Question 1

Thermal aging rule of thumb for solid dielectrics As the operating temperature of a solid dielectric increases by 10 °C, how does the rate of thermal deterioration (aging) typically change?

Accepted Answer

Introduction / Context: Designers of insulation systems use empirical thermal-life rules to estimate aging. A common engineering heuristic, derived from Arrhenius-type behavior, states that every 10 °C rise roughly doubles the rate of deterioration for many insulating materials (the “10-degree rule”). Given Data / Assumptions: Solid organic dielectrics under steady thermal stress below decomposition temperatures. Chemical degradation follows Arrhenius kinetics with an effective activation energy. Comparison is approximate; exact factors vary by material and temperature range. Concept / Approach: The Arrhenius relation for reaction rate k is k = k0 exp(−Ea/(R T)). A small temperature increase ΔT changes k by a factor exp(Ea/R * (1/T − 1/(T+ΔT))). For typical activation energies of polymer aging, ΔT ≈ 10 °C leads to a factor near 2 in the practical operating range. Hence lifetime L, inversely related to rate, roughly halves per 10 °C rise. Step-by-Step Solution: Assume Arrhenius: k ∝ exp(−Ea/(R T)).Compute ratio k(T+10)/k(T) ≈ exp(Ea/R * (1/T − 1/(T+10))).For typical Ea → ratio ≈ 2 → aging rate doubles. Verification / Alternative check: IEC thermal index testing and Montsinger’s rule in transformer insulation practice reflect the same trend: 10 °C rise ≈ 2× aging rate (though factors from ~1.8 to ~2.5 are observed). Why Other Options Are Wrong: Values 1.5×, 3×, or 4× can occur for specific materials/temperatures, but the widely used general rule is “approximately doubles”. Common Pitfalls: Treating the rule as exact for all ranges; always consult material-specific thermal life data for critical designs. Final Answer: It approximately doubles

Question 2

Information obtained from Hall coefficient of a single-carrier semiconductor Measuring the Hall coefficient RH of a semiconductor that has only one dominant type of charge carrier provides which information?

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Introduction / Context: The Hall effect is a fundamental characterization tool for semiconductors. By applying a magnetic field perpendicular to current flow, a transverse Hall voltage develops whose sign and magnitude reveal properties of the carriers. Given Data / Assumptions: Single dominant carrier type (electrons or holes). Uniform sample with known thickness and applied current. Magnetic field perpendicular to current. Concept / Approach: For a single-carrier semiconductor, the Hall coefficient is RH = E_H / (J B) = ± 1/(q n), with the sign positive for holes and negative for electrons. Thus, measuring RH immediately indicates carrier sign. Its magnitude gives n = 1/(|q| |RH|). If both electrons and holes contribute appreciably, RH becomes more complex; but the question specifies one type dominates, so RH yields both sign and density. Step-by-Step Solution: Measure Hall voltage V_H, compute RH from geometry and applied J, B.Determine sign of RH → electron (−) or hole (+).Compute n = 1/(|q| |RH|) to obtain carrier density. Verification / Alternative check: Mobility μ can be found only if conductivity σ is also measured, using σ = q n μ. The Hall measurement alone gives sign and density; pairing with σ yields mobility. Why Other Options Are Wrong: (a) and (b) are incomplete; (d) contradicts Hall theory; (e) requires σ as well, not available from RH alone. Common Pitfalls: Ignoring multi-carrier effects or geometry corrections; ensure single dominant carrier and accurate thickness measurement. Final Answer: Both sign and density of the charge carrier

Question 3

Force between parallel current-carrying conductors Evaluate the statement: “When two long, parallel conductors carry currents in the same direction, the force between them is repulsive.”

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Introduction / Context: The magnetic interaction between parallel currents is a cornerstone of electromagnetism and defines the ampere in classical terms. The direction of force depends on whether the currents are co-directional or counter-directional. Given Data / Assumptions: Two long, straight, parallel conductors separated by distance r. Currents I1 and I2 flow in the same direction. Quasi-static fields; neglect edge effects. Concept / Approach: The magnetic field around conductor 1 at the location of conductor 2 is B1 = μ0 I1 /(2π r). Conductor 2 experiences magnetic force per unit length f = I2 × B1, direction given by the right-hand rule. For currents in the same direction, the force is attractive; for opposite directions, it is repulsive. Hence the given statement claiming repulsion for same-direction currents is false. Step-by-Step Solution: Compute B1 at conductor 2: B1 = μ0 I1 /(2π r).Force per unit length: f = I2 B1 = μ0 I1 I2 /(2π r).Direction from right-hand rule → attraction for same directions. Verification / Alternative check: Symmetry implies conductor 1 experiences equal and opposite force. This mutual attraction underpins the classical ampere definition and many busbar spacing calculations. Why Other Options Are Wrong: (c) direction does not depend on equality of currents; (d) AC does not change instantaneous force law (time-average remains attractive for in-phase currents); (e) sign is independent of spacing (magnitude changes with r). Common Pitfalls: Confusing magnetic force direction with electric force between like charges; here, current direction determines magnetic interaction. Final Answer: False

