)

Explanation

The source supplies current through the brushes and commutator to the loop, and interaction between the loop field and permanent-magnet field produces torque. Permanent magnets provide the magnetic field but do not supply the loop current.

21. Which description correctly identifies a single-phase induction motor?

A. An AC motor with a single-phase stator supply and a squirrel-cage rotor receiving induced current

B. An AC motor with a three-phase stator supply and a rotor connected directly to the source

C. A DC motor with a two-phase stator supply and a wound rotor receiving current from brushes

D. A synchronous motor with a single-phase rotor supply and a commutator-fed stator

Answer: (A)

Explanation

A single-phase induction motor receives single-phase AC at its stator, while alternating stator flux induces current in its squirrel-cage rotor. A three-phase stator supply describes a three-phase induction motor rather than this machine.

22. What sequence occurs after a single-phase induction motor is energized?

A. The stator produces alternating flux, rotor bars receive induced current, and the rotor produces its own alternating flux

B. The rotor produces flux first, the stator receives induced current, and the supply flux then disappears

C. The supply produces mechanical rotation first, after which the rotor bars create the stator flux

D. The stator produces steady flux, the rotor receives direct current, and the rotor becomes a permanent magnet

Answer: (A)

Explanation

Stator excitation creates alternating flux, which induces current in the squirrel-cage rotor bars; those currents produce the rotor’s alternating flux. Rotor flux therefore follows stator excitation rather than initiating it.

23. Why does a single-phase induction motor require an initial rotation or starting aid?

A. Its rotor must be connected to a separate DC source before it can turn

B. Its rotor has no conductive bars to carry induced current

C. Its stator cannot produce any magnetic flux from a single-phase supply

D. Its alternating fluxes do not produce starting torque from rest

Answer: (D)

Explanation

At rest, the two alternating fluxes do not create a net starting torque, so the motor cannot start by itself. An external initial rotation can break this condition and allow the motor to accelerate.

24. How does a shaded-pole motor develop a rotating resultant flux?

A. A capacitor makes the unshaded-pole flux lag the shaded-pole flux

B. A separate three-phase supply makes the shaded and unshaded poles rotate together

C. A commutator reverses rotor current so both pole fluxes remain exactly in phase

D. Current in the short-circuited shaded coil makes its flux lag the unshaded-pole flux

Answer: (D)

Explanation

Alternating flux induces current in the short-circuited shaded coil, causing shaded-pole flux to lag the unshaded-pole flux. The phase difference between these fluxes produces a rotating resultant flux and starting torque.

25. What distinguishes a separately excited DC generator from a self-excited generator?

A. Its field winding receives energy from an independent external voltage source

B. Its shunt field winding is connected across both generator terminals

C. Its field winding receives energy from the generator’s own output terminals

D. Its series field winding is connected between the armature and the load

Answer: (A)

Explanation

A separately excited generator uses an external source to energize its field, while its output depends on speed and field current. A self-excited generator instead derives field excitation from its own electrical output.

26. How is the field winding connected in a series generator compared with a shunt generator?

A. Both field windings have high resistance and are connected in parallel with the armature and external load

B. Both field windings have low resistance and are connected in series with the armature and external load

C. The series field has low resistance and carries armature and load current, while the shunt field has high resistance and is connected across the armature

D. The series field has high resistance and is connected across the armature, while the shunt field has low resistance and carries load current

Answer: (C)

Explanation

A series generator places its low-resistance field in series with the armature and load, whereas a shunt generator places its high-resistance field across the armature. The alternative arrangements reverse these defining connections.

27. Which connection correctly describes a short-shunt compound generator?

A. The shunt field is connected across the series field, and the series field is connected across the armature

B. The shunt field is connected across the armature, and the series field is connected in series with the load

C. The shunt field is connected across both armature and series field, and the series field bypasses the load

D. The shunt field and series field are both connected across the armature without a series path through the load

Answer: (B)

Explanation

In a short-shunt compound generator, the shunt field is placed across the armature while the series field remains in series with the load. A long-shunt connection instead places the shunt field across both the armature and series field.

28. How does a self-excited DC generator obtain field-winding excitation?

A. It uses electrical output produced by the generator itself

B. It uses an independent battery connected only during full-load operation

C. It uses mechanical power supplied directly to the field winding

D. It uses a separate constant-voltage source outside the generator

Answer: (A)

Explanation

A self-excited generator derives field-winding excitation from its own output and may be series, shunt, or compound connected. Using an independent external voltage source describes separate excitation rather than self-excitation.

