Question 1

Electromagnetic generator basics: In a generator, which component delivers electrical power to the external circuit (i.e., where the induced voltage and current are produced)?

Accepted Answer

Introduction / Context: Electric generators convert mechanical energy into electrical energy using electromagnetic induction. Knowing the role of each component clarifies troubleshooting and design: the field creates the magnetic flux, the armature cuts the flux to generate voltage, and the brushes/commutator or slip rings transfer power to the load. Given Data / Assumptions: Conventional DC or AC generator construction is considered. The magnetic field is provided by field windings or permanent magnets. The armature conductors move relative to the magnetic flux. Concept / Approach: By Faraday’s law, an emf is induced when conductors experience changing magnetic flux linkage. In a generator, the armature winding is the location of these conductors, so induced voltage appears across its terminals and it delivers power to the external circuit. The field winding provides the magnetic field but does not deliver the generated output; the commutator/brush assembly serves as a mechanical rectifier or a sliding contact, not as the power-producing element. Step-by-Step Solution: Identify element cutting flux: the armature conductors rotate (or move linearly) within the magnetic field.Apply induction principle: emf is generated in those moving conductors.Therefore, the armature is the component that produces and supplies electrical power to the load. Verification / Alternative check: In practical machines, measuring open-circuit voltage at the armature terminals confirms that the generated emf is on the armature side; changes in field current vary the voltage by changing flux, but the power path remains through the armature. Why Other Options Are Wrong: Field winding: Creates magnetic flux; it controls but does not deliver generated output power. Commutator/brush: Provide mechanical/electrical interface; they transfer the armature's generated power but do not generate it. Common Pitfalls: Confusing control of generated voltage (field excitation) with the location where power is generated. Remember: flux from the field, emf in the armature, delivery via commutator/brushes or slip rings. Final Answer: armature

Question 2

Hall effect sensor fundamentals In practical electronics and mechatronics, a Hall effect sensor is best described as which type of device?

Accepted Answer

Introduction / Context: Hall effect sensors are ubiquitous in modern electronics, from brushless DC motor commutation to proximity detection and current sensing. They exploit the Hall effect, where a transverse voltage appears across a current-carrying conductor or semiconductor placed in a magnetic field that is perpendicular to the current flow. Given Data / Assumptions: The question asks for a functional characterization of a Hall effect sensor. Real-world Hall sensors are solid-state, contactless devices. Typical output levels are small and require amplification or conditioning. Concept / Approach: When charge carriers moving through a conductor experience a magnetic field, the Lorentz force deflects them, producing a measurable Hall voltage across the device. Because the measurement relies on a magnetic field rather than mechanical contact, Hall sensors provide noncontact detection of position, speed, or current. Integrated Hall switches and linear Hall ICs include on-chip amplifiers, regulators, and temperature compensation to deliver clean digital or analog outputs. Step-by-Step Solution: Identify operating principle: transverse Hall voltage proportional to magnetic flux density and current.Relate to applications: speed pickups, gear-tooth sensing, BLDC commutation, and current sensing via magnetic field around a conductor.Note key advantage: noncontact sensing reduces wear and allows sealed designs.Select the option that captures this essence: “is a noncontacting magnetic sensor.” Verification / Alternative check: Compare to alternatives: reed switches require magnetic actuation with contacts; inductive sensors use eddy currents; Hall sensors directly measure magnetic flux via the Hall effect without mechanical contact. Why Other Options Are Wrong: Exists only in theory: contradicted by extensive commercial use.Can operate only a few times: devices are solid-state and highly durable.Produces very large voltages: Hall voltages are typically millivolt-level without amplification. Common Pitfalls: Confusing Hall sensors with magnetoresistive sensors or reed relays; all detect magnetic fields but with different physics and output characteristics. Final Answer: is a noncontacting magnetic sensor

Question 3

Calculate magnetic reluctance from geometry and permeability A core has length l = 0.045 m, cross-sectional area A = 0.015 m², and material permeability μ = 2500 µWb/(At·m). What is the reluctance S = l / (μ * A)?

