Question 1

Magnetic pole interaction — like vs. unlike poles: If the south poles of two bar magnets are brought close together, will the force between them be attractive?

Accepted Answer

Introduction / Context: Magnetostatics principles mirror electric charges: like poles repel, unlike poles attract. This rule underlies basic applications from magnetic separation to motor stator–rotor interactions and the design of magnetic couplings. Given Data / Assumptions: Two permanent bar magnets are considered. We are bringing south poles near each other in free space. No ferromagnetic shielding or complex materials are involved. Concept / Approach: Magnetic field lines emerge externally from the north pole and enter the south pole. Like poles produce opposing field patterns when brought together, raising magnetic reluctance and resulting in a repulsive force. Attraction occurs when a north is near a south, because field lines can connect directly, lowering energy in the field configuration. Step-by-Step Solution: Identify pole pairing: south–south is a like-pole scenario.Recall rule: like poles repel; unlike poles attract.Infer force direction: repulsive, not attractive.Conclude the statement claiming attraction is incorrect. Verification / Alternative check: Hands-on test with two bar magnets shows clear repulsion when trying to bring identical poles together. Field visualization using iron filings also demonstrates field-line crowding and opposition between like poles. Why Other Options Are Wrong: “Correct” contradicts fundamental behavior. Suggestions that weak fields, distance alone, or soft iron composition flip the rule are not applicable for two permanent magnets with poles aligned like-to-like. Common Pitfalls: Mistaking magnet–ferromagnet interactions (magnet attracts soft iron regardless of pole) for magnet–magnet interactions (which obey like-repel, unlike-attract). Final Answer: Incorrect

Question 2

Power generation principle — induced EMF: Does the operation of an electrical generator fundamentally rely on electromagnetic induction (changing magnetic flux linking a conductor)?

Accepted Answer

Introduction / Context: All practical electrical generators—whether in power plants, portable alternators, or bicycle dynamos—convert mechanical energy into electrical energy via electromagnetic induction. Faraday’s law states that a voltage is induced when the magnetic flux linking a circuit changes with time, which happens through motion, changing field strength, or both. Given Data / Assumptions: We consider conventional rotating machines (AC alternators and DC generators). Conductors (windings) move relative to a magnetic field or the field varies with time. Slip rings/commutators and excitation methods may differ, but induction principle is common. Concept / Approach: Faraday’s law: e = –N * dΦ/dt. A rotating coil in a stationary magnetic field produces a sinusoidal change in flux linkage and therefore an alternating induced voltage. DC generators use commutation to rectify the induced AC internally. In large alternators, the field is typically on the rotor and the stator contains the armature windings to deliver power efficiently, but induction is still the core mechanism. Step-by-Step Solution: Create changing flux linkage by rotation or field variation.Use windings with N turns to scale induced EMF (e ∝ N).Collect generated voltage via slip rings (AC) or commutator (DC).Deliver electrical power to a load; mechanical input balances electrical output plus losses. Verification / Alternative check: Observe that stationary conductors in a static field generate no EMF (dΦ/dt = 0). Starting rotation immediately yields measurable voltage proportional to speed and field strength, confirming induction. Why Other Options Are Wrong: DC vs. AC does not change the underlying induction; speed may vary and still generate voltage; permanent magnets are not required—electromagnets are common in large machines. Common Pitfalls: Confusing “induction motor” with “induction” as a general principle; all generators rely on flux change regardless of excitation source. Final Answer: Correct

Question 3

Magnetic flux direction inside a magnet: Within the body of a permanent magnet, does magnetic flux go from the north pole to the south pole, or in the opposite internal direction?

Accepted Answer

Introduction / Context: Field-line conventions help visualize magnetic circuits in transformers, motors, and sensors. While external flux lines are often drawn from north to south, the internal path within the magnet itself is opposite, completing a closed loop through the magnetic material and the surrounding space (the magnetic circuit). Given Data / Assumptions: The statement claims that inside the magnet, flux flows from the north pole to the south pole. We consider a standard bar magnet in free space. We use the conventional direction of field lines. Concept / Approach: Conventional magnetic field-line representation: outside the magnet, lines emerge from the north pole and enter the south pole. Inside the magnet, lines continue from the south pole back to the north pole, forming closed loops. Thus, internal direction is south to north, the opposite of the external direction. This is consistent with the idea that magnetic flux has no beginning or end; it forms continuous closed paths. Step-by-Step Solution: Recall external direction: north → south outside the magnet.Determine internal direction: south → north inside the magnet.Recognize closed-loop nature: flux lines form continuous circuits.Conclude: the statement is incorrect about the internal direction. Verification / Alternative check: Visualize with iron filings or use a simulator: the pattern consistently shows external north-to-south and internal south-to-north, closing the loop through the magnet’s body. Why Other Options Are Wrong: Claiming correctness reverses the internal path. Magnet length, electromagnet vs. permanent magnet, or air gaps do not change the fundamental loop direction. Common Pitfalls: Assuming the external direction applies everywhere, ignoring the requirement that magnetic field lines are continuous within a magnetic circuit. Final Answer: Incorrect