29. Which set lists the three important characteristics of a DC shunt motor?

A. Torque versus armature current, speed versus armature current, and speed versus torque

B. Power versus armature resistance, voltage versus speed, and flux versus load resistance

C. Speed versus field resistance, torque versus frequency, and current versus terminal voltage

D. Torque versus field voltage, current versus frequency, and voltage versus power factor

Answer: (A)

Explanation

The principal characteristics relate torque to armature current, speed to armature current, and speed to torque. The other sets use variables that do not define the standard characteristic curves for a DC shunt motor.

30. If the flux of a DC motor remains constant while armature current doubles, what happens to armature torque?

A. It becomes four times larger because torque varies with the square of armature current

B. It decreases by half because constant flux limits the available electromagnetic torque

C. It doubles because armature torque is proportional to flux multiplied by armature current

D. It remains unchanged because torque depends on speed rather than armature current

Answer: (C)

Explanation

The motor relation is $T_a \propto \Phi I_a$, so constant flux makes torque directly proportional to armature current. A squared-current relationship does not describe the armature torque law for this motor.

31. Why is the torque-versus-armature-current characteristic of a DC shunt motor approximately a straight line through the origin?

A. The armature current is approximately constant, making torque directly proportional to field flux

B. The back emf is approximately constant, making torque inversely proportional to armature current

C. The armature resistance is approximately constant, making torque proportional to the square of current

D. The shunt-field flux is approximately constant, making torque directly proportional to armature current

Answer: (D)

Explanation

With nearly constant shunt-field flux, the relation $T_a \propto \Phi I_a$ reduces to torque being proportional to armature current. This produces a straight-line characteristic passing through the origin.

32. Why does the speed of a DC shunt motor decrease only slightly when its load increases?

A. The field flux rises substantially and compensates for a large decrease in back emf

B. The back emf and flux decrease marginally because of armature resistance drop and armature reaction

C. The applied voltage increases automatically to offset the added mechanical load

D. The armature current falls sharply, reducing the torque required from the motor

Answer: (B)

Explanation

Increased load causes effects that slightly reduce back emf and flux, through armature resistance drop and armature reaction, so speed falls only modestly. The motor does not normally respond by automatically increasing its applied voltage.

33. How is the transformer open-circuit test conducted?

A. A rated current is forced through the high-voltage side while the low-voltage side is connected to a resistive load

B. Rated-frequency voltage is applied gradually to the high-voltage side while the low-voltage side remains open

C. Rated-frequency voltage is applied gradually to the low-voltage side while the high-voltage side remains open

D. A low voltage is applied to the low-voltage side while the high-voltage side is short-circuited at rated current

Answer: (C)

Explanation

The open-circuit test applies rated-frequency voltage to the low-voltage winding with the high-voltage winding open, while instrument readings are recorded at rated low-voltage voltage. Applying voltage to the high-voltage side with the other winding shorted describes the short-circuit test.

34. What procedure is used for the transformer short-circuit test?

A. A rated-frequency source is connected to the low-voltage side while the high-voltage side carries no current

B. Rated voltage is applied to both windings while the transformer supplies its normal external load

C. Voltage is applied gradually to the high-voltage side with the low-voltage side shorted until rated high-voltage current flows

D. Voltage is applied gradually to the low-voltage side with the high-voltage side open until rated low-voltage voltage is reached

Answer: (C)

Explanation

During the short-circuit test, the low-voltage winding is shorted and voltage is gradually applied to the high-voltage winding until rated high-voltage current flows. The low-voltage-side open-circuit procedure belongs to the open-circuit test.

35. What does the wattmeter reading approximately represent during a transformer short-circuit test?

A. The full-load copper loss because the applied voltage and core loss are negligible

B. The no-load loss because the high-voltage winding carries negligible current

C. The full-load core loss because the applied voltage produces rated magnetic flux

D. The transformer output power because the shorted winding supplies rated load voltage

Answer: (A)

Explanation

The short-circuit test uses a low applied voltage, so core loss is negligible, while rated current produces the relevant winding losses. Therefore, the wattmeter reading is approximately the full-load copper loss.