Accepted Answer

Introduction / Context: Reluctance in magnetic circuits is analogous to resistance in electric circuits. It quantifies the opposition to establishing magnetic flux and depends on geometry and permeability. Accurately converting units and applying S = l / (μ * A) is a common design step for transformers, inductors, and magnetic sensors. Given Data / Assumptions: Length l = 0.045 m. Area A = 0.015 m². Permeability μ = 2500 µWb/(At·m) = 2500 × 10^-6 Wb/(At·m) = 0.0025 Wb/(At·m). Uniform cross-section and material; fringing neglected. Concept / Approach: Magnetic reluctance is S = l / (μ * A). Units check: l in m; μ in Wb/(At·m); A in m². Then μ * A is Wb/At, and l divided by that yields At/Wb, the correct unit for reluctance. Step-by-Step Solution: Convert permeability: μ = 2500 µWb/(At·m) = 0.0025 Wb/(At·m).Compute μ * A: 0.0025 * 0.015 = 0.0000375 Wb/At.Reluctance S = l / (μ * A) = 0.045 / 0.0000375 = 1200 At/Wb.Therefore, S = 1200 At/Wb. Verification / Alternative check: Order-of-magnitude sanity: Shorter length or larger area should reduce S; here l is modest and A is not huge, giving a mid-range S around 10^3 At/Wb, which is reasonable. Why Other Options Are Wrong: 833.33 µAt/Wb: wrong magnitude and unit prefix; result must be in At/Wb, not micro-At/Wb.0.27 At/Wb: inconsistent with computed μ * A.More information needed: not true; formula has all required quantities. Common Pitfalls: Mishandling micro (µ) conversions or mixing area units; always convert µWb to Wb before calculation. Final Answer: 1200 At/Wb

Question 4

Compute flux density from flux and area A magnetic field has total flux Φ = 3000 µWb through a uniform cross-sectional area A = 0.25 m². What is the flux density B = Φ / A (expressed conveniently)?

Accepted Answer

Introduction / Context: Flux density B expresses magnetic flux per unit area and is central to transformer core sizing and inductor design. Converting micro-webers to webers and presenting B in a convenient unit (tesla, millitesla, or microtesla) is a routine calculation in magnetics problems. Given Data / Assumptions: Total flux Φ = 3000 µWb = 3000 × 10^-6 Wb = 0.003 Wb. Area A = 0.25 m². Uniform flux distribution across the stated area. Concept / Approach: Use B = Φ / A. The SI unit of B is tesla (T) = Wb/m². After computing B in tesla, convert to practical subunits if needed to match answer formatting. Step-by-Step Solution: Convert flux: Φ = 0.003 Wb.Compute: B = Φ / A = 0.003 / 0.25 = 0.012 T.Convert: 0.012 T = 12 mT = 12,000 µT.Therefore, B = 12,000 µT. Verification / Alternative check: Dimensional check: Wb/m² yields tesla; numerical scale is reasonable for modest lab magnets or transformer cores below saturation. Why Other Options Are Wrong: 83,330 T: astronomically high; likely a unit misconversion.0 T: incorrect unless Φ = 0.More information needed: not required—Φ and A fully determine B. Common Pitfalls: Forgetting to convert µWb to Wb or misplacing decimal points when expressing results in µT or mT. Final Answer: 12,000 µT

Question 5

Recognize electromechanical devices by construction Which electromagnetic device uses carbon brushes and a mechanical commutator as part of its normal operation?