Question 4

Semiconductor doping basics: effect of adding a trivalent impurity When a trivalent (group III) impurity such as boron, aluminium, or gallium is introduced into a tetravalent semiconductor (for example silicon or germanium), the resulting semiconduc…

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Introduction / Context: Doping is the intentional introduction of impurity atoms to control carrier concentrations in semiconductors. Understanding how donor and acceptor dopants change conduction is foundational for diodes, transistors, and integrated circuits. Given Data / Assumptions: Host crystal is tetravalent (e.g., Si or Ge). Impurity is trivalent (group III), such as B, Al, or Ga. Thermal equilibrium and moderate doping (nondegenerate) are assumed. Concept / Approach: A trivalent atom has three valence electrons. In a tetrahedrally coordinated lattice requiring four covalent bonds, one bond around a trivalent atom lacks an electron, creating an electron vacancy called a hole. Such dopants are called acceptors because they can accept an electron from the valence band, leaving behind mobile holes that carry positive charge. Step-by-Step Solution: A trivalent dopant contributes one fewer valence electron than the host requires for bonding.This deficit forms an acceptor level slightly above the valence band edge.At ordinary temperatures, electrons thermally excite from the valence band into the acceptor level.This process leaves mobile holes in the valence band, increasing hole concentration p.Since p ≫ n after such doping, holes are majority carriers and the material is p-type. Verification / Alternative check: Mass-action law states n * p = n_i^2. If acceptors raise p well above n_i, then n = n_i^2 / p becomes small, confirming holes as majority carriers and electrons as minority carriers. Why Other Options Are Wrong: n type: Requires donors (group V), not acceptors. Intrinsic: Undoped, with n ≈ p. Neutral: Doping purposefully shifts carrier balance. Degenerate semiconductor: Implies very heavy doping that moves the Fermi level into a band, not assumed here. Common Pitfalls: Confusing valence count (3 vs 5) and mixing up donors with acceptors; assuming doping always increases electrons rather than considering the dopant’s valence relative to the host. Final Answer: p type

Question 5

Frequency response of dielectric polarizability: when is the imaginary part of αe zero? In an alternating electric field of angular frequency ω, the complex electronic polarizability αe(ω) has a real and an imaginary part. For which frequency limits…

Accepted Answer

Introduction / Context: Dielectric response to a time-varying field is commonly described by a complex polarizability αe(ω) or complex permittivity ε*(ω). The imaginary component represents energy dissipation (dielectric loss), while the real component represents stored energy. Recognizing where the loss vanishes helps interpret relaxation and resonance behavior. Given Data / Assumptions: Linear, time-invariant response with a standard dispersion model (e.g., Debye-type for orientational or Lorentz-type for bound electrons/ions). We focus on qualitative limits: static (ω = 0) and very high frequency (ω → ∞). No extrinsic conduction or electrode polarization effects considered. Concept / Approach: In canonical dispersion models, the imaginary part of the response is linked to phase lag between polarization and field. At DC (ω = 0), there is no phase lag for a relaxed system, so the loss term tends to zero. At extremely high frequencies (ω → ∞), massive or bound charges cannot follow the rapidly varying field, so the response amplitude tends to a limiting value with negligible phase lag, again driving the imaginary part to zero. Between these limits, around a characteristic time constant (Debye) or near a resonance (Lorentz), the imaginary part peaks. Step-by-Step Solution: Recognize that αe(ω) = α′(ω) − j α″(ω).At ω = 0, polarization fully follows the field without lag ⇒ α″(0) = 0.At ω → ∞, charges cannot respond to rapid changes ⇒ lag goes to zero and α″(∞) → 0.Hence α″(ω) = 0 in both limiting cases: DC and infinite frequency. Verification / Alternative check: For Debye relaxation, ε″(ω) = (εs − ε∞) * ωτ / (1 + ω^2 τ^2). This tends to zero as ω → 0 or ω → ∞, with a maximum at ωτ = 1. Analogous statements hold for αe(ω). Why Other Options Are Wrong: Only ω = 0 or only ω → ∞ state one limit and miss the other; ω = ω0 corresponds to resonance where losses peak, not vanish; “any finite value” ignores the characteristic peak and decay behavior. Common Pitfalls: Confusing conductivity loss (σ/ωε0 contribution) with dipolar loss; assuming constant nonzero loss across frequencies. Final Answer: ω = 0 and ω → ∞

Question 6

Magnetism laboratory evidence: do Bitter powder patterns prove ferromagnetic domains? State whether the following is correct: "Bitter powder (colloidal) patterns provide experimental proof of the existence of magnetic domains in ferromagnets."