Question 4

Electromagnetic induction condition: Will a stationary conductor in a static (unchanging) magnetic field have a voltage induced across it?

Accepted Answer

Introduction / Context: Faraday’s law governs when and how an electromotive force (EMF) is induced. Understanding the required conditions—motion of the conductor, time-varying magnetic fields, or changing circuit area—is crucial in generator design, sensor placement, and EMI troubleshooting. Given Data / Assumptions: A straight conductor is stationary in space. The magnetic field present is static (no change in magnitude or direction over time). No change in loop area or orientation occurs. Concept / Approach: Faraday’s law in words: the induced EMF around a closed path equals the negative time rate of change of magnetic flux linking that path. If neither the field nor the geometry/orientation changes, the magnetic flux is constant and its time derivative is zero, so no EMF is induced. Motional EMF requires relative velocity between conductor and field (v × B). Transformer EMF requires dB/dt (time-varying field). Neither condition is met here. Step-by-Step Solution: Assess flux linkage: Φ is constant because B and geometry are constant.Compute rate of change: dΦ/dt = 0 → induced EMF e = 0.Check motional EMF: v = 0 for a stationary conductor → e_motional = B * L * v = 0.Conclude: no induced voltage under the stated conditions. Verification / Alternative check: Start moving the conductor, rotate it, or vary the magnetic field with time (e.g., AC excitation). A measurable EMF immediately appears, confirming the necessity of changing flux linkage. Why Other Options Are Wrong: Material choice (copper vs. others), extreme field strength, or temperature does not matter if dΦ/dt = 0 and v = 0; without change, no EMF is induced. Common Pitfalls: Assuming a strong static field alone causes EMF; forgetting that only a change in flux linkage—by motion or time-varying fields—creates induced voltage. Final Answer: Incorrect

Question 5

Electromagnetic induction: a straight conductor cuts magnetic flux lines at some rate. Evaluate the claim that the induced electromotive force (emf) is directly proportional to the rate at which flux is cut (i.e., the time rate of change of magnetic…

Accepted Answer

Introduction / Context: Induction is one of the bedrock principles of electrical engineering. When a conductor experiences a changing magnetic environment, an electromotive force (emf) is produced. This question asks whether the induced emf is directly proportional to the rate at which magnetic flux is cut by the conductor, which is a practical phrasing of Faraday’s law. Given Data / Assumptions: The conductor and magnetic field are arranged so that the conductor cuts magnetic flux lines (either by motion or by a time-varying field). Lenz’s law establishes the polarity opposing the causative change. We neglect parasitic capacitances and nonlinear magnetic effects for first-order reasoning. Concept / Approach: Faraday’s law states that induced emf, e_ind, is proportional to the negative time derivative of the magnetic flux linkage. For a single conductor moving with velocity v across a uniform field B and conductor length l, the motional form is e_ind = B * l * v. More generally, e_ind = d(lambda)/dt, where lambda is flux linkage. Both expressions show a direct proportionality to the rate of cutting or the rate of change of flux linkage. Step-by-Step Solution: 1) Identify the situation: a conductor, a magnetic field, and relative change (motion or time variation).2) Apply Faraday’s law: e_ind ∝ d(phi_linkage)/dt.3) For motional induction: e_ind = B * l * v → directly proportional to rate at which flux lines are cut.4) Conclude the statement is consistent with electromagnetic induction theory. Verification / Alternative check: Doubling conductor speed v or doubling field B doubles the induced emf, confirming the direct proportionality and matching experiment. Why Other Options Are Wrong: Incorrect: contradicts Faraday’s law. True only when resistance is zero: resistance affects current, not the proportionality of induced emf to the rate of change of flux. True only for AC fields: a moving conductor in a steady DC field also induces emf; AC is not required. Common Pitfalls: Confusing induced emf with induced current (the latter also depends on circuit resistance/impedance). Assuming flux must be sinusoidal; any time rate of change qualifies. Final Answer: Correct

Question 6

Hall effect geometry: in a standard Hall-effect sensor, the magnetic field (B), the current through the Hall element (I), and the measured Hall voltage (V_H) are mutually orthogonal (each at right angles to the other). Assess this statement.