36. What is the defining function of a transformer?

A. It generates electrical energy from a magnetic field while fixing both output voltage and output current

B. It transfers electrical energy between circuits by electromagnetic induction while changing voltage and current magnitudes without changing frequency

C. It converts alternating current into direct current while maintaining the same voltage and current magnitudes

D. It transfers mechanical energy between shafts by magnetic coupling while changing the supply frequency

Answer: (B)

Explanation

A transformer is a static device that transfers energy between circuits through electromagnetic induction, changing voltage and current magnitudes while preserving frequency. It does not operate as a mechanical converter, frequency changer, or AC-to-DC rectifier.

37. Which energy conversion correctly distinguishes a DC generator from a DC motor?

A. A generator converts mechanical energy to electrical energy, while a motor converts DC electrical energy to mechanical energy

B. A generator converts DC electrical energy to mechanical energy, while a motor converts mechanical energy to electrical energy

C. A generator converts thermal energy to electrical energy, while a motor converts electrical energy to magnetic energy

D. A generator converts magnetic energy to mechanical energy, while a motor converts mechanical energy to thermal energy

Answer: (A)

Explanation

A DC generator produces electrical energy from mechanical input, whereas a DC motor produces mechanical output from DC electrical input. The second choice reverses these two machine functions.

38. How are the field windings connected in shunt, series, and compound DC motors?

A. Shunt field parallel to the armature, series field in series with it, and compound motors using both connections

B. Shunt field in series with the armature, series field parallel to it, and compound motors using parallel fields

C. Shunt and series fields both parallel to the armature, with compound motors using two series windings

D. Shunt and series fields both in series with the armature, with compound motors using two parallel windings

Answer: (A)

Explanation

A shunt motor has a parallel field winding, a series motor has a series field winding, and a compound motor combines both arrangements. The second choice interchanges the defining connections of shunt and series motors.

39. What does the commutator accomplish in a DC generator?

A. It increases the magnetic flux produced by the field winding and raises the generated voltage

B. It connects the field winding directly to the supply so that the machine operates as a motor

C. It converts the alternating current induced in the armature into unidirectional current at the terminals

D. It mechanically supports the armature core while allowing the rotor to turn inside the stator

Answer: (C)

Explanation

The commutator reverses armature connections at suitable instants, making the terminal current unidirectional even though the armature conductors carry alternating current. Increasing field flux is a function of the field system, not the commutator.

40. Which combination correctly describes the functions of the yoke and pole shoes in a DC machine?

A. The yoke supplies external resistance, while pole shoes connect the armature winding to the commutator segments

B. The yoke provides mechanical strength and carries flux, while pole shoes support coils and spread flux across the air gap

C. The yoke carries the armature current, while pole shoes convert alternating armature current into direct terminal current

D. The yoke controls rotor speed, while pole shoes provide electrical insulation between the armature and field winding

Answer: (B)

Explanation

The yoke is both a structural member and a magnetic-flux path, while pole shoes hold the field coils and distribute flux over the air gap. Current conversion and external resistance are associated with other parts or circuits.

41. Which expression gives the induced RMS emf per phase of a synchronous generator when coil-span and distribution effects are included?

A. $E_{ph}=2.22K_cK_df\Phi T_{ph}$

B. $E_{ph}=4.44K_cK_d\frac{f\Phi}{T_{ph}}$

C. $E_{ph}=4.44f\Phi T_{ph}$

D. $E_{ph}=4.44K_cK_df\Phi T_{ph}$

Answer: (D)

Explanation

The synchronous-generator emf equation includes the factor 4.44 and both correction factors, $K_c$ and $K_d$, multiplying frequency, flux per pole, and series turns per phase. Omitting the correction factors gives an incomplete form, while changing the numerical factor or placing turns in the denominator is incorrect.

42. What operating characteristic distinguishes an induction motor from a synchronous motor?

A. An induction motor operates without a stationary stator and uses two rotating magnetic fields

B. An induction motor runs asynchronously and does not operate at synchronous speed

C. An induction motor runs synchronously and maintains exact synchronous speed under load

D. An induction motor converts mechanical input to electrical output through a commutated armature

Answer: (B)

Explanation

An induction motor is asynchronous, so its rotor does not run at synchronous speed, and its main parts are a stationary stator and rotating rotor. Running at synchronous speed describes a synchronous motor rather than an induction motor.