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Introduction / Context: Ferromagnetic materials are subdivided into regions called domains where magnetic moments align coherently. Visualizing these domains experimentally confirms the domain theory that explains hysteresis, coercivity, and magnetic microstructure effects. Given Data / Assumptions: Bitter powder (or colloidal) method uses fine magnetic particles in a liquid. Particles gather where stray field gradients at the surface are large, typically at domain boundaries. Observation is made under a microscope on a polished, magnetized specimen. Concept / Approach: Domains minimize total magnetic energy by balancing exchange, anisotropy, and magnetostatic energies. At a sample surface, adjacent domains with different magnetization directions produce nonuniform stray fields. Colloidal magnetic particles experience forces toward maxima of field gradient and deposit preferentially along domain walls, thereby tracing the domain pattern. Step-by-Step Solution: Prepare a polished ferromagnetic surface (e.g., Fe, Ni).Apply a suspension containing fine magnetic particles (Bitter solution).Under a microscope, observe particle accumulation along lines reflecting strong field gradients.Interpret these lines as domain boundaries or closure domains. Verification / Alternative check: Other techniques such as Kerr microscopy (magneto-optic Kerr effect), Lorentz TEM, and magnetic force microscopy also reveal domain structures, corroborating Bitter patterns and ensuring the phenomenon is not an artifact. Why Other Options Are Wrong: False: Contradicted by decades of microscopy. Below Néel temperature: Refers to antiferromagnets, not typical ferromagnetic domain imaging. Paramagnets: Do not exhibit stable domain structures. Crystalline defects only: While defects can pin domain walls, the patterns specifically trace magnetic field gradient zones. Common Pitfalls: Confusing domain walls with grain boundaries; assuming particles map magnetization direction rather than wall locations (they primarily reveal field gradients). Final Answer: True

Question 7

Microscopic current loop: magnetic moment and angular velocity Assertion (A): For an electron of charge magnitude e revolving in a circular orbit of radius R with angular velocity ω, the orbital magnetic dipole moment is μ = 0.5 * e * ω * R^2. Reaso…

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Introduction / Context: A moving charge constitutes a current loop and therefore produces a magnetic dipole moment. The classical electron-in-orbit model, though simplified, builds intuition for relationships among current, area, magnetic moment μ, and orbital angular momentum L. Given Data / Assumptions: Electron of charge magnitude e circulates in a circle of radius R with angular velocity ω. We consider magnitudes (signs from negative charge can be treated separately). Nonrelativistic classical picture is sufficient for this relation. Concept / Approach: The effective current is i = q * f = q * (ω / 2π). The loop area is A = π R^2. Thus, μ = i * A = (q ω / 2π) * π R^2 = (q ω R^2) / 2. For electron magnitude e, μ = 0.5 * e * ω * R^2 (direction opposite to L due to negative charge). Angular momentum is L = m * v * R = m * ω * R^2. μ and L are proportional, not equal: μ = (q / 2m) * L (Bohr magneton concept). Step-by-Step Solution: Compute current: i = e * ω / (2π).Compute area: A = π R^2.Magnetic moment: μ = i * A = (e ω R^2) / 2.Compare with L: L = m ω R^2 ⇒ μ = (e / 2m) * L (proportional, not equal). Verification / Alternative check: In atomic physics, μ/L ratio equals e/(2m), leading to the Bohr magneton μB = e ħ / (2m) for one quantum of angular momentum ħ. The equality claim is therefore incorrect. Why Other Options Are Wrong: A is correct by direct calculation; R is false because it asserts equality instead of proportionality; the other combinations contradict these facts. Common Pitfalls: Ignoring the factor 1/2 from current–frequency relation; forgetting the electron’s negative charge reverses μ direction relative to L. Final Answer: A is true but R is false

Question 8

Choosing dielectrics for capacitors: which permittivity is preferable? For maximizing the capacitance of a given capacitor geometry, it is better to use a dielectric material having:

Accepted Answer

Introduction / Context: Capacitance depends on geometry and on the dielectric constant (relative permittivity εr) of the material between electrodes. Selecting the appropriate dielectric directly affects energy storage, device size, and performance across frequency. Given Data / Assumptions: Parallel-plate or fixed geometry capacitor concept. Linear, isotropic dielectrics considered for simplicity. Breakdown strength and loss tangent are not the focus of this single-factor question. Concept / Approach: The capacitance of a parallel-plate capacitor is C = ε0 εr A / d, where ε0 is vacuum permittivity, εr is the relative permittivity, A is plate area, and d is separation. For fixed A and d, increasing εr increases C proportionally, allowing higher energy storage (E = 0.5 C V^2) at the same voltage or a smaller component for the same C. Step-by-Step Solution: Identify C dependence: C ∝ εr for fixed geometry.For higher C, choose larger εr materials.Therefore, “high permittivity” is preferable when maximizing capacitance alone. Verification / Alternative check: Practical high-εr materials include ferroelectric ceramics (e.g., barium titanate-based). Conversely, PTFE or air has εr near 2–1 and produces smaller C for the same geometry. Why Other Options Are Wrong: Low or air-like εr: Lowers capacitance. Neither high nor low: Offers no advantage in C. Negative ε: Not suitable for ordinary static capacitors; appears in metamaterials and plasmonic contexts, not conventional dielectrics. Common Pitfalls: Ignoring trade-offs: high εr materials may have higher loss or lower breakdown; however, the question asks only about maximizing C. Final Answer: high permittivity

Question 9

Total angular momentum sources in atoms and the scale of magnetic moments Assertion (A): The angular momentum of an atom arises from three contributions: orbital motion of electrons, electron spin, and nuclear spin. Reason (R): The magnetic moment d…

Accepted Answer

Introduction / Context: Atomic angular momentum and magnetic moments underpin spectroscopy (Zeeman, hyperfine structure), magnetic resonance (NMR, EPR), and selection rules. Distinguishing the relative magnitudes of electronic and nuclear contributions is essential. Given Data / Assumptions: Total atomic angular momentum J arises from electron orbital (L), electron spin (S), and nuclear spin (I). Magnetic moments scale with charge-to-mass ratios and fundamental constants. Concept / Approach: Electrons carry charge −e and small mass m_e, giving a large gyromagnetic ratio. The characteristic electronic magnetic moment scale is the Bohr magneton μB = eħ/(2m_e). Nuclear moments scale with the nuclear magneton μN = eħ/(2m_p), which is smaller than μB by the mass ratio m_e/m_p ≈ 1/1836. Consequently, electron spin moments dominate over nuclear spin moments by roughly three orders of magnitude. Step-by-Step Solution: A is evaluated: J indeed combines L, S, and I (vector-coupled).R is evaluated: claims nuclear spin magnetic moment ≫ electron spin magnetic moment.Compare scales: μN ≈ μB / 1836 ⇒ nuclear spin moments are much smaller.Therefore A is true; R is false. Verification / Alternative check: Hyperfine splittings (nuclear-spin–electron interaction) are much smaller than fine-structure splittings (electron spin–orbit), consistent with the smaller nuclear moment. Why Other Options Are Wrong: Any option accepting R as true contradicts well-established magneton scales; denying A ignores standard angular momentum composition in atomic physics. Common Pitfalls: Confusing magnitude with qualitative importance (nuclear spins are crucial in NMR despite smaller moments); overlooking vector coupling and selection rules. Final Answer: A is true but R is false

Question 10

Polarization mechanisms in monatomic solid insulators In a solid insulating material composed of only one kind of atom (monatomic crystal or solidified rare gas), which polarization mechanisms can exist under an applied electric field?

Accepted Answer

Introduction / Context: Polarization mechanisms determine a dielectric’s relative permittivity and loss behavior. The nature of available mechanisms depends on the microscopic structure: atomic species present, bonding type, and whether permanent dipoles exist. Given Data / Assumptions: Material contains only one kind of atom (e.g., solidified noble gas or elemental monatomic crystal with no ionic sublattices). No permanent molecular dipoles are present. Small-signal, linear response to an applied field. Concept / Approach: Electronic polarization involves a slight displacement of electron clouds relative to nuclei and exists in essentially all materials. Ionic polarization requires at least two oppositely charged ion species moving relative to each other in a lattice (e.g., Na+ and Cl−), which is impossible in monatomic solids. Permanent (orientational) polarization requires pre-existing dipoles (e.g., polar molecules like H2O), also absent in monatomic solids. Step-by-Step Solution: Assess composition: single atomic species → no ionic pairs.Assess molecular structure: no molecules → no permanent dipoles to orient.Therefore, only electronic polarization can occur under an electric field. Verification / Alternative check: Measured εr values for monatomic solids (e.g., solid Ar, Ne) are close to 1 and show minimal dispersion, consistent with electronic polarization only. Why Other Options Are Wrong: Electronic and ionic: Requires at least two ion types. Electronic, ionic, and permanent: Requires polar molecules and ionic sublattices. Electronic and permanent: Permanent dipoles do not exist in monatomic solids. None: Electronic polarization always exists to some extent. Common Pitfalls: Equating “single atom type” with “no polarization at all”; forgetting that even noble gases polarize electronically. Final Answer: electronic only

Question 11

Classification of the Schrödinger equation How should the time-independent Schrödinger equation for a typical quantum system be classified?

