Question 1

In electromagnetic theory, what is the correct SI unit for magnetic permeability (μ), keeping full dimensional clarity?

Accepted Answer

Introduction / Context: Understanding units in electromagnetics is essential for interpreting formulas and designing components such as inductors, transformers, and magnetic cores. Magnetic permeability, denoted μ, links magnetic flux density B to magnetic field strength H through B = μ * H. Choosing the correct SI unit ensures dimensional consistency in calculations and specifications. Given Data / Assumptions: Permeability is defined by B = μ * H. B (flux density) is measured in tesla, T, which equals Wb/m^2. H (magnetic field strength) is measured in ampere-turns per meter (At/m) or equivalently A/m when turns are absorbed into current. Concept / Approach: The unit of μ follows directly from μ = B / H. Substitute the SI base units for B and H to derive μ’s unit symbolically, then match with the listed options. Keep ampere-turns (At) explicit to align with classic magnetics notation used in many textbooks and data sheets. Step-by-Step Solution: Start with μ = B / H.Use B units: B = Wb / m^2.Use H units: H = At / m.Compute units of μ: μ units = (Wb / m^2) / (At / m) = (Wb / m^2) * (m / At) = Wb / (At * m). Verification / Alternative check: Henry (H) is Wb/A. The widely used engineering unit form for permeability is H/m. Since H = Wb/A and At approximates A when turns = 1, H/m = (Wb/A)/m = Wb/(A·m). With At explicitly shown, Wb/(At·m) is the consistent expression used in many magnetics contexts. Why Other Options Are Wrong: At/Wb: This is the unit of magnetic reluctance, not permeability. Wb: Weber alone measures flux, not a material property per length. At/m: This is the unit for field strength H, not permeability. Common Pitfalls: Confusing permeability μ with permittivity ε; ε has units F/m, while μ has units H/m. Dropping the per-meter factor; permeability is a material property per length dimension. Final Answer: Wb/At × m

Question 2

Electromagnetic induction concept check: What is the induced voltage across a stationary conductor placed in a stationary magnetic field?

Accepted Answer

Introduction / Context: Faraday’s law of electromagnetic induction explains how changing magnetic environments produce electric potentials. This question distinguishes the necessary conditions for an induced voltage from static scenarios where no induction occurs. Given Data / Assumptions: The conductor is stationary. The magnetic field is static (unchanging in time and space relative to the conductor). No motion or time-varying flux is present. Concept / Approach: Faraday’s law states that the induced electromotive force (emf) in a circuit equals the negative time rate of change of magnetic flux linking the circuit: emf = - dΦ/dt. Motional emf requires relative motion v between the conductor and magnetic field: emf = B * l * v (for a straight conductor orthogonal to B and motion). If both the field and conductor are stationary, and flux is constant, dΦ/dt = 0 and v = 0, so the induced voltage is zero. Step-by-Step Solution: Identify the induction mechanism: time-varying flux or conductor motion is required.Given there is no motion (v = 0), motional emf term is zero.Given the field is stationary, dΦ/dt = 0, transformer emf term is zero.Therefore, total induced voltage = 0 V. Verification / Alternative check: Practical lab confirmation: With a coil or bar held fixed inside a constant magnetic field and no time variation or movement, a voltmeter reads 0 V. Induction readings appear only when the field changes or the conductor moves (e.g., rotating a loop in a generator). Why Other Options Are Wrong: Increased/Decreased: Suggests a nonzero starting value, which does not exist here. Reversed in polarity: Polarity reversal occurs when motion direction or field direction flips while emf is nonzero; not applicable in a wholly static case. Common Pitfalls: Assuming any magnetic field automatically induces voltage—induction requires change or motion. Confusing static flux presence with flux change. Final Answer: zero

Question 3

Wirewound core behavior: For a given magnetic core with a coil, what happens to flux density when the coil current is increased?