43. Which construction correctly distinguishes squirrel-cage and phase-wound induction-motor rotors?

A. A squirrel-cage rotor uses commutator segments, whereas a phase-wound rotor uses permanent magnets and brushes

B. A squirrel-cage rotor has insulated three-phase windings with slip rings, whereas a phase-wound rotor has bars shorted by end rings

C. A squirrel-cage rotor has external resistance built into its bars, whereas a phase-wound rotor has an unconnected open winding

D. A squirrel-cage rotor has uninsulated bars shorted by end rings, whereas a phase-wound rotor has insulated three-phase windings with slip rings

Answer: (D)

Explanation

Squirrel-cage rotors use uninsulated copper or aluminium bars joined by end rings, while phase-wound rotors use insulated three-phase windings connected to slip rings and possible external resistance. Slip rings and external resistance therefore characterize the phase-wound design, not the cage rotor.

44. What is meant by the peak inverse voltage of a diode?

A. The greatest forward voltage a diode can apply before its current becomes zero

B. The average voltage delivered by a diode during one complete rectifier cycle

C. The greatest reverse voltage a diode can withstand before Zener breakdown or reverse-voltage damage

D. The reverse voltage required to make a diode conduct with a low forward resistance

Answer: (C)

Explanation

Peak inverse voltage is the maximum reverse voltage the diode can tolerate before entering the Zener region or suffering damage. It is not a forward-voltage rating or an average rectifier output value.

45. What happens to a PN junction when it is forward biased?

A. The depletion region widens and majority carriers are blocked from crossing the junction

B. The depletion region narrows and majority carriers cross the junction in a heavy flow

C. The depletion region disappears because minority carriers are removed from both sides

D. The junction becomes reverse biased as electrons and holes move away from the interface

Answer: (B)

Explanation

Forward bias promotes recombination near the junction, reduces depletion width, and permits substantial majority-carrier conduction. Widening the depletion region is associated with reverse bias, making that description the closest misconception.

46. How does increasing temperature generally affect the forward characteristics of a diode?

A. The forward characteristic remains unchanged because thermal voltage has no effect on diode behavior

B. The forward characteristic becomes less ideal because the reverse saturation current decreases substantially

C. The forward characteristic becomes more resistive because majority carriers lose their ability to cross the junction

D. The forward characteristic becomes more nearly ideal because the rise in reverse saturation current dominates

Answer: (D)

Explanation

As temperature rises, reverse saturation current increases and its effect dominates the relatively smaller percentage change in thermal voltage, making the forward characteristic more nearly ideal. The reverse saturation current increases rather than decreases with temperature.

47. Which application is directly associated with operational amplifiers?

A. Generating only fixed-frequency digital clock signals

B. Converting mechanical motion into hydraulic pressure

C. Storing data permanently in semiconductor memory

D. Combining integrators, differentiators, and summing amplifiers

Answer: (D)

Explanation

Operational amplifiers support applications such as integrators, differentiators, summing amplifiers, multipliers, and various types of amplification. Digital clock generation, permanent memory storage, and hydraulic conversion are not listed operational-amplifier applications.

48. Which combination best describes typical operational-amplifier resistance parameters?

A. Low input resistance and very high output resistance

B. Very high input resistance and relatively low output resistance

C. Moderate input resistance and moderate output resistance

D. Equal input and output resistance across normal operation

Answer: (B)

Explanation

Typical operational amplifiers have input resistance ranging from $10^5$ to $10^{13}\ \Omega$, while output resistance is approximately 10 to 100 $\Omega$. The reverse relationship is the documented confusion because the input resistance is high and the output resistance is comparatively low.

49. What does a digital-to-analog converter produce from a binary input?

A. A continuously amplified digital waveform

B. A reversed sequence of logic-state transitions

C. A binary number representing a measured signal

D. An analog voltage or current

Answer: (D)

Explanation

A digital-to-analog converter transforms a binary number into an analog voltage or current. Producing a binary output from an analog signal describes an analog-to-digital converter instead.

50. An ideal inverting amplifier uses $R_i=2\ \text{k}\Omega$ and $R_f=10\ \text{k}\Omega$. What is its voltage gain?

A. $-0.2$

B. $+0.2$

C. $-5$

D. $+5$

Answer: (C)

Explanation

The ideal inverting-amplifier relation is $A_v=-\frac{R_f}{R_i}$, so $A_v=-\frac{10}{2}=-5$. A positive result would ignore the phase inversion, while $0.2$ reverses the resistor ratio.