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Introduction / Context: The Schrödinger equation is the central equation of nonrelativistic quantum mechanics. Correctly classifying it helps students select appropriate mathematical tools (separation of variables, eigenfunction expansions, boundary-value methods) for solving physical problems. Given Data / Assumptions: Time-independent form (TISE) for a particle of mass m in potential V(r): typically written as −(ħ^2 / 2m) ∇^2 ψ + V ψ = E ψ. Spatial coordinates involve more than one independent variable in 3D. Linearity with respect to the wavefunction ψ. Concept / Approach: TISE involves spatial derivatives through the Laplacian ∇^2. Because the function depends on spatial coordinates (x, y, z), the equation is a partial differential equation (PDE). It is also linear in ψ: superposition applies. While “linear equation” and “differential equation” are also true descriptors, “partial differential equation” is the most precise single classification among the options. Step-by-Step Solution: Identify derivative type: second-order spatial derivatives → differential equation.Multiple independent variables → partial differential equation.Operator acts linearly on ψ → linear PDE.Hence, the best single choice is “partial differential equation”. Verification / Alternative check: Examples: Particle in a box, hydrogen atom, and harmonic oscillator are solved using PDE techniques (separation of variables) in appropriate coordinate systems. Why Other Options Are Wrong: Linear equation: True but incomplete; not as specific. Differential equation: True but does not specify “partial.” None / nonlinear integral: Contradicted by the standard form of TISE. Common Pitfalls: Confusing “ordinary” with “partial” differential equations; overlooking linearity due to the presence of the potential term (it multiplies ψ and preserves linearity). Final Answer: partial differential equation

Question 12

Lattice specific heat and the Debye temperature At sufficiently high temperature, the specific heat of a crystalline metal approaches a constant (the Dulong–Petit limit). The characteristic temperature that marks this transition scale is called the:

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Introduction / Context: The temperature dependence of specific heat reveals how lattice vibrations (phonons) are excited. The Debye model improves on classical Dulong–Petit by predicting low-temperature T^3 behavior and recovery of the classical limit at high temperature. Given Data / Assumptions: Crystalline metal approximated by a 3D elastic continuum with a cutoff frequency (Debye frequency). At low T, only low-frequency phonons are excited; at high T, the phonon spectrum is saturated. Concept / Approach: Debye introduced a characteristic temperature θD (Debye temperature) corresponding to the highest vibrational frequency (ωD) of the solid via k_B θD = ħ ωD. The specific heat at constant volume approaches 3R per mole when T ≫ θD because all vibrational modes are equally populated (equipartition). Below θD, quantum effects suppress high-frequency mode occupation, reducing specific heat. Step-by-Step Solution: Recognize the high-T constant limit is the Dulong–Petit limit (≈ 3R).Identify θD as the characteristic scale separating quantum and classical regimes.Therefore, the temperature referred to is the Debye temperature. Verification / Alternative check: Experimental plots of C_v/T versus T show saturation toward 3R beyond material-specific θD values (e.g., Al, Cu have θD of order a few hundred kelvin). Why Other Options Are Wrong: Curie temperature: Magnetic order transition (ferromagnets). Neel temperature: Antiferromagnetic order. Transition temperature: Nonspecific term. Kondo temperature: Scale for magnetic impurity scattering in metals, not lattice specific heat saturation. Common Pitfalls: Confusing magnetic transition temperatures with lattice vibrational scales; assuming saturation occurs at a universal temperature rather than material-dependent θD. Final Answer: Debye temperature

Question 13

Magnetic shielding principle Decide whether the following statement is correct: "For effective magnetic shielding of low-frequency or static magnetic fields, a ferromagnetic material (high-permeability alloy) is required to provide a low-reluctance …