Accepted Answer

Introduction / Context: Different electromagnetic devices have distinct internal constructions that reveal their function. Identifying which machines employ brushes and a commutator helps differentiate DC machines from AC machines and from passive actuators like relays and solenoids. Given Data / Assumptions: Commutator: a segmented copper cylinder switched by brushes. Brushes: typically carbon blocks that make sliding contact. We compare common devices: speaker, DC generator, relay, solenoid. Concept / Approach: In DC machines (motors and generators), the commutator mechanically rectifies the internal alternating EMF induced in the armature windings, providing unidirectional current at the terminals. Brushes conduct between the rotating commutator and the external circuit. Other devices listed do not require commutation by brushes: relays and solenoids are stationary actuators; loudspeakers use a voice-coil and permanent magnet without sliding electrical contacts. Step-by-Step Solution: Recall DC generator (or motor) anatomy: armature, commutator, brushes, field.Function: convert mechanical to electrical (generator) or electrical to mechanical (motor) with DC at terminals.Match feature: only the DC generator in the list uses brushes and a commutator.Select: “A dc generator.” Verification / Alternative check: AC machines use slip rings (not commutators) for rotor currents in wound-rotor designs; speakers, relays, and solenoids have no sliding DC commutation system. Why Other Options Are Wrong: Speaker: uses a flexible cone and a voice coil driven by AC.Relay: uses stationary contacts actuated by a magnetic field; no commutator.Solenoid: a coil with a moving plunger; no brushes. Common Pitfalls: Confusing slip rings with commutators; slip rings provide continuous AC connection, while commutators segment and rectify. Final Answer: A dc generator

Question 6

Practical significance of the Hall effect Select the accurate statement about the Hall effect and its engineering uses.

Accepted Answer

Introduction / Context: The Hall effect links magnetic fields, current flow, and charge carriers to produce a measurable transverse voltage. Far from being merely academic, it underpins many practical sensors and measurement instruments in automotive, industrial, and consumer electronics. Given Data / Assumptions: Hall voltage is typically small (millivolts) and needs amplification. Hall elements are made from semiconductors with high carrier mobility. Applications include position, speed, proximity, and current sensing. Concept / Approach: A Hall sensor outputs a voltage proportional to magnetic flux density and the current through the sensor element. Integrated devices combine the Hall plate with amplifiers, comparators, and temperature compensation to deliver robust digital (switch) or analog (linear) outputs. This allows contactless measurement, high reliability, and operation in harsh environments where optical or mechanical sensors would fail. Step-by-Step Solution: Identify the key property: Hall voltage reflects magnetic field strength.Map to applications: gear-tooth sensing, brushless motor commutation, current probes, and proximity switches.Evaluate options: only one affirms widespread sensor usage without exaggeration.Choose: “is used in various sensor applications.” Verification / Alternative check: Catalogs from sensor manufacturers list Hall-effect product families for automotive crankshaft sensing, wheel speed (ABS), and linear position encoders. Why Other Options Are Wrong: No practical applications: plainly false.Thousands of volts: Hall voltages are small; system-level outputs are conditioned, not raw high voltage.Basis for solar cells: photovoltaic effect, not Hall effect, governs solar cells. Common Pitfalls: Confusing Hall sensors with magnetoresistive or fluxgate sensors; while all detect magnetic fields, their physics and performance differ. Final Answer: is used in various sensor applications

Question 7

Electromagnetic induction with a moving loop When a closed conducting loop is moved through a magnetic field, what occurs regarding induced voltage, current, and polarity?

Accepted Answer

Introduction / Context: Faraday’s law and Lenz’s law govern electromagnetic induction: motion of a conductor in a magnetic field produces an electromotive force (EMF). If the conductor forms a closed loop, this EMF drives a current whose direction opposes the change that created it. Understanding these principles is fundamental to generators, inductive sensors, and eddy-current braking. Given Data / Assumptions: Closed wire loop (complete circuit). Uniform magnetic field intersecting the loop area. Finite motion causing change in magnetic flux linkage. Concept / Approach: Faraday’s law: induced EMF magnitude is proportional to the rate of change of flux linkage. Lenz’s law: induced current direction opposes the flux change. Polarity therefore depends on whether the loop enters or leaves the field and on the direction of motion relative to the field lines. Step-by-Step Solution: Move loop so that flux through it changes → EMF appears across the loop.Because loop is closed, EMF drives an induced current.Reverse motion or flip field orientation → polarity reverses accordingly.Thus, all three statements (voltage induced, current induced, polarity depends on motion) are correct. Verification / Alternative check: Generator action: rotating coils in a magnetic field produce AC because polarity alternates as the coil turns, perfectly illustrating motion-dependent polarity. Why Other Options Are Wrong: Choosing only one statement ignores the complete behavior of EMF, current, and motion-dependent polarity. Common Pitfalls: Assuming motion is required even when flux changes due to time-varying fields can also induce EMF; both relative motion and time-varying fields work by changing flux linkage. Final Answer: All of the above

Question 8

Identify device with a plunger (movable iron core) Which electromagnetic device uses a cylindrical iron plunger that moves when the coil is energized?