51. What distinguishes a bipolar junction transistor from a unipolar device?

A. It amplifies signals without using doped semiconductor regions

B. It operates through both electrons and holes

C. It uses a single carrier type in its semiconductor regions

D. It contains two terminals rather than three

Answer: (B)

Explanation

A bipolar junction transistor is a three-terminal doped-semiconductor device that uses both electrons and holes for amplification or switching. A unipolar device relies primarily on one carrier type, so it does not have the defining bipolar behavior.

52. In an NPN transistor operating in the forward-active region, what happens to injected electrons?

A. They remain confined to the emitter because both junctions oppose their motion

B. They move from the emitter through the thin base and are swept into the collector

C. They travel from the base into the emitter after the collector junction is forward biased

D. They move from the collector through the base and are absorbed by the emitter

Answer: (B)

Explanation

Forward-active operation injects electrons from the forward-biased emitter into the thin base, where they diffuse and are swept into the collector by the reverse-biased collector-base junction. The reverse direction and the stated junction conditions do not describe NPN forward-active operation.

53. Which statement correctly describes the regions of a BJT?

A. The collector lies between the emitter and a base located externally

B. The emitter and collector are adjacent with no intervening base region

C. The heavily doped base lies outside the emitter and collector

D. The lightly doped base lies physically between the emitter and collector

Answer: (D)

Explanation

A BJT contains emitter, base, and collector regions, and the base is lightly doped and located between the other two regions. The alternatives misplace the base or omit the required three-region structure.

54. Which bias relationship turns on an NPN transistor?

A. The base is at a lower potential than the emitter

B. The base is at a higher potential than the emitter

C. The collector is at a lower potential than the base

D. The base and emitter are held at identical potentials

Answer: (B)

Explanation

An NPN transistor turns on when its base potential exceeds its emitter potential. A lower base potential relative to the emitter describes the turn-on relationship for a PNP transistor, not an NPN transistor.

55. Which gain combination characterizes the common-emitter BJT configuration?

A. Current gain without voltage gain

B. Both voltage gain and current gain

C. Neither voltage gain nor current gain

D. Voltage gain without current gain

Answer: (B)

Explanation

The common-emitter configuration provides both voltage gain and current gain. Common-base has voltage gain without current gain, whereas common-collector has current gain without voltage gain.

56. In a common-base BJT configuration, where are the input and output applied?

A. Input between emitter and collector; output between base and ground

B. Input between base and ground; output between emitter and collector

C. Input between collector and base; output between emitter and base

D. Input between emitter and base; output between collector and base

Answer: (D)

Explanation

The base is the common terminal, with the input applied between emitter and base and the output taken between collector and base. Reversing the input and output locations does not match the common-base arrangement.

57. How does a BJT behave in cutoff compared with saturation?

A. Cutoff has both junctions reverse biased and acts open; saturation has both forward biased and acts closed

B. Cutoff has both junctions forward biased and acts closed; saturation has both reverse biased and acts open

C. Cutoff has one junction forward biased and amplifies; saturation has one reverse biased and blocks

D. Cutoff has both junctions unbiased; saturation has one junction disconnected from the circuit

Answer: (A)

Explanation

In cutoff, both junctions are reverse biased, producing approximately zero collector current and open-switch behavior; in saturation, both junctions are forward biased and the transistor acts as a closed switch. The second option reverses the defining bias conditions and switch states.

58. If a BJT has $\alpha=0.98$, what is its approximate current gain $\beta$?

A. $0.0196$

B. $49$

C. $98$

D. $1.02$

Answer: (B)

Explanation

Using $\beta=\frac{\alpha}{1-\alpha}$ gives $\beta=\frac{0.98}{0.02}=49$. The value $0.0196$ comes from multiplying rather than applying the stated relationship, while the other values do not satisfy the conversion.

59. What is the primary function of a transducer in a measurement or control system?

A. Store a measured signal for later digital processing

B. Convert an electrical signal into a proportional physical quantity

C. Convert a physical quantity into a proportional electrical signal

D. Amplify an electrical signal without changing its physical form

Answer: (C)

Explanation

A transducer converts a physical quantity or another form of energy into a proportional electrical signal for measurement or control. An inverse transducer performs the reverse conversion, so it does not describe the primary function here.