Accepted Answer

Introduction / Context: Coil current and magnetic flux density are closely linked through magnetomotive force and the material’s magnetic response. This question tests understanding of how increasing current influences flux in a core before saturation effects dominate. Given Data / Assumptions: Fixed core geometry and material. Increased current through the coil turns. Operation not deep into saturation (linear region assumption). Concept / Approach: Magnetomotive force (mmf) is F = N * I, measured in ampere-turns. Magnetic field strength H in a core of mean length l is H = F / l = N * I / l. Flux density B is B = μ * H, where μ is the core’s permeability. Thus, as I increases, H increases, and therefore B increases proportionally in the linear region. Step-by-Step Solution: Compute mmf: F = N * I (increases with I).Relate field strength: H = F / l so H increases as I increases.Relate flux density: B = μ * H ⇒ B increases when H increases (for constant μ). Verification / Alternative check: Using a B-H curve of the core material, small increases in H (from more current) lead to increases in B until approaching saturation, where the slope reduces. Measurements with a Hall probe or fluxmeter corroborate this in lab experiments. Why Other Options Are Wrong: Causes no change: Contradicts B = μ * H and H ∝ I. Reverses flux lines: Reversal requires reversing current direction, not merely increasing magnitude. Decreases flux density: Would require reduced current or reduced permeability. Common Pitfalls: Ignoring core saturation, which can limit further B increase at very high I but does not cause a decrease by itself. Confusing flux direction (set by current direction) with magnitude (set by current size). Final Answer: increases the flux density

Question 4

Flux density change with area: If the magnetic flux remains constant but the cross-sectional area of the field region increases, how does the flux density B change?

Accepted Answer

Introduction / Context: Flux density is a spatial measure of magnetic flux distribution. This question probes the relationship between total flux (Φ), cross-sectional area (A), and flux density (B) in magnetics design and analysis. Given Data / Assumptions: Total magnetic flux Φ is constant. Cross-sectional area A increases. Uniform flux distribution over area for simplicity. Concept / Approach: Flux density is defined as B = Φ / A for a uniformly distributed flux. With Φ held constant, B is inversely proportional to A. Hence, when A increases, B must decrease to keep the product B * A equal to Φ. Step-by-Step Solution: Start from the definition: B = Φ / A.Hold Φ constant; consider A → k * A with k > 1.Then B_new = Φ / (k * A) = (1 / k) * (Φ / A) = B / k, which is smaller. Verification / Alternative check: Dimensional reasoning: Weber per square meter (Wb/m^2) falls as the area denominator grows, consistent with field lines spreading out over a larger cross-section. Practical example: widening an air gap area reduces B if the same magnet produces the same Φ. Why Other Options Are Wrong: Increases/Doubles: Would require Φ to rise proportionally, not true here. Remains the same: Only possible if Φ increases exactly with A, which contradicts the given condition. Common Pitfalls: Confusing flux Φ (total lines) with flux density B (lines per unit area). Assuming nonuniform distributions; while real cores can be nonuniform, the inverse relation holds locally. Final Answer: decreases

Question 5

Inside a permanent magnet, what is the conventional direction of magnetic field lines?

Accepted Answer

Introduction / Context: Magnetic field line conventions are crucial for interpreting diagrams and understanding flux paths in machines and magnetic circuits. The internal direction differs from the external direction around a bar magnet, forming a closed loop. Given Data / Assumptions: Standard physics convention for magnetic field lines. Bar magnet model with identifiable north and south poles. Concept / Approach: Magnetic field lines emerge from the north pole, travel through the surrounding space, and re-enter the magnet at the south pole. Inside the magnet, they continue from the south pole back to the north pole to complete a continuous closed loop, consistent with ∇ · B = 0 (no magnetic monopoles). Step-by-Step Solution: Outside the magnet: direction is from north to south.At the south pole: lines enter the magnet.Inside the magnet: lines run from south to north to close the loop. Verification / Alternative check: Iron filings experiments and field mapping with a Hall probe show the external N→S direction and confirm the internal S→N path. In magnetic circuit textbooks, loop drawings consistently show this convention. Why Other Options Are Wrong: From north to south: Correct outside the magnet, not inside. Front to back / Back to front: These lack reference to poles and are not standard descriptors. Common Pitfalls: Assuming the same N→S direction both outside and inside; forgetting the closed-loop nature of B. Confusing electric field line conventions with magnetic ones. Final Answer: from south to north

Question 6

Faraday–Lenz intuition: When the speed of a conductor moving through a magnetic field is increased, how does the induced voltage respond?