Accepted Answer

Introduction / Context: Electromagnetic actuators convert electrical energy into mechanical motion. The terms “relay” and “solenoid” are often used loosely, but a solenoid specifically refers to a coil that creates a linear motion of a ferromagnetic plunger or armature when energized. Given Data / Assumptions: Movable iron core (plunger) feature highlighted. We contrast with speaker, DC generator, and relay. Assume conventional electromechanical constructions. Concept / Approach: Energizing a solenoid coil establishes a magnetic field that pulls a ferromagnetic plunger into the coil, producing linear motion used for latching, valves, and push-pull mechanisms. While many relays also move an armature, the classic plunger configuration with a tubular coil is emblematic of a solenoid actuator, not a loudspeaker or DC generator. Step-by-Step Solution: Identify the key hallmark: a sliding ferromagnetic plunger drawn into a coil.Map the hallmark to device type: characteristic of solenoids.Differentiate: a relay moves contacts via a levered armature; a speaker moves a cone with a voice coil; a DC generator rotates an armature.Select: “A solenoid.” Verification / Alternative check: Product datasheets for linear solenoids show coil, sleeve, and plunger assemblies designed for strokes in millimeters to centimeters, confirming the definition. Why Other Options Are Wrong: Speaker: has a flexible cone and voice coil, not a plunger.DC generator: rotary device with commutator and brushes.Relay: uses contacts and an armature; not typically a sliding plunger inside a tubular coil. Common Pitfalls: Assuming every coil-actuated device is a solenoid; relays are optimized for switching contacts, whereas solenoids generate linear force and stroke. Final Answer: A solenoid

Question 9

Identify device by mechanical element Which electromagnetic device incorporates a flexible cone as its primary moving element for converting electrical signals to sound?

Accepted Answer

Introduction / Context: Loudspeakers convert electrical audio signals into acoustic pressure waves. The classic electrodynamic speaker uses a flexible cone (diaphragm) that moves air in response to current through a voice coil situated in a static magnetic field. Given Data / Assumptions: Item uses a flexible cone (diaphragm). We compare with DC generator, relay, and solenoid. Conventional speaker construction assumed. Concept / Approach: An audio signal through the voice coil produces a force F = B * I * L that moves the coil. The coil is attached to a cone suspended by a surround and spider, allowing axial motion. As the cone vibrates, it compresses and rarefies air, producing sound corresponding to the input waveform. None of the other listed devices use a flexible cone in their operation. Step-by-Step Solution: Identify unique mechanical part: flexible cone.Associate with device: electrodynamic loudspeaker.Exclude alternatives: DC generator (commutator and brushes), relay (contacts and armature), solenoid (plunger).Select: “A speaker.” Verification / Alternative check: Exploded diagrams of speakers show cone, dust cap, voice coil former, spider, surround, and permanent magnet assembly. Why Other Options Are Wrong: DC generator: rotating armature and commutator, no cone.Relay: switches contacts; no diaphragm for acoustic output.Solenoid: linear actuator; no flexible acoustic radiator. Common Pitfalls: Confusing “speaker” with “microphone”; although related by transduction, microphones usually do not use large flexible cones except in some dynamic designs, and their purpose is opposite (sound to electricity). Final Answer: A speaker

Question 10

Compute magnetomotive force (MMF) from turns and current For a coil with N = 150 turns carrying current I = 2 A, what is the magnetomotive force F = N * I (in ampere-turns)?