Accepted Answer

Introduction / Context: The Hall effect underpins magnetic field sensors and current probes. Understanding the orientation of the magnetic field, current, and Hall voltage is crucial for correct device mounting and signal interpretation. Given Data / Assumptions: A rectangular Hall plate carries current along one axis. A magnetic field is applied perpendicular to the current path. Charge carriers experience the Lorentz force and accumulate on the sides, producing a transverse voltage. Concept / Approach: In the Hall effect, carriers with drift velocity v_d moving in magnetic field B experience a force q*(v_d × B). This deflects carriers sideways, creating a transverse electric field and hence a measurable Hall voltage across the plate. The canonical setup has: current along x, magnetic flux density along z, and Hall voltage along y, making the three directions mutually orthogonal. Step-by-Step Solution: 1) Drive current I through the Hall element along one axis (e.g., x-direction).2) Apply magnetic field B perpendicular to current (e.g., z-direction).3) Carriers are deflected sideways, building an electric field along the remaining orthogonal axis (e.g., y-direction).4) The resulting Hall voltage V_H appears across the sides orthogonal to both I and B. Verification / Alternative check: Swapping the sign of B reverses the polarity of V_H. Rotating the sensor so that B is parallel to current collapses V_H to near zero, confirming the cross-product geometry. Why Other Options Are Wrong: Incorrect: contradicts the fundamental Hall geometry. True only for semiconductors or only for alternating fields: the orthogonality requirement is geometric and applies broadly; semiconductors are preferred for sensitivity, not for geometry. Common Pitfalls: Mounting a sensor with the wrong orientation; mixing up lead designations; expecting Hall voltage when B is parallel to current (it is maximized when perpendicular). Final Answer: Correct

Question 7

Magnetic circuits: a ferromagnetic material such as iron provides a high-permeability path that can redirect or concentrate magnetic lines of force. Judge this statement.

Accepted Answer

Introduction / Context: Transformers, inductors, and magnetic shields exploit high-permeability materials to guide magnetic flux. The statement claims that iron can change the course (path) of magnetic lines of force, which reflects practical magnetic circuit design. Given Data / Assumptions: Iron or similar ferromagnets have relative permeability much greater than air. We consider fields below saturation for linear reasoning. No permanent magnetization is required to demonstrate flux guidance. Concept / Approach: Magnetic flux prefers paths of least reluctance, analogous to current choosing paths of least resistance. Since ferromagnetic materials have high permeability, their reluctance is low, attracting and guiding flux. C-cores, E-cores, and closed magnetic paths in transformers exemplify this principle by confining and redirecting field lines to improve coupling and reduce leakage. Step-by-Step Solution: 1) Compare permeabilities: mu_iron » mu_air implies lower reluctance in iron.2) Flux lines crowd into the iron, following the high-permeability path.3) The presence and shape of the iron core redirect the flux lines.4) Therefore, iron changes the course of magnetic lines of force. Verification / Alternative check: Field plots (finite-element simulations) and simple iron filings experiments show flux concentration near iron pieces and redirection along cores and shields. Why Other Options Are Wrong: Incorrect: ignores basic magnetic circuit behavior. Requires permanent magnetization or cryogenic temperatures: flux guidance is due to permeability, not necessarily due to remanent magnetization or extreme temperatures. Common Pitfalls: Equating “lines of force” with physical strings; they are visualizations of field direction and density. Forgetting saturation, which limits flux guidance when B approaches material limits. Final Answer: Correct

Question 8

Units and dimensions: identify whether the unit of magnetic flux density (B) is the weber, or whether the weber actually measures magnetic flux (Φ) while the tesla is the unit for magnetic flux density.