Accepted Answer

Introduction / Context: Motional electromotive force (emf) underpins generators and many sensors. The induced voltage depends on geometry, field strength, and the relative speed between a conductor and the magnetic field. Recognizing these proportionalities helps predict system behavior when mechanical speed changes. Given Data / Assumptions: Uniform magnetic field B. A straight conductor of effective length l moving with speed v perpendicular to B for maximum effect. Linear, non-saturating regime without end effects. Concept / Approach: The motional emf formula is emf = B * l * v * sin(θ), with θ the angle between velocity and the field. For the perpendicular case (θ = 90°, sin θ = 1), emf ∝ v. Increasing speed v therefore increases the induced voltage proportionally. Step-by-Step Solution: Assume perpendicular motion: θ = 90°.Write emf relation: emf = B * l * v.As v increases → emf increases linearly. Verification / Alternative check: Generator behavior: Raising rotor speed increases generated voltage at constant field excitation. Similarly, moving a wire faster through a field yields higher galvanometer deflection (higher induced emf) in classroom demonstrations. Why Other Options Are Wrong: Remains constant: Only true if speed and effective flux cutting remain constant. Decreases or Reaches zero: Opposite of the speed dependence; zero would require v = 0 or no effective flux cutting. Common Pitfalls: Forgetting the sin(θ) factor; with non-perpendicular motion, the increase still scales with the perpendicular component of v. Confusing transformer emf (from dΦ/dt) with motional emf (from v across B). Final Answer: increases

Question 7

Magnetic materials: The property describing a material’s ability to stay magnetized after the magnetizing force is removed is called

Accepted Answer

Introduction / Context: Magnetic material properties determine performance in permanent magnets, memory elements, and motors. It is important to distinguish among permeability, reluctance, hysteresis, and retentivity when selecting or analyzing magnetic materials. Given Data / Assumptions: After removing the magnetizing force (H → 0), some materials retain a nonzero flux density B. We are naming the specific property that quantifies this behavior. Concept / Approach: Retentivity (or residual flux density, Br) measures how much magnetization remains when the magnetizing force is reduced to zero. Hysteresis refers to the lag between B and H and the energy loss in one cycle, visualized as the hysteresis loop. Reluctance is the magnetic analog of resistance (opposition to flux). Permeability measures how readily a material supports the formation of magnetic flux. Step-by-Step Solution: Identify the phenomenon: nonzero B when H = 0 after magnetization.Match to property name: retentivity (residual magnetism).Differentiate from hysteresis: hysteresis is the entire loop behavior, not just the retained magnetism. Verification / Alternative check: Refer to a B-H curve: At H = 0 after driving to saturation, the vertical intercept is Br, which reflects retentivity. Materials like hard ferrites have large Br; soft magnetic materials have small Br to allow easy demagnetization. Why Other Options Are Wrong: Hysteresis: Describes loop energy loss and lag, not specifically “remaining magnetization”. Reluctance: Unit At/Wb; relates to flux opposition in a magnetic path. Permeability: Unit H/m; governs B = μ * H, not residual magnetism after H is removed. Common Pitfalls: Using “hysteresis” when the question asks about retained magnetization explicitly. Confusing retentivity (Br) with coercivity (Hc), which is the reverse field needed to reduce B to zero. Final Answer: retentivity

Question 8

Magnetomotive force (mmf): For a 75-turn coil carrying 4 A, what is the mmf developed by the winding?