Accepted Answer

Introduction / Context: Magnetomotive force (MMF) is the magnetic analog of electromotive force in circuits. It drives magnetic flux through a magnetic path and is directly proportional to the number of turns and the current through those turns. Calculating MMF is a foundational step in designing inductors, solenoids, and transformers. Given Data / Assumptions: Number of turns N = 150 turns. Current I = 2 A. Use F = N * I with F in ampere-turns (At). Concept / Approach: The greater the product of turns and current, the stronger the magnetizing force. While material properties and geometry affect resulting flux, MMF itself is purely a function of windings and current, independent of core type in the basic calculation. Step-by-Step Solution: Write the formula: F = N * I.Substitute: F = 150 * 2 = 300 At.Therefore, MMF = 300 ampere-turns. Verification / Alternative check: Units check: turns × amperes → ampere-turns; no conversions needed. Why Other Options Are Wrong: 13.33 mAt: off by orders of magnitude; milli-At inappropriate here.75 At: corresponds to 150 * 0.5 A, not given.152 At: not any simple combination of the provided values. Common Pitfalls: Confusing MMF with magnetic field intensity H; H also depends on magnetic path length: H = F / l. Here only MMF is requested. Final Answer: 300 At

Question 11

Magnetic material behavior under cyclic magnetization What is the name of the phenomenon in which the magnetization of a material lags behind the applied magnetizing force when cycled, producing a characteristic loop on the B–H curve?

Accepted Answer

Introduction / Context: When ferromagnetic materials are driven through cycles of magnetization, the relationship between magnetic flux density (B) and magnetizing force (H) does not follow a single line. Instead, a loop forms because the material’s response lags the excitation. This phenomenon is called hysteresis and is central to core-loss analysis in transformers and inductors. Given Data / Assumptions: Cyclic magnetization of a ferromagnetic core. Observation of a closed loop in the B–H plane. Steady temperature and frequency for qualitative description. Concept / Approach: Hysteresis arises due to domain wall motion and irreversible processes at the microscopic level. As H increases and decreases, domains reorient with energy loss, so the path on the B–H plot during increasing H differs from the path during decreasing H. The area of the loop equals energy dissipated per cycle per unit volume, contributing to core heating and efficiency limits. Step-by-Step Solution: Apply increasing H: B rises toward saturation.Reduce H to zero: B does not return to zero, leaving remanence (Br).Reverse H: coercive field (Hc) needed to reduce B to zero.Complete cycle: loop encloses an area equal to hysteresis energy loss per cycle. Verification / Alternative check: Compare materials: soft magnetic cores (silicon steel, ferrites) have narrow loops (low loss), while hard magnetic materials (permanent magnets) have wide loops with high coercivity and retentivity. Why Other Options Are Wrong: Induction: general term for EMF generation, not lag behavior.Retentivity: the residual magnetization Br after removing H, a feature within hysteresis but not the lag itself.Reluctance: opposition to magnetic flux, a circuit parameter, not a cyclic lag effect. Common Pitfalls: Confusing coercivity, remanence, and hysteresis; remember: the loop is hysteresis, its width relates to coercivity, and the intercept at H=0 is remanence. Final Answer: Hysteresis

Question 12

In electromagnetic theory and SI units, what is the standard unit used to express magnetic flux density (symbol B), as encountered in sensors, motors, and transformers?

Accepted Answer

Introduction / Context: Magnetic flux density, commonly denoted by B, quantifies how much magnetic flux passes through a given area. It is central to the analysis of transformers, electric machines, magnetic sensing, and electromagnetic compatibility. Using the correct SI unit avoids confusion when converting between systems such as SI (International System) and cgs (centimeter–gram–second) units. Given Data / Assumptions: We are identifying the SI unit for magnetic flux density B. Context includes typical applications such as motors, relays, and transformers. We distinguish between related quantities: flux (Φ), magnetomotive force (MMF), and flux density (B). Concept / Approach: Magnetic flux density B is defined as magnetic flux Φ per unit area A: B = Φ / A. In SI, magnetic flux Φ is measured in webers (Wb), and area is in square meters (m^2). Therefore, the SI unit of B is Wb/m^2. This compound unit is named the tesla (T). The cgs unit gauss (G) is sometimes encountered; 1 T equals 10,000 G, but gauss is not an SI unit. Magnetomotive force (measured in ampere-turns) and flux (measured in webers) are different physical quantities. Step-by-Step Solution: Identify the physical quantity: B = magnetic flux density.Identify base relation: B = Φ / A (flux per unit area).Insert SI units: Φ in webers (Wb), A in m^2 → B in Wb/m^2.Recognize derived SI name: 1 Wb/m^2 is defined as 1 tesla (T). Verification / Alternative check: Check dimensional consistency: a tesla also equals newton per ampere-meter (N/(A·m)), consistent with Lorentz force F = B * I * l for a conductor in a magnetic field. This confirms that tesla is the correct SI unit for B. Why Other Options Are Wrong: Magnetomotive force (MMF) is measured in ampere-turns, not a unit of B. A weber is a unit of flux Φ, not flux density B. A maxwell is a cgs unit of flux (1 maxwell = 10^-8 Wb), not B. Gauss is a cgs unit of B, not SI. Common Pitfalls: Mixing cgs and SI units or treating flux (Wb) and flux density (T) as interchangeable. Always specify whether a value refers to Φ or B to avoid calculation errors. Final Answer: a tesla