Accepted Answer

Introduction / Context: Accurate units prevent design and documentation errors. Magnetic flux density B and magnetic flux Φ are closely related but measured in different SI units. The statement claims the unit of B is the weber, which needs verification. Given Data / Assumptions: SI unit system is used. Magnetic flux Φ has units of weber (Wb). Magnetic flux density B is flux per unit area. Concept / Approach: By definition, B = Φ / A. In SI, Φ is measured in weber and area in square meters, so B is measured in weber per square meter. One weber per square meter is defined as one tesla (T). Therefore, the unit of magnetic flux density is the tesla, not the weber. The weber measures total flux, not flux density. Step-by-Step Solution: 1) Start with B = Φ / A.2) Substitute units: B units = Wb / m^2.3) Recognize that 1 T = 1 Wb/m^2 by SI definition.4) Conclude the statement “B unit is weber” is incorrect; it should be tesla. Verification / Alternative check: Standard SI tables list tesla (T) as the unit for B and weber (Wb) for Φ. Conversions and dimensional analysis confirm this relationship. Why Other Options Are Wrong: Correct: would assert the incorrect unit. cgs vs SI: in cgs (gauss for B), but in SI, the correct unit is tesla; the claim is still wrong in SI context. AC vs DC does not change the unit system. Common Pitfalls: Confusing flux with flux density; mixing SI and cgs units (tesla vs gauss). Final Answer: Incorrect

Question 9

Distinguishing Hall voltage from motional emf: is the Hall voltage simply the voltage created by a conductor moving through a magnetic field, or is it specifically a transverse voltage produced in a current-carrying conductor in a magnetic field due…

Accepted Answer

Introduction / Context: Both the Hall effect and motional emf involve magnetic fields and voltages, but they arise from different physical setups. This item tests whether the Hall voltage is correctly described. Given Data / Assumptions: A Hall element with current driven through it. Magnetic field applied perpendicular to the current. No requirement for the bulk conductor to move relative to the field. Concept / Approach: The Hall voltage occurs because moving charge carriers (due to current) experience a Lorentz force in a magnetic field, which pushes them to one side of the conductor and creates a transverse voltage. In contrast, motional emf is generated when a conductor physically moves through a magnetic field lines (e = B * l * v) even without a prior current. Therefore, describing the Hall voltage as the voltage created by a conductor moving through a magnetic field conflates it with motional emf and is incorrect. Step-by-Step Solution: 1) Define Hall effect: V_H arises in a current-carrying slab when B is applied orthogonally.2) Define motional emf: a voltage generated by a moving conductor cutting flux lines.3) Recognize that Hall voltage does not require the slab to translate; current flow is sufficient.4) Conclude the statement misidentifies Hall voltage and is incorrect. Verification / Alternative check: Stationary Hall sensors on PCBs measure magnetic fields with no mechanical motion; this directly proves that motion of the bulk conductor is not required for V_H. Why Other Options Are Wrong: Correct: would wrongly equate Hall voltage with motional emf. Restrictions to high fields or metals are irrelevant; Hall sensors often use semiconductors at modest fields. Common Pitfalls: Using motional emf formulas to predict Hall sensor outputs; ignoring the need for an applied current in Hall measurements. Final Answer: Incorrect

Question 10

Magnetic materials: the ability of a material to retain magnetization after the magnetizing force is removed is called “retentivity.” Confirm or refute this definition.

Accepted Answer

Introduction / Context: Magnetic materials are characterized by properties such as permeability, saturation, remanence, retentivity, and coercivity. Understanding these terms is essential for selecting core materials and designing magnetic devices. Given Data / Assumptions: Materials are ferromagnetic or ferrimagnetic and exhibit hysteresis. We consider quasi-static magnetization cycles to define properties. Terminology follows standard SI/engineering usage. Concept / Approach: Retentivity refers to how well a material retains magnetic flux density after the external magnetizing field is removed. The measured residual magnetization is called remanence (or remanent flux density), and a material with high retentivity strongly holds that remanence. Coercivity is the reverse field needed to bring flux back to zero. The statement maps correctly to these definitions: the “ability to maintain magnetization” aligns with retentivity. Step-by-Step Solution: 1) Magnetize the specimen to saturation with an external field.2) Remove the field and observe the remaining flux density (remanence).3) The material’s tendency to keep that remanence is its retentivity.4) Materials with high retentivity are suitable for permanent magnets; low retentivity suits transformer cores. Verification / Alternative check: Hysteresis loops show vertical intercept (remanence) and loop shape; materials with wide loops exhibit high retentivity and coercivity, matching this definition. Why Other Options Are Wrong: Incorrect: conflicts with standard magnetic terminology. Limiting to permanent magnets or room temperature is unnecessary; retentivity is a property measurable across conditions. Common Pitfalls: Confusing retentivity with coercivity; equating “remanence” (a value) with “retentivity” (a material tendency/capability). Final Answer: Correct

Question 11

Electromagnetism basics: does an electric current flowing through a conductor produce a magnetic field around that conductor? Evaluate the statement with reference to right-hand-rule intuition.