Accepted Answer

Introduction / Context: Magnetomotive force (mmf) is the driving “cause” of magnetic flux in a magnetic circuit, analogous to electromotive force in electric circuits. Correctly computing mmf is essential when sizing coils and predicting magnetic field strength in cores and air gaps. Given Data / Assumptions: Number of turns N = 75. Coil current I = 4 A. We neglect leakage and assume steady DC current. Concept / Approach: The definition of mmf is F = N * I, with units ampere-turns (At). This simple relation underlies the calculation of field strength H = F / l (where l is magnetic path length) and subsequent flux density B = μ * H. Step-by-Step Solution: Write the mmf formula: F = N * I.Insert values: F = 75 * 4.Compute: F = 300 ampere-turns (At). Verification / Alternative check: If the mean magnetic path length were known, we could find H = F / l and then B = μ * H to corroborate physical magnitudes. The linear proportionality in F confirms that doubling turns or current doubles mmf. Why Other Options Are Wrong: 18.75 At or 30 At: These result from mistakenly multiplying or dividing by partial factors (e.g., using 0.25 A or 7.5 turns). 187 At: A rounding or arithmetic slip; 75 * 4 is exactly 300. Common Pitfalls: Forgetting the “turns” factor; mmf is not just current. Unit confusion: mmf is At, not volts or amperes alone. Final Answer: 300 At

Question 9

Magnetic pole interaction: When the north poles of two bar magnets are brought close together, what happens?

Accepted Answer

Introduction / Context: Basic magnetic interactions mirror electrostatic analogies: like poles repel and unlike poles attract. Recognizing these interactions is critical in designing magnetic couplings, holding fixtures, and understanding motor forces. Given Data / Assumptions: Two bar magnets with clearly identified north poles. Magnets are brought close together in free space. Concept / Approach: Field lines emerge from each north pole and seek to enter a south pole. When two north poles face each other, their field lines conflict, creating a region of high magnetic pressure and resulting in a repulsive mechanical force pushing the magnets apart. Step-by-Step Solution: Identify pole orientation: N facing N.Recall rule: like poles repel, unlike poles attract.Therefore, the mechanical interaction is repulsion. Verification / Alternative check: Hands-on demonstration with two bar magnets: attempt to push two north poles together and feel resistance. Field visualization with iron filings shows crowding and deflection, consistent with repulsive force lines. Why Other Options Are Wrong: A force of attraction: Occurs only for N–S facing alignment. No force: Magnetic fields exert forces on magnetic dipoles; null only at very large separations. A downward force: Direction depends on how magnets are constrained; the fundamental interaction is repulsive, not direction-specific like “downward”. Common Pitfalls: Confusing electric charge rules with magnetic poles but the rule is analogous: like repels, unlike attracts. Misinterpreting “downward” as a universal direction; magnetic forces act along the line of centers between poles. Final Answer: a force of repulsion

Question 10

Magnetomotive force (MMF) calculation: A coil of 8 turns carries a steady current of 3 A. Determine the MMF in ampere–turns (At).

Accepted Answer

Introduction / Context: Magnetomotive force (MMF) is a fundamental quantity in electromagnetics and electrical machines. It represents the 'driving force' that establishes magnetic flux in a magnetic circuit, analogous to how a voltage source drives electric current in an electric circuit. Given Data / Assumptions: Number of turns, N = 8. Current through the coil, I = 3 A (direct current, steady state). Air gaps, core properties, and leakage are irrelevant for MMF calculation; only N and I matter. Concept / Approach: The definition of MMF for a simple coil is MMF = N * I, expressed in ampere–turns (At). This linear relation comes from summing the magnetizing effect of each turn carrying current I around the magnetic path. Step-by-Step Solution: Use MMF formula: MMF = N * ISubstitute: MMF = 8 * 3Compute: MMF = 24 At Verification / Alternative check: If the current were doubled or the turns doubled, MMF would double accordingly, confirming proportionality to both N and I. The units ampere–turns reflect 'A per turn' summed across turns, consistent with the definition. Why Other Options Are Wrong: 240 At and 2,400 At incorrectly multiply by extra powers of 10. 2.4 At divides instead of multiplies. 0.375 At would correspond to I/N rather than N*I, which is incorrect. Common Pitfalls: Confusing MMF with magnetic flux or flux density, or forgetting that MMF depends only on turns and current, not on core dimensions or permeability. Final Answer: 24 At

Question 11

Flux density from flux and area: In a magnetic field, a flux of 600 µWb passes through an area of 0.1 m^2. Calculate the flux density B.