Question 13

On a magnetization (B–H) characteristic, which two physical quantities are plotted against each other to describe how a material responds to an applied magnetic field?

Accepted Answer

Introduction / Context: The B–H curve (magnetization curve) is a foundational tool in electromagnetics and power engineering. It captures how a ferromagnetic material's flux density B responds to an applied magnetizing force H. Designers use it to choose cores, estimate saturation, evaluate hysteresis, and calculate losses in transformers and inductors. Given Data / Assumptions: B denotes magnetic flux density, typically in tesla (T). H denotes magnetizing force (magnetic field strength), in A/m. The graph summarizes material behavior, including linear region, knee (saturation onset), and hysteresis. Concept / Approach: B–H graphs plot B on the vertical axis and H on the horizontal axis. As H increases, magnetic domains align and B rises until approaching saturation. When H is cycled, the material exhibits a hysteresis loop whose area reflects core loss. Permeability μ relates B and H by B = μ * H in the linear region, but μ itself can be nonlinear and is not directly plotted; instead, it is inferred from the slope dB/dH. Step-by-Step Solution: Identify axes: vertical = B (flux density), horizontal = H (magnetizing force).Interpret slope: slope = dB/dH = μ (instantaneous or differential).Note features: initial magnetization, knee, and saturation.Understand hysteresis: loop area corresponds to energy loss per cycle. Verification / Alternative check: Datasheets and core catalogs publish B–H curves or related hysteresis loops. Calculations of inductor volt-seconds and transformer excitation current rely on B–H behavior. Why Other Options Are Wrong: Reluctance, permeability, and magnetomotive force are derived quantities; they are not the standard axes of the B–H plot. Flux (Φ) is related to B but is not used on the basic B–H characteristic. Common Pitfalls: Assuming a constant μ across all H; most ferromagnets are nonlinear, and μ varies with operating point. Always consult the relevant B–H or core data for accurate design. Final Answer: Flux density and magnetizing force

Question 14

In direct-current (dc) motor nomenclature, the terms “series” and “parallel (shunt)” refer to which internal connection within the motor?

Accepted Answer

Introduction / Context: DC motors are classified by how their field windings are connected relative to the armature: series, shunt (parallel), and compound. These configurations strongly affect torque–speed behavior, starting current, and regulation, which is why correct terminology matters in troubleshooting and selection. Given Data / Assumptions: We are distinguishing common dc motor types by internal wiring. Armature is the rotating winding; field coil(s) create the magnetic field. External controllers, batteries, and mechanical couplings are not the focus of “series” and “shunt.” Concept / Approach: In a series dc motor, the field winding is connected in series with the armature, so the same current flows through both. This yields high starting torque but poor speed regulation. In a shunt (parallel) dc motor, the field winding is connected in parallel with the armature across the supply, producing more constant speed. Compound motors combine both windings to balance torque and regulation. Step-by-Step Solution: Identify the internal elements: field coil(s) and armature.Series motor: I_field = I_armature (same series current).Shunt motor: V_field = V_armature (both across supply).Therefore, “series” vs “parallel” refers to field–armature connection. Verification / Alternative check: Torque–speed curves confirm behavior: series motors accelerate aggressively under reduced load; shunt motors maintain speed more tightly. Wiring diagrams corroborate the field–armature connection difference. Why Other Options Are Wrong: Motor–controller wiring, battery connections, and shaft couplings are external concerns and do not define series vs shunt motor types. Brush–commutator connections are intrinsic to all brushed dc motors and do not differentiate configurations. Common Pitfalls: Assuming a series motor is universally better due to high torque; its poor speed regulation can be dangerous without proper load. Final Answer: the connection of the field coil and armature windings

Question 15

In practical electromechanical control, which electromagnetic device explicitly contains a hinged or pivoted armature that is pulled by a coil to make or break electrical contacts?