Accepted Answer

Introduction / Context: Everyday components like inductors, transformers, and even simple wires rely on the coupling between electric current and magnetic fields. The statement asks whether a current-carrying conductor produces a magnetic field around it, a fundamental concept from Ampere’s law. Given Data / Assumptions: Steady current flows in a long, straight conductor. Space around the conductor is free space (or uniform medium) for simplicity. Quasi-static conditions (no radiation effects). Concept / Approach: Ampere’s circuital law states that the line integral of magnetic field H around a closed path equals current enclosed. For a straight wire, the magnetic field circles the conductor with magnitude proportional to current and inversely proportional to radial distance (B ∝ I / r). The right-hand rule gives the direction of B around the wire when the thumb points with current. Step-by-Step Solution: 1) Consider a straight conductor with current I.2) Apply Ampere’s law: ∮H·dl = I_enclosed → circular field lines form around the wire.3) Determine direction via right-hand rule: thumb along I; curled fingers show B direction.4) Conclude a magnetic field is indeed produced around the conductor. Verification / Alternative check: Compass deflection near a current-carrying wire demonstrates the presence and direction of the magnetic field; turning current off removes the deflection. Why Other Options Are Wrong: Incorrect: contradicts Ampere’s law. Restrictions to AC or superconductors are unfounded; DC currents also produce magnetic fields, and superconductivity is not required. Common Pitfalls: Assuming only coils create fields; even a single straight wire produces a field. Forgetting the 1/r spatial dependence that weakens the field with distance. Final Answer: Correct

Question 12

Ferromagnetism clarified: do ferromagnetic materials contain a permanent, pre-existing macroscopic magnetic field even without magnetization, or do they contain magnetic domains that may be randomly oriented so net field is usually zero until aligne…

Accepted Answer

Introduction / Context: Ferromagnetism is often simplified to “strongly magnetic materials,” but the internal physics involves domains whose orientations determine the net magnetization. This question tests whether ferromagnets have an inherent macroscopic magnetic field without magnetization. Given Data / Assumptions: Unmagnetized ferromagnets have many domains. Domain orientations are largely random in the unmagnetized state. External fields, mechanical stress, or prior magnetization can align domains. Concept / Approach: In the unmagnetized state, ferromagnetic domains point in different directions, so their magnetic moments cancel macroscopically and produce nearly zero external field. Only after magnetization (by an external field or other means) do domains align sufficiently to produce a net macroscopic magnetic field. Therefore, saying ferromagnetic materials have pre-existing magnetic fields is misleading; they have the capacity for strong magnetization due to exchange interactions and domain structure. Step-by-Step Solution: 1) Consider domains as microscopic regions with aligned spins.2) In unmagnetized materials, domains are randomly oriented → net magnetization ~ 0.3) Apply magnetizing force to align domains → net field appears.4) Conclude the blanket statement about pre-existing fields is incorrect. Verification / Alternative check: Hysteresis measurements show remanence after magnetization; prior to magnetization, net moment is minimal, confirming no significant macroscopic field exists initially. Why Other Options Are Wrong: Correct: would imply a permanent macroscopic field without magnetization, not generally true. Material- or temperature-limited caveats do not change the basic domain argument; above Curie temperature, ferromagnets lose ferromagnetic ordering entirely. Common Pitfalls: Equating “ferromagnetic” with “always magnetic externally.” Confusing domain presence with net magnetization. Final Answer: Incorrect

Question 13

Induction conditions: for a stationary conductor placed in a stationary (unchanging) magnetic field, what is the induced voltage across the conductor?