Accepted Answer

Introduction / Context: Flux density B quantifies how much magnetic flux passes per unit area and is a central concept in electromagnetics, affecting induced voltages and magnetic forces. It connects the physical flux to field intensity via the area considered. Given Data / Assumptions: Total magnetic flux, phi = 600 µWb = 600 * 10^-6 Wb. Area, A = 0.1 m^2. Uniform flux distribution is assumed over the area for a straightforward calculation. Concept / Approach: By definition, B = phi / A. Convert all units to SI before dividing. 1 tesla (T) equals 1 Wb/m^2. You can also express the result in microtesla (µT) for convenience, where 1 T = 10^6 µT. Step-by-Step Solution: Convert flux: phi = 600 µWb = 600 * 10^-6 Wb = 6 * 10^-4 Wb.Compute B: B = phi / A = (6 * 10^-4) / 0.1 = 6 * 10^-3 T.Convert to µT: 6 * 10^-3 T = 6 * 10^3 µT = 6,000 µT. Verification / Alternative check: A quick order-of-magnitude check: micro–weber over a tenth of a square meter should yield milli–tesla or thousands of microtesla, consistent with 6,000 µT. Why Other Options Are Wrong: 600 µT is a factor of 10 too small; 6 T and 600 T are far too large given the small flux and moderate area; 60 µT is two orders too small. Common Pitfalls: Missing the micro (10^-6) prefix or forgetting to divide by area correctly. Always standardize units first to avoid errors. Final Answer: 6,000 µT

Question 12

Induced EMF from changing flux: A 250-turn coil is in a magnetic field where flux changes at 8 Wb/s. Find the induced voltage across the coil.

Accepted Answer

Introduction / Context: Faraday's law of electromagnetic induction states that a changing magnetic flux linking a coil induces an electromotive force (EMF) proportional to both the rate of change of flux and the number of turns. This principle underpins transformers, generators, and many sensors. Given Data / Assumptions: Number of turns, N = 250. Rate of change of flux, d(phi)/dt = 8 Wb/s (assumed uniform linkage of all turns). Ideal conditions: negligible resistance and leakage for EMF magnitude calculation. Concept / Approach: Induced EMF magnitude is given by E = N * (d(phi)/dt). Polarity is determined by Lenz's law, but only magnitude is asked here. Units are consistent: Wb/s times turns gives volts. Step-by-Step Solution: Use E = N * (d(phi)/dt).Substitute: E = 250 * 8.Compute: E = 2,000 V. Verification / Alternative check: Sanity check: 10 Wb/s with 200 turns would give 2,000 V; here 8 * 250 produces exactly the same magnitude, confirming the arithmetic. Why Other Options Are Wrong: 3,125 V and 1,000 V are incorrect products. 31.25 V is much too small (confuses division). 250 V would correspond to 1 Wb/s, not 8 Wb/s. Common Pitfalls: Mixing up flux and flux density, or forgetting that all turns must be linked by the changing flux for the simple formula to apply. Final Answer: 2,000 V

Question 13

Identify the proper unit of magnetic reluctance for a magnetic circuit.

Accepted Answer

Introduction / Context: Reluctance is the magnetic analogue of electrical resistance. It quantifies how strongly a magnetic circuit opposes the establishment of magnetic flux for a given magnetomotive force (MMF). Understanding its unit clarifies relationships in magnetic circuits. Given Data / Assumptions: Reluctance symbol: script R (often Rm or ℜ). Definition: ℜ = MMF / phi. MMF in ampere–turns (At), flux phi in webers (Wb). Concept / Approach: From the definition ℜ = MMF / phi, the unit must be (ampere–turns) per weber, i.e., At/Wb. Other magnetic quantities have different SI units (e.g., tesla for flux density, Wb for flux, H/m for permeability). Step-by-Step Solution: Start with ℜ = MMF / phi.Units: MMF in At; phi in Wb.Therefore, unit(ℜ) = At/Wb. Verification / Alternative check: Also, using ℜ = l / (µ * A) leads to units: m / (H/m * m^2) = m / (H * m) = 1/H. Since 1 H = Wb/A, 1/H = A/Wb. Considering turns as unitless count, this is consistent with At/Wb in practical usage. Why Other Options Are Wrong: Tesla is flux density, At/m is field strength unit (H), and Wb is flux. H/m is permeability, not reluctance. Common Pitfalls: Confusing reluctance with reluctivity or permeability, and mixing up the roles of H, B, and phi in magnetic circuits. Final Answer: At/Wb

Question 14

Effect of speed on induced EMF: In a basic direct-current (DC) generator, if the loop suddenly rotates faster, what happens to the induced voltage?