Accepted Answer

Introduction / Context: Magnetic shielding is essential in precision instrumentation, medical imaging, and metrology. The physical mechanism depends strongly on frequency: static and low-frequency fields are best mitigated by high-permeability paths, while high-frequency fields often rely on conductive shields and eddy currents. Given Data / Assumptions: We consider static or low-frequency magnetic fields (quasi-DC). Shielding enclosure can be made from high-μ alloys (e.g., mu-metal). Superconducting options (Meissner effect) are not required or practical for most applications. Concept / Approach: A ferromagnetic shield presents a material with very high relative permeability μr. Magnetic flux prefers low-reluctance paths, so the shield attracts and guides flux lines around the protected volume. This is analogous to providing a shunt path in magnetic circuits. For AC at higher frequencies, conductive materials create eddy currents that oppose incident fields (skin effect), but for static/slow variations, permeability dominates shield performance. Step-by-Step Solution: Identify the field regime: static/low-frequency.Select materials with high μr to reduce magnetic reluctance.Design geometry (thickness, continuity, multiple layers) to minimize leakage and saturation.Thus, a ferromagnetic material is appropriate and effective. Verification / Alternative check: Practical enclosures for magnetometers or CRTs historically use mu-metal. Superconducting shields work via flux expulsion but require cryogenics; diamagnets have μr slightly less than 1 and are far less effective than high-μ alloys. Why Other Options Are Wrong: False: Contradicts standard practice. True only for RF: RF shielding is often dominated by conductivity and skin depth, not high μ alone. Only superconductors: Impractical and unnecessary for most uses. Only diamagnets: Very weak effect (μr ≈ 1). Common Pitfalls: Confusing electric-field shielding (Faraday cages) with magnetic shielding; overlooking saturation of high-μ materials at large flux densities. Final Answer: True

Question 14

Intrinsic semiconductor at room temperature — comparing conducting electrons to valence electrons In an intrinsic (undoped) semiconductor at room temperature, how does the number of conducting electrons compare with the total number of valence elect…

Accepted Answer

Introduction / Context: This question checks conceptual understanding of intrinsic (pure, undoped) semiconductors such as silicon and germanium at room temperature. The focus is on the relative populations: the total valence electrons bound in covalent bonds versus the much smaller population of thermally excited carriers (electrons and holes) that participate in conduction. Given Data / Assumptions: Material is intrinsic (no intentional doping). Temperature is around room temperature (approximately 300 K). Electrons are promoted from the valence band to the conduction band by thermal energy, creating electron–hole pairs. Concept / Approach: In an intrinsic semiconductor, the number of electrons in the conduction band equals the number of holes in the valence band, denoted by n_i. The vast majority of electrons remain in valence-band bonding states. Because the band gap is on the order of electron-volts, only a tiny fraction of electrons has sufficient thermal energy to cross the gap at 300 K. Thus, n_i is many orders of magnitude smaller than the total number of valence electrons per unit volume. Step-by-Step Solution: Recall: intrinsic carrier concentration n_i for Si at 300 K is roughly 10^10 cm^−3; for Ge it is roughly 10^13 cm^−3 (order of magnitude).Compare with atomic density ~ 5 × 10^22 cm^−3 and four valence electrons per atom for Si/Ge → total valence electrons ~ 2 × 10^23 cm^−3.Compute fraction: n_i / (valence electrons) ~ 10^10 / 10^23 = 10^−13 for Si (extremely small).Therefore, the thermally excited conducting electrons constitute a very small fraction of the total valence electrons. Verification / Alternative check: The exponential dependence n_i ∝ exp(−E_g / (2kT)) ensures that for band gaps of ~1 eV at 300 K, n_i is tiny compared to the density of valence electrons. This holds broadly for common semiconductors. Why Other Options Are Wrong: “Almost equal” or “about half” ignore the band-gap barrier. “Small as compared to” is imprecise; the correct emphasis is “a very small fraction.” “Exactly zero” is incorrect because finite temperature always generates some carriers. Common Pitfalls: Confusing intrinsic with extrinsic (doped) behavior. Underestimating the effect of the exponential dependence on band gap. Final Answer: Is a very small fraction of the number of valence electrons

Question 15

Relaxation time versus viscosity in polar liquids As the viscosity of a liquid increases, how does the dielectric relaxation time (characteristic time constant for dipole reorientation) change?

Accepted Answer

Introduction / Context: Dielectric relaxation time characterizes how fast dipoles in a polar liquid reorient to follow a changing electric field. Understanding its dependence on viscosity connects molecular kinetics to macroscopic dielectric behavior (Debye relaxation). Given Data / Assumptions: Polar molecules experiencing rotational diffusion in a viscous medium. Low to moderate fields so that linear response applies. Temperature and molecular size held fixed while viscosity varies. Concept / Approach: According to the Debye model, rotational relaxation time τ is proportional to the product of viscosity η and effective molecular volume V divided by thermal energy. A common form is τ ∝ η * a^3 / (k_B * T), where a is an effective molecular radius. As viscosity increases, rotational motion is hindered, so the time to reorient increases. Step-by-Step Solution: Relate torque balance and rotational drag: higher η → larger rotational friction.Debye relation: τ = (4π * η * a^3) / (k_B * T) (proportionality shown).Thus, when η increases while T and a remain constant, τ increases proportionally. Verification / Alternative check: Empirical dielectric spectroscopy shows that cooling (which raises viscosity) lengthens relaxation times dramatically, consistent with the theoretical dependence. Why Other Options Are Wrong: “Remains constant” contradicts the friction dependence. “Decreases” is opposite to observed behavior. “First decreases then increases” lacks physical basis here. “Becomes zero” is impossible; higher viscosity slows dynamics but does not eliminate them entirely. Common Pitfalls: Confusing translational diffusion with rotational reorientation; both slow with viscosity. Ignoring temperature effects which can mask the sole effect of viscosity in real systems. Final Answer: Increases