Accepted Answer

Introduction / Context: Induced voltages require change—either motion relative to a magnetic field or a time-varying magnetic field. This question probes the special case where both the conductor and the magnetic field are stationary. Given Data / Assumptions: The conductor does not move (velocity v = 0). The magnetic field is constant in time and space (dB/dt = 0). No change in flux linkage with the conductor occurs. Concept / Approach: Faraday’s law indicates induced emf is related to the time rate of change of magnetic flux linkage. Motional emf requires nonzero velocity v across B (e = B * l * v). Transformer emf requires time-varying flux (e ∝ dΦ/dt). With both v = 0 and dΦ/dt = 0, there is no mechanism to produce an induced voltage, so the induced voltage is zero. Step-by-Step Solution: 1) Check for motional induction: v = 0 → e_motional = B * l * v = 0.2) Check for transformer induction: dΦ/dt = 0 → e_transformer = dΦ/dt = 0.3) Sum of induced effects = 0.4) Therefore, the induced voltage is 0 V. Verification / Alternative check: An experiment with a stationary wire and a permanent magnet at rest shows no measurable emf; moving either the wire or the magnet produces a measurable voltage, confirming the need for change. Why Other Options Are Wrong: “Usually very large”: contradicts the requirement for motion or time variation. “Varies with time”: no time variation exists in this setup. “Depends on field strength”: field strength alone without motion or change does not induce a voltage across a stationary conductor. Common Pitfalls: Assuming a strong static magnet automatically induces emf; confusing static magnetic force effects with induction requirements. Final Answer: equals 0 V

Question 14

Magnetic properties metrics: after the magnetizing force is removed, the ratio of the remaining magnetic flux (remanent flux) to the saturated magnetic flux is termed what property of the material?

Accepted Answer

Introduction / Context: Hysteresis loops capture several key magnetic properties: saturation, remanence, retentivity, and coercivity. The question asks for the term describing how much magnetization remains as a fraction of saturation once the external magnetizing force is removed. Given Data / Assumptions: The material has been driven to magnetic saturation. The external field is then reduced to zero. The remaining magnetization (remanence) is observed and compared to saturation. Concept / Approach: When the magnetizing field H is removed after saturation, the flux density B does not drop to zero for ferromagnets; it falls to a residual value called remanent flux density (or remanence). A material’s tendency to retain this magnetization is called retentivity. Coercivity, by contrast, is the reverse field required to reduce B to zero. Saturation is the maximum B level; hysteresis refers to the looped relationship between B and H over a magnetization cycle, not a specific ratio. Step-by-Step Solution: 1) Drive the material to saturation → B = B_sat.2) Remove the magnetizing field → B = B_rem (remanence).3) Compare B_rem to B_sat; the material’s ability to maintain B_rem relates to retentivity.4) Identify the term: retentivity describes the retention capability (often discussed via remanence magnitude or normalized ratio). Verification / Alternative check: Typical permanent-magnet materials exhibit high remanence and high coercivity, indicating high retentivity; soft magnetic materials (e.g., transformer steels) show low coercivity/retentivity to minimize hysteresis losses. Why Other Options Are Wrong: Saturation: indicates the maximum B, not the retained portion. Hysteresis: the loop behavior overall, not the specific retention measure. Coercivity: the required reverse field to demagnetize, not the retained fraction. Common Pitfalls: Mixing up remanence (a value) with retentivity (a property describing how well the material holds that value); confusing coercivity and retentivity. Final Answer: retentivity

Question 15

Electromagnetic relays — identifying the moving part: In an electromechanical relay used in electronics and control circuits, which component is the primary movable element that shifts position when the coil is energized to open or close contacts?

Accepted Answer

Introduction / Context: Relays are widely used to isolate control signals from power circuits, to switch loads, and to implement interlocks. Understanding the internal construction helps technicians diagnose failures and select appropriate parts. A key element is the component that physically moves when the coil is energized. Given Data / Assumptions: We are discussing a conventional electromagnetic relay. A coil creates a magnetic field when energized by current. Mechanical contacts change state due to a movable piece attracted by the magnetic field. Concept / Approach: When the relay coil is energized, magnetic flux is produced in the iron core. This flux attracts a pivoted or spring-loaded movable piece called the armature. The armature motion opens or closes the relay contacts (such as SPDT or DPDT). Some relay designs include a plunger-type mechanism, but in standard terminology the main movable member in many relays is still referred to as the armature that actuates contacts. Step-by-Step Solution: Identify what energizing the coil produces: magnetic flux in the core.Recognize the part that the flux pulls toward the core: the armature.Link the armature’s motion to contact operation (NO to NC, etc.).Therefore, the primary movable element is the armature. Verification / Alternative check: Exploded diagrams and datasheets for electromagnetic relays label the moving member as the armature, while the stationary piece is the yoke/core. Plunger styles are a subtype, but the moving magnetic element is still categorized as an armature assembly driving contacts. Why Other Options Are Wrong: Plunger: A plunger is a specific mechanical style often used in solenoids; in relay terminology, the general movable member that operates contacts is the armature. Cone: Not a standard relay part. Brushes: Found in motors and generators, not in typical sealed relays. Common Pitfalls: Confusing solenoid plungers with relay armatures; assuming “armature” only applies to motors. In relays, armature denotes the moving magnetic element that actuates contacts. Final Answer: armature