Accepted Answer

Introduction / Context: In electromechanical energy conversion, the induced EMF in a rotating conductor cutting magnetic flux depends on rotational speed. This principle governs generator output behavior with changing speed at constant field strength. Given Data / Assumptions: Same magnetic flux per pole (field strength unchanged). Only the mechanical speed is increased. Basic DC generator with commutation maintaining output polarity per half-turn in the usual manner. Concept / Approach: The magnitude of induced EMF in a generator is proportional to the rate of flux cutting, which is proportional to angular velocity omega. In simplified form, E ∝ phi * omega. With phi constant, increasing speed raises E proportionally. Step-by-Step Solution: Assume field flux phi is constant.Increase speed: omega → k * omega, k > 1.Then E_new = k * E_old (direct proportionality). Verification / Alternative check: Practical machines show higher terminal voltage at higher speed if field and load remain unchanged (until regulation or saturation effects intervene), consistent with the theoretical relation. Why Other Options Are Wrong: 'Remains unchanged' contradicts E ∝ speed. 'Reverses polarity' would require reversing rotation direction or field polarity, not merely increasing speed. 'Decreases' is opposite to the physical relationship. 'Drops to zero' has no basis unless the field collapses. Common Pitfalls: Confusing AC generator phase effects with DC commutation, or assuming internal regulation keeps voltage constant, which is not implied here. Final Answer: increases

Question 15

Electromagnet behavior: When the current through a coil is reversed, how does the magnetic field of the electromagnet respond?

Accepted Answer

Introduction / Context: The polarity of an electromagnet is determined by the direction of current through its windings. Understanding how field direction responds to current reversal is essential in devices like relays, solenoids, and rotating machines. Given Data / Assumptions: Single coil carrying current I. Current reverses to −I while magnitude may remain the same. Core material and geometry unchanged. Concept / Approach: Using the right-hand rule (for conventional current), curling fingers in the direction of coil current gives the direction of magnetic field lines through the core. Reversing current flips the field direction end-for-end, swapping the north and south poles of the electromagnet. Step-by-Step Solution: Initial current I → field points from core south to north.Reverse current −I → field flips direction 180 degrees.The resulting field has the same magnitude (if |I| unchanged) but opposite polarity. Verification / Alternative check: This is consistent with MMF = N * I: MMF changes sign with current sign, hence field direction reverses. Magnetic flux promptly reorients following the new magnetizing force. Why Other Options Are Wrong: 'Remains unchanged' ignores current direction. 'Expands' or 'collapses permanently' misstate transient effects; no permanent collapse occurs on simple reversal. 'Field intensity doubles' is unsupported without doubling current magnitude. Common Pitfalls: Confusing current magnitude changes with direction changes; mixing electron flow and conventional current without a consistent rule can cause sign errors. Final Answer: direction of the magnetic field reverses

Question 16

Compute reluctance: A magnetic material has length l = 0.07 m, cross-sectional area A = 0.014 m^2, and permeability µ = 4,500 µWb/(At·m). Find the reluctance ℜ in At/Wb.