60. Which statement correctly distinguishes active and passive transducers?

A. Active transducers require external power, while passive ones generate output independently

B. Active transducers measure motion, while passive ones measure electrical quantities

C. Active transducers generate output without external power, while passive ones require it

D. Active transducers produce digital signals, while passive ones produce analog signals

Answer: (C)

Explanation

Active transducers generate a proportional voltage or current without an external power source, whereas passive transducers require external power and respond through parameter changes. The distinction is based on power requirements, not on whether the output is analog or digital.

61. Which classification set includes the principal ways transducers can be categorized?

A. Active or passive, analog or digital, primary or secondary, and direct or inverse

B. Primary or secondary, wired or wireless, resistive or capacitive, and static or dynamic

C. Continuous or sampled, voltage or current, high or low range, and manual or automatic

D. Powered or unpowered, thermal or optical, mechanical or magnetic, and local or remote

Answer: (A)

Explanation

Transducers may be classified by transduction principle, signal form, measurement stage, and conversion direction using the listed pairs. The other sets combine some relevant terms with categories that are not identified as the principal classification scheme.

62. For a parallel-plate capacitive transducer, what happens to capacitance when the plate separation increases while the other variables remain constant?

A. It decreases because capacitance is inversely proportional to separation

B. It increases because capacitance is directly proportional to separation

C. It remains constant because separation does not enter the capacitance relation

D. It changes unpredictably because capacitance depends only on plate material

Answer: (A)

Explanation

The relation $C=\frac{\varepsilon_r\varepsilon_0A}{d}$ shows that capacitance decreases as plate separation $d$ increases. Capacitance is directly proportional to overlap area, which is a different dependence.

63. How can a capacitive transducer measure linear displacement?

A. By converting displacement directly into a piezoelectric charge

B. By changing plate separation or the area of overlap

C. By changing temperature to alter the dielectric coefficient

D. By changing the plate material while fixing its geometry

Answer: (B)

Explanation

A capacitive transducer measures linear displacement by changing either the distance between its plates or their overlapping area. Altering temperature or plate material is not the stated displacement-measurement mechanism.

64. Which application is most appropriate for a piezoelectric transducer?

A. Measuring displacement through a change in plate overlap area

B. Measuring resistance changes caused by steady temperature variation

C. Measuring a long-term static load with no dynamic variation

D. Measuring a rapidly changing force during an impact

Answer: (D)

Explanation

Piezoelectric transducers use materials such as quartz to measure dynamic forces, including impact, oscillation, and high-speed compression or tension. The other choices describe static-force or capacitive and resistive measurement methods rather than the stated piezoelectric applications.

65. What sequence describes the signal path in a digital storage oscilloscope?

A. Voltage generation, frequency multiplication, analog storage, and pulse shaping

B. Memory storage, digital-to-analog conversion, attenuation, and analog display

C. Attenuation and amplification, analog-to-digital conversion, memory storage, and display

D. Mechanical displacement, resistance measurement, filtering, and magnetic storage

Answer: (C)

Explanation

A digital storage oscilloscope first conditions the input through attenuation and amplification, converts it into digital data with an analog-to-digital converter, and stores it for display or processing. The other sequences omit or reverse the essential acquisition stages.

66. What does a current transformer provide for measurement of a high-current circuit?

A. A low secondary voltage proportional to the high primary voltage

B. A high secondary current proportional to the low primary current

C. A low secondary current proportional to the high primary current

D. A constant secondary current independent of the primary current

Answer: (C)

Explanation

A current transformer produces a low secondary current proportional to the high current in its primary winding. A potential transformer instead reduces voltage, so a voltage-based description does not define a current transformer.

67. To what approximate current level is a protective current transformer expected to remain accurate?

A. About 50 times its normal full-load current

B. About 1.25 times its normal full-load current

C. About 5 times its normal full-load current

D. About 20 times its normal full-load current

Answer: (D)

Explanation

A protective current transformer is expected to remain accurate up to about 20 times its normal full-load current. Measuring-instrument current transformers have the lower accuracy range of approximately 125% of normal full-load current.

68. What is the purpose and connection of a potential transformer?

A. It steps up line voltage in series for measurement by a high-range ammeter

B. It steps down line current in series for measurement by a low-range voltmeter

C. It stores line energy in parallel for measurement by a high-range wattmeter

D. It steps down line voltage in parallel for measurement by a low-range voltmeter

Answer: (D)

Explanation

A potential transformer is a step-down transformer connected in parallel across a line, producing a reduced voltage suitable for a low-range voltmeter. A current transformer is the device associated with reducing current and series connection.