Question 16

Magnetic field terminology in electromagnetism: A collection of magnetic lines of force that extend externally from the north pole to the south pole of a magnet is called the magnetic __________.

Accepted Answer

Introduction / Context: In magnetic circuits and electromagnetic devices, we visualize fields using lines of force. Knowing the correct nomenclature is essential for analyzing transformers, motors, sensors, and magnetic materials in electronics and power engineering. Given Data / Assumptions: A permanent magnet or an energized electromagnet is present. We refer to the external field that emerges from the north pole and re-enters at the south pole. Standard magnetic circuit terminology is used. Concept / Approach: The total quantity of magnetic field lines (conceptually) in a region is called magnetic flux, typically symbolized by phi and measured in webers. Flux density B quantifies flux per unit area and is measured in tesla. Other terms like reluctance, hysteresis, and retentivity describe different magnetic properties, not the lines themselves. Step-by-Step Solution: Identify what is being described: lines of force from north to south outside the magnet.Recall the standard term for the total field: magnetic flux.Match the term to the provided options.Select “flux.” Verification / Alternative check: Textbooks and standards define flux (weber) as the measure of the magnetic field’s presence, while flux density B (tesla) is flux per area. This aligns with the description of lines of force. Why Other Options Are Wrong: Reluctance: Magnetic “resistance” of a path, analogous to electrical resistance. Retentivity: Ability to retain magnetization (residual magnetism). Hysteresis: The lagging effect of magnetization relative to the applied magnetizing force, producing the hysteresis loop. Common Pitfalls: Mixing flux (quantity of field) with flux density (field strength per area) or confusing material properties with field descriptions. Final Answer: flux

Question 17

Electromechanical meter movement identification: Which electromagnetic device consists of a light coil that rotates in a permanent magnetic field on jeweled bearings to indicate current or voltage?

Accepted Answer

Introduction / Context: Analog panel meters and galvanometers use precise electromagnetic mechanisms to display current and voltage. Recognizing the specific movement type helps with calibration knowledge, scaling, and interpreting meter response characteristics such as sensitivity and damping. Given Data / Assumptions: Device has a coil that can rotate in a permanent magnetic field. It uses jeweled bearings for low friction and high sensitivity. Used for indicating electrical quantities on a scale. Concept / Approach: The classic moving-coil meter is the d’Arsonval (or Weston) movement. Current in the coil generates torque in the magnetic field, causing rotation proportional to the measured current (or proportional to voltage via series resistance). The core features—moving coil, permanent magnet, jeweled bearings—clearly identify this movement. Step-by-Step Solution: Match the description (moving coil, permanent magnet, jeweled bearings) to known devices.Recognize that speakers have voice coils but are not jeweled or scaled indicators.Relays and automotive sensors do not use jeweled moving-coil indicators.Select d’Arsonval movement. Verification / Alternative check: Any analog multimeter teardown or galvanometer description confirms the moving-coil d’Arsonval structure with jeweled pivots and restoring springs. Why Other Options Are Wrong: Permanent-magnet speaker: Uses a voice coil and cone to produce sound, not a metering mechanism. SPDT relay: An electromechanical switch, not a precision indicating movement. Crankshaft position sensor: Typically magnetic/inductive or Hall-effect, not a jeweled moving coil. Common Pitfalls: Confusing any “coil + magnet” device with a meter movement; the jeweled pivot and calibrated pointer are key identifiers. Final Answer: d’Arsonval movement

Question 18

Faraday’s law in practice: If the speed of a straight conductor moving through a uniform magnetic field is increased while field strength and geometry remain the same, how does the induced voltage (emf) change?