Accepted Answer

Introduction / Context: Reluctance determines how much magnetomotive force is required to establish a given magnetic flux in a magnetic path. It mirrors electrical resistance in Ohm’s law, with the magnetic counterpart given by Hopkinson’s law: MMF = ℜ * phi. Given Data / Assumptions: Length, l = 0.07 m. Area, A = 0.014 m^2. Permeability, µ = 4,500 µWb/(At·m) = 4,500 * 10^-6 Wb/(At·m) = 0.0045 Wb/(At·m). Uniform cross-section and material properties; linear operation (no saturation considered). Concept / Approach: The reluctance formula is ℜ = l / (µ * A), with units At/Wb. Substitute SI-consistent values, being careful with the micro (10^-6) prefix in permeability. Step-by-Step Solution: Convert µ: µ = 4,500 µWb/(At·m) = 0.0045 Wb/(At·m).Compute µA: µ * A = 0.0045 * 0.014 = 6.3 * 10^-5 Wb/At.Compute ℜ: ℜ = l / (µA) = 0.07 / (6.3 * 10^-5) ≈ 1.111 * 10^3 At/Wb.Therefore, ℜ ≈ 1111 At/Wb. Verification / Alternative check: Dimensional check: m / (Wb/(At·m) * m^2) = m / (Wb/At * m) = At/Wb, correct unit for reluctance. Why Other Options Are Wrong: 11 and 111 At/Wb underestimate by factors of 100 and 10. 1 At/Wb is far too small. 7 At/Wb has no basis in the calculation. Common Pitfalls: Failing to convert µ from micro-units, or using µ_r instead of µ (absolute) in the formula. Always ensure SI consistency. Final Answer: 1111 At/Wb

Question 17

Solenoid actuator principle: When a solenoid is energized, what physical cause moves the plunger?

Accepted Answer

Introduction / Context: Solenoids are electromechanical actuators that convert electrical energy into linear motion. Understanding what actually exerts force on the plunger is key for troubleshooting and design in automation and control systems. Given Data / Assumptions: Coil supplied with current, creating a magnetic field. Movable ferromagnetic plunger aligned with the coil axis. Standard solenoid construction with return spring omitted from consideration here. Concept / Approach: The energized coil establishes an electromagnetic field. The ferromagnetic plunger experiences a force pulling it into the region of higher magnetic field density to minimize magnetic reluctance. The cause is the interaction of the induced magnetic field with the plunger’s material, not merely the presence of voltage or current without a field. Step-by-Step Explanation: Current through coil → magnetic field H is produced.Magnetization of plunger → induced poles create attraction towards the coil center.Mechanical motion occurs as magnetic energy is minimized by increasing flux linkage. Verification / Alternative check: Reversing current reverses polarity, but the plunger is still pulled in, demonstrating force depends on field magnitude and geometry rather than polarity for typical designs. Why Other Options Are Wrong: Permanent magnetic field is not necessary in standard solenoids. 'Varying voltage' and 'steady current' are electrical conditions; the physical mover is the resulting electromagnetic field. Electrostatic attraction is negligible here compared to magnetic forces. Common Pitfalls: Confusing cause (field) with electrical conditions (voltage/current). The field creates the force that moves the plunger. Final Answer: an electromagnetic field

Question 18

Reluctance from MMF and flux: A wire with 40 turns carries 900 mA. If the resulting magnetic flux is 400 µWb, find the magnetic circuit reluctance.

Accepted Answer

Introduction / Context: In magnetic circuits, reluctance relates the applied magnetomotive force (MMF) to the resulting flux, analogous to resistance relating voltage to current. When MMF and flux are known, reluctance directly characterizes the magnetic path's opposition to flux. Given Data / Assumptions: Number of turns, N = 40. Current, I = 900 mA = 0.9 A. Flux, phi = 400 µWb = 400 * 10^-6 Wb = 4 * 10^-4 Wb. Uniform linkage and steady-state conditions. Concept / Approach: Reluctance is ℜ = MMF / phi, where MMF = N * I in ampere–turns. Compute MMF, then divide by flux to obtain ℜ in At/Wb. Step-by-Step Solution: Compute MMF: N * I = 40 * 0.9 = 36 At.Convert flux: phi = 400 µWb = 4 * 10^-4 Wb.Compute ℜ: ℜ = 36 / (4 * 10^-4) = 36 / 0.0004 = 90,000 At/Wb. Verification / Alternative check: Unit check: At/Wb is correct for reluctance. The value is large, consistent with modest flux produced by a sizeable MMF, indicating a relatively reluctant magnetic path. Why Other Options Are Wrong: 14,400, 9,000, and 1,440 At/Wb result from misplacing decimal points or incorrect flux conversion. 144 At/Wb is orders of magnitude too small. Common Pitfalls: Forgetting that 400 µWb is 4e-4 Wb; using 400e-6 incorrectly in a calculator can lead to factor-of-10 errors. Always standardize units first. Final Answer: 90,000 At/Wb

Question 19

Induced current from induced voltage: A coil has an induced voltage of 200 mV. A 120 Ω resistor is connected across the coil. Determine the induced current.