Accepted Answer

Introduction / Context: The term “armature” is used across several electromagnetic devices, but in control and switching applications it most commonly describes the moving iron piece in a relay that opens or closes contacts under coil excitation. Understanding this helps technicians quickly identify components and failure modes in control panels and automation systems. Given Data / Assumptions: We focus on the control/switching context where an armature physically actuates contacts. Devices considered: speaker, dc generator, relay, solenoid, induction motor. Terminology follows common industrial practice. Concept / Approach: A relay consists of a coil, a magnetic core, and a movable ferromagnetic armature linked to spring-loaded contacts. When energized, magnetic attraction pulls the armature, changing the contact state (NO/NC). While generators also have an armature (the current-carrying rotor or stator winding), the question targets the device where the armature is the moving yoke that explicitly makes/breaks contacts in a control circuit. Step-by-Step Solution: Identify device with a hinged moving iron piece controlling contacts → relay.Speaker cones are driven by a voice coil but lack a hinged armature for switching contacts.Solenoids have a plunger; some literature loosely says “armature,” but standard control switching function is that of a relay.Induction motor rotors do not use a relay-style armature mechanism. Verification / Alternative check: Relay datasheets explicitly label the moving part as an “armature,” showing exploded diagrams with coil, yoke, and armature linked to contacts. Why Other Options Are Wrong: DC generators have an armature in the machine-theory sense, but not the hinged contact-actuating piece intended here. Speakers and induction motors do not have relay-style armatures making electrical contact changes. Solenoids actuate a plunger, not switching contacts by a hinged armature as the primary function. Common Pitfalls: Confusing machine-theory “armature” (generator/motor winding) with the relay’s mechanical armature. Context determines the correct interpretation. Final Answer: A relay

Question 16

A magnetic field has a flux density of 5000 µT across a uniform cross-sectional area of 300 mm². Calculate the total magnetic flux Φ present (state the result in webers or submultiples).

Accepted Answer

Introduction / Context: Relating magnetic flux density (B) to total flux (Φ) through an area is a basic step in magnetic circuit calculations for transformers, inductors, and sensors. Flux determines induced voltages and is central to Faraday’s law and core saturation considerations. Given Data / Assumptions: B = 5000 µT (microtesla). A = 300 mm^2 (square millimeters), area is uniform and perpendicular to flux lines. Flux Φ = B * A for uniform fields. Concept / Approach: Use the relation Φ = B * A. Convert all quantities to SI units: tesla (T) for B and square meters (m^2) for area. Then multiply to get webers (Wb), the SI unit of flux. Step-by-Step Solution: Convert B: 5000 µT = 5000 * 10^-6 T = 0.005 T.Convert A: 300 mm^2 = 300 * 10^-6 m^2 = 3.0 * 10^-4 m^2.Compute Φ: Φ = B * A = 0.005 * 3.0 * 10^-4 = 1.5 * 10^-6 Wb.Express in submultiples: 1.5 * 10^-6 Wb = 1.5 µWb. Verification / Alternative check: Check units: T * m^2 = (Wb/m^2) * m^2 = Wb; magnitude is small and consistent with milli/µT fields and mm^2 areas. Why Other Options Are Wrong: milliweber values (0.15–16.67 mWb) are three to four orders of magnitude larger than the correct microlevel result. 5.0 µWb assumes A = 1000 mm^2 (not given). Common Pitfalls: Forgetting unit conversions (µT to T; mm^2 to m^2) leads to answers off by factors of 10^3 or 10^6. Always convert before multiplying. Final Answer: 1.5 µWb

Question 17

According to Lenz’s law and basic induction, when a magnetic field linked to a coil collapses, what is the polarity relationship of the induced voltage relative to the original cause (the force creating the field)?