Accepted Answer

Introduction / Context: Generators, sensors, and motion-based energy harvesting rely on electromagnetic induction. Faraday’s law tells us how induced voltage depends on motion. This question checks conceptual understanding of how speed affects induced emf when a conductor cuts magnetic flux lines. Given Data / Assumptions: Uniform magnetic field, constant flux density. Conductor length and orientation are constant. Only the speed of motion is changed. Concept / Approach: For a straight conductor of length L moving at velocity v perpendicular to magnetic flux density B, the induced emf is E = B * L * v. More generally, Faraday’s law states emf is proportional to the rate of change of flux linkage. If everything else is fixed, increasing v increases the rate of flux cutting and therefore increases the induced voltage proportionally. Step-by-Step Solution: Recall induced emf relation: E ∝ rate of flux cutting.With B and L fixed, E = B * L * v.Increase v ⇒ E increases linearly.Hence, the induced voltage increases. Verification / Alternative check: Generator equations and lab demonstrations (sliding conductor on rails in a magnetic field) show emf rises with speed under constant field conditions and geometry. Why Other Options Are Wrong: Not be affected / decrease / equal 0 V: All contradict E ∝ v for fixed B and L. Common Pitfalls: Forgetting that only the velocity has changed; mixing up induced voltage with induced current (which also depends on circuit resistance and back-effects). Final Answer: increase

Question 19

Magnetic material property definition: Which term describes a material’s ability to remain magnetized (retain residual magnetism) after the external magnetizing force has been removed?

Accepted Answer

Introduction / Context: Magnetic materials are characterized by several properties that influence transformer cores, memory devices, sensors, and permanent magnets. Knowing exactly what each term means helps in selecting materials and interpreting B-H curves. Given Data / Assumptions: No specific numeric data; this is a definition question. The material has been magnetized and the external field is subsequently removed. Concept / Approach: Retentivity (also called remanence when referring to flux density) measures how well a material retains magnetization after the magnetizing force (H) is reduced to zero. Hysteresis describes the lag between B and H and the loop area related to energy loss; reluctance is the opposition to flux; induction is the process of generating magnetic effects from applied fields or currents. Only retentivity directly defines the ability to remain magnetized. Step-by-Step Solution: Recognize the question asks about “remaining magnetized” after removal of H.Recall that the property is called retentivity or remanence.Match to the closest option wording: Retentivity.Select Retentivity. Verification / Alternative check: Examine a B-H hysteresis loop: when H returns to zero, B remains at Br (remanent flux density), illustrating retentivity. Why Other Options Are Wrong: Hysteresis: Describes the loop behavior and energy loss, not retention magnitude alone. Reluctance: Analogous to magnetic resistance, not retention. Induction: General phenomenon of field or emf generation, not magnetization retention. Common Pitfalls: Using “hysteresis” as a catch-all for magnetic behavior; the precise property of staying magnetized is retentivity/remanence. Final Answer: Retentivity

Question 20

AC coil behavior — field direction change: When the current through a coil reverses direction in an AC excitation scenario, what happens to the direction of the magnetic field around the coil?

Accepted Answer

Introduction / Context: Inductors and transformers rely on time-varying magnetic fields. Understanding how magnetic field direction relates to current direction is foundational for applying right-hand rules, analyzing AC behavior, and predicting induced voltages in nearby coils. Given Data / Assumptions: A single coil carrying current that can reverse (as in AC). Geometry and coil orientation are fixed. We consider the near field around the conductor/coil. Concept / Approach: The magnetic field direction is directly tied to current direction by the right-hand rule. If current reverses, the field direction also reverses. The field does not vanish permanently; it simply changes sign as current changes sign. Magnitude depends on current magnitude; direction depends on current polarity. Step-by-Step Solution: Use the right-hand rule: thumb for current, curled fingers for field direction.Reverse current ⇒ thumb points opposite ⇒ field curls opposite.Therefore, the magnetic field direction reverses with current reversal.Select the option stating reversal of direction. Verification / Alternative check: Transformer operation depends on this; the alternating current causes alternating flux that induces voltage in secondary windings with corresponding polarity reversals each half cycle. Why Other Options Are Wrong: Does not change / only increases: Contradict the right-hand rule. Disappears: Only true at the instant current passes through zero; it returns with opposite direction immediately after. Common Pitfalls: Confusing instantaneous zero crossings (momentary field collapse) with permanent disappearance; forgetting the field orientation flips with current polarity. Final Answer: reverses direction

Magnetism and Electromagnetism Questions