Accepted Answer

Introduction / Context: Ohm’s law directly relates voltage, current, and resistance in a simple resistive load. When a coil produces an induced voltage and a resistor is connected across its terminals, the resulting current is set by I = V / R (ignoring coil resistance for this calculation, as typical in such questions). Given Data / Assumptions: Induced voltage, V = 200 mV = 0.2 V. Load resistance, R = 120 Ω. Assume purely resistive load and negligible source impedance for this basic computation. Concept / Approach: Apply Ohm’s law: I = V / R. Be careful with milli- prefixes to avoid unit mistakes. The answer will be in amperes; convert to milliamperes for a convenient final value. Step-by-Step Solution: Convert V: 200 mV = 0.2 V.Compute I: I = 0.2 / 120 = 0.001666… A.Express in mA: 0.001666… A ≈ 1.67 mA ≈ 1.7 mA. Verification / Alternative check: Reverse check: V = I * R ≈ 1.7 mA * 120 Ω = 0.204 V (rounding explains slight difference), consistent with the given voltage. Why Other Options Are Wrong: 120 mA would require 14.4 V across 120 Ω. 16 mA and 12 mA correspond to 1.92 V and 1.44 V, respectively—far larger than 0.2 V. 0.17 mA is off by a factor of 10 due to unit slip. Common Pitfalls: Not converting millivolts to volts, or misreading 200 mV as 2 V or 0.02 V. Always convert before dividing. Final Answer: 1.7 mA

Question 20

Electromagnetic field basics in circuit context: an electromagnetic field exists only under which condition?

Accepted Answer

Introduction / Context: In introductory electrical engineering, learners often mix up when electric fields, magnetic fields, and full electromagnetic fields appear around conductors. This question targets the everyday circuit context: when does an electromagnetic field (with a magnetic component tied to charge motion) exist around a wire or device? Understanding this is vital for power electronics, signal integrity, and electromagnetic compatibility. Given Data / Assumptions: The discussion is in the context of practical circuits and conductors. Conductors are assumed to be surrounded by free space or dielectric materials. We focus on the presence of current and its role in producing magnetic fields and thus an electromagnetic field around the conductor. Concept / Approach: In circuit practice, a steady current produces a steady magnetic field encircling the conductor (right-hand rule). The combination of the electric potential distribution (due to charges) and the magnetic field (due to moving charges) is commonly referred to as the electromagnetic field surrounding the conductor. While a static voltage establishes an electric field, the distinctive magnetic component that completes the electromagnetic picture appears with current flow. Step-by-Step Solution: Recognize that current (motion of charges) produces a magnetic field around a wire.A static voltage alone produces mainly an electric field; the magnetic component is negligible without charge motion.Therefore, an electromagnetic field in the common circuit sense exists whenever current flows. Verification / Alternative check: Observe a current-carrying conductor with a magnetic compass; deflection occurs only when current flows, confirming the magnetic component. With only a disconnected high-voltage source and no conduction path, there is an electric field but no surrounding magnetic field due to current. Why Other Options Are Wrong: Voltage: Voltage alone does not guarantee a magnetic field; it primarily establishes an electric field. An increasing current / decreasing current: These describe time variation (which changes the field magnitude), but a magnetic field exists even for steady current. None of the above: Incorrect because current is the right condition in basic circuit context. Common Pitfalls: Equating any presence of voltage with a full electromagnetic field including a magnetic component. Assuming only changing current creates fields; in fact, steady current creates a steady magnetic field. Final Answer: current

Magnetism and Electromagnetism Questions