Question 16

Eddy-current loss dependence on flux density If B_max is the maximum flux density in a magnetic core, how does the eddy-current power loss vary with B_max (keeping frequency and lamination thickness constant)?

Accepted Answer

Introduction / Context: Eddy-current losses arise from circulating currents induced in conductive core materials by time-varying magnetic flux. Recognizing the correct dependence on flux density is essential for core design, lamination choices, and loss estimation. Given Data / Assumptions: Laminated core with fixed lamination thickness t and electrical conductivity. Sinusoidal excitation at fixed frequency f. Maximum flux density B_max is the variable of interest. Concept / Approach: Classical eddy-current loss per unit volume follows P_e ∝ f^2 * t^2 * B_max^2 for sinusoidal excitation. The square dependence on B_max comes from Faraday’s law (induced emf proportional to dB/dt) and Joule heating proportional to current squared. Step-by-Step Solution: Induced emf ∝ dΦ/dt ∝ A * dB/dt ∝ A * ω * B_max.Eddy current density J ∝ induced emf / resistance ∝ ω * B_max (for fixed geometry).Power density P ∝ J^2 * ρ_eff^−1 → P_e ∝ (ω^2) * B_max^2.At fixed f and t, P_e ∝ B_max^2. Verification / Alternative check: Standard core-loss separation uses Steinmetz-like forms: total loss = hysteresis (∝ f * B_max^n with n ≈ 1.6–2.0) plus eddy-current (∝ f^2 * B_max^2). Holding f constant isolates the B_max^2 term for eddy currents. Why Other Options Are Wrong: Linear or cubic dependencies do not follow from the classical derivation. The 1.6 exponent pertains to hysteresis loss, not eddy-current loss. Independence from B_max contradicts Faraday induction. Common Pitfalls: Mixing hysteresis and eddy-current exponents. Ignoring the role of lamination thickness and frequency in the full expression. Final Answer: Proportional to B_max^2

Question 17

Thermal classes of solid dielectrics by temperature limit According to standard insulation (thermal) classes used for solid dielectrics, how many categories are recognized by their permissible temperature limits?

Accepted Answer

Introduction / Context: Electrical insulation systems are grouped into thermal classes, each with a maximum temperature limit for reliable operation. Recognizing the number of categories helps designers select appropriate materials for transformers, machines, and cables. Given Data / Assumptions: Common industrial practice per IEC/IEEE thermal classes. Classes typically identified as Y, A, E, B, F, H, and C. Each class corresponds to a different temperature ceiling. Concept / Approach: Traditional classification enumerates seven thermal classes for solid insulation systems. Although exact temperature limits vary slightly by standard and update, the canonical count remains seven, covering a broad temperature range from basic (Y) to very high temperature (C). Step-by-Step Solution: List the classes: Y, A, E, B, F, H, C.Count them → 7 categories.Conclude that the number of categories, by common standards, is seven. Verification / Alternative check: Many textbooks and datasheets cite these seven categories; designers map winding temperature rises and ambient conditions to these limits to ensure insulation life. Why Other Options Are Wrong: Options 4, 5, 6, and 8 do not match the commonly recognized set of classes. While some documents add subclasses or application-specific groupings, the canonical list is seven. Common Pitfalls: Confusing thermal classes with flammability or tracking-resistance ratings. Assuming newer standards eliminated classes—most still reference the seven-class schema. Final Answer: 7

Question 18

Electronic polarizability versus atomic size for rare gases If R is the effective radius of the electron cloud around the nucleus of a rare-gas atom, the electronic polarizability α of the atom is proportional to:

Accepted Answer

Introduction / Context: Electronic polarizability measures how easily an atom's electron cloud is distorted by an external electric field. For spherical, closed-shell (rare-gas) atoms, α depends strongly on atomic size. Given Data / Assumptions: Spherically symmetric electron cloud (noble-gas atom). Linear, small-field response. Effective radius R characterizes cloud size. Concept / Approach: Simple models (e.g., classical oscillator or uniformly charged sphere) indicate the restoring force scales with charge distribution and size, yielding polarizability proportional to volume. Since volume scales as R^3, electronic polarizability α ∝ R^3. Step-by-Step Solution: Model electron cloud as a sphere of radius R.Under field E, displacement δ creates dipole p proportional to displaced charge times δ.Restoring force scales with distribution size; linearization gives p = α E with α ∝ volume ∝ R^3. Verification / Alternative check: Empirical polarizabilities increase down the noble-gas group (He < Ne < Ar < Kr < Xe), consistent with increasing atomic size and the α ∝ R^3 trend. Why Other Options Are Wrong: R or R^2 underestimate the size dependence; R^4 overestimates it. 1/R contradicts observed periodic trends. Common Pitfalls: Confusing polarizability with dipole moment; α links p and E (p = αE). Ignoring that closed shells still polarize even without permanent dipoles. Final Answer: R^3

Question 19

Hysteresis loss dependence on flux density If B_max is the maximum flux density, how does magnetic hysteresis power loss depend on B_max (for a given material and frequency range using a Steinmetz-type relation)?

Accepted Answer

Introduction / Context: Hysteresis loss is associated with irreversible domain wall motion and magnetization reversal in ferromagnetic cores. Engineers often use empirical Steinmetz equations to relate loss to flux density and frequency. Given Data / Assumptions: Fixed material with given hysteresis characteristics. Frequency in a range where classical Steinmetz parameters apply. We compare dependence on B_max only. Concept / Approach: The classical Steinmetz relation expresses hysteresis power loss per unit volume as P_h ∝ f * B_max^n where n is typically around 1.6 for many steels (actual values vary with material and processing). Holding f constant isolates the dependence P_h ∝ B_max^n with n ≈ 1.6. Step-by-Step Solution: Adopt Steinmetz form: P_h = k_h * f * B_max^n.With f fixed, P_h ∝ B_max^n.For common lamination steels, n ≈ 1.6 → P_h ∝ B_max^1.6. Verification / Alternative check: Manufacturer core-loss curves plotted on log–log axes show slopes near 1.6 in the hysteresis-dominated regime, confirming the exponent. Why Other Options Are Wrong: Linear, quadratic, or cubic dependencies do not reflect the empirical hysteresis scaling across typical operating ranges. Independence from B_max contradicts basic magnetization physics. Common Pitfalls: Confusing hysteresis (≈ B^1.6) with eddy-current loss (∝ B^2 at fixed f). Assuming a universal exponent; it is material-dependent but commonly near 1.6. Final Answer: Proportional to B_max^1.6

Question 20

Do many compounds show semiconducting behavior? Evaluate the statement: “Many inorganic and some organic compounds also exhibit semiconducting properties.”

Accepted Answer

Introduction / Context: Semiconductivity is not limited to elemental silicon and germanium. A wide range of compounds, alloys, and even organic materials display semiconducting behavior, underpinning modern electronics and optoelectronics. Given Data / Assumptions: Semiconducting behavior means a band gap in a practical range with controllable carrier concentrations. Includes binary and ternary compounds, alloy semiconductors, and certain organic systems. Concept / Approach: Compound semiconductors (e.g., III–V such as GaAs, InP; II–VI such as CdS, ZnSe) are central to high-speed electronics, LEDs, laser diodes, and photodetectors. Ternaries/quaternaries (AlGaAs, InGaN, HgCdTe) tailor band gaps across the spectrum. Some organic semiconductors (polyacetylene derivatives, small-molecule systems) enable flexible electronics and OLEDs. Step-by-Step Solution: Identify examples: GaAs (direct band gap), GaN (wide band gap), CdTe (solar cells), SiC (power devices).Describe functionality: direct band gaps enable efficient light emission; tailored gaps suit detectors and photovoltaics.Acknowledge organics: conjugated polymers and small molecules show semiconducting transport via π–π stacking and hopping mechanisms. Verification / Alternative check: Commercial technologies—LED lighting (InGaN), laser diodes (AlGaAs/GaAs), high-electron-mobility transistors (GaN/AlGaN)—demonstrate the ubiquity of compound semiconductors. Why Other Options Are Wrong: “False” and “only elements” ignore widespread industrial practice. “Only at absolute zero” is meaningless for conduction. “Only under illumination” confuses photoconductivity with intrinsic/extrinsic conduction mechanisms. Common Pitfalls: Assuming silicon dominates all applications; in RF, power, and optoelectronics, compounds often outperform Si. Equating organic semiconductors with insulators; many organics have useful mobility. Final Answer: True

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