Accepted Answer

Introduction / Context: Lenz’s law provides the “minus sign” in Faraday’s induction law, indicating that induced effects oppose the change that produced them. This rule explains why inductors generate a flyback voltage when current is interrupted and why energy stored in magnetic fields tries to sustain current flow. Given Data / Assumptions: A coil with collapsing magnetic flux linkage (dΦ/dt < 0). We examine the polarity of the induced EMF. Linear behavior and standard sign conventions. Concept / Approach: Faraday’s law gives magnitude proportional to |dΦ/dt|. Lenz’s law sets polarity so that the induced current’s magnetic field opposes the change in flux. During collapse, the induced EMF polarizes itself to keep current flowing in the original direction, thereby opposing the reduction in flux—hence it is opposite to the original driving “force.” Step-by-Step Solution: Field decreases → induced EMF appears.Induced EMF polarity causes current that attempts to maintain original flux.Therefore, induced voltage is opposite to the cause of the original field.This manifests as a reverse-polarity “kick” across an inductor when switched off. Verification / Alternative check: Observe relay coils without snubbers: switch-off generates a high opposite-polarity spike. Adding a flyback diode clamps this spike by providing a path for the energy to dissipate safely. Why Other Options Are Wrong: Independence or randomness contradicts the deterministic nature of Lenz’s law. “Identical” polarity would reinforce change and violate energy conservation. Stationary force is irrelevant; change in flux is the trigger for induction. Common Pitfalls: Confusing “opposes the change” with “opposes the steady value.” The induced effect resists change, not necessarily the original field itself if it were steady. Final Answer: opposite to the force creating the field

Question 18

Using the electrical–magnetic circuit analogy, permeability (a material property that facilitates magnetic flux) is most closely analogous to which electrical quantity (the inverse counterpart to resistance)?

Accepted Answer

Introduction / Context: Magnetic circuits can be analyzed using an analogy to electric circuits. In this analogy, magnetomotive force (MMF) corresponds to voltage, flux Φ to current, reluctance to resistance, and permeance to conductance. Understanding where permeability fits helps engineers reason about core materials and flux paths. Given Data / Assumptions: We use the standard analogy: Φ ↔ I, MMF ↔ V, reluctance ℜ ↔ R, permeance ↔ G. Permeability μ is a material property appearing in permeance: P = μ * A / l. We are choosing the electrical quantity that plays the inverse role of resistance. Concept / Approach: Reluctance ℜ = l / (μ * A) opposes magnetic flux, directly analogous to resistance R = l / (σ * A) in conductors. Its inverse is permeance P = μ * A / l, which is analogous to electrical conductance G = σ * A / l. Since permeability μ multiplies into permeance, μ aligns with the “conductive” side of the analogy. Thus, the closest electrical counterpart—in the sense of being inverse to resistance—is conductance. Step-by-Step Solution: Write magnetic reluctance: ℜ = l / (μ * A).Write electrical resistance: R = l / (σ * A).Take inverses: P = 1/ℜ = μ * A / l and G = 1/R = σ * A / l.Therefore, permeability (μ) corresponds to conductance-type behavior (via permeance). Verification / Alternative check: Higher μ reduces reluctance just as higher conductivity increases conductance (reducing resistance). Both promote “flow” (flux for magnetics; current for electrics). Why Other Options Are Wrong: Resistance is analogous to reluctance, the opposite role of permeability. Voltage and current correspond to MMF and flux, respectively, not to μ. Capacitance does not map directly into this particular magnetic property analogy. Common Pitfalls: Confusing permeability with permeance. Permeability is a material property (like conductivity), while permeance is a geometric/material combination (like conductance). Final Answer: Conductance

Question 19

The ability of a material to maintain a magnetized state (without the presence of a magnetizing force) is called hysteresis.

Accepted Answer

False

Question 20

The unit of magnetic flux is the weber.

Accepted Answer

True

Magnetism and Electromagnetism Questions