Question 1

Electromagnetics — assertion–reason on electron acceleration and current density Assertion (A): An electron of charge −e and mass m placed in a uniform electric field E experiences an acceleration a = −(e/m) * E. Reason (R): In a material containing…

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Introduction / Context: This item checks two cornerstone relations in electromagnetics and solid-state physics: Newton’s second law for a point charge in an electric field, and the drift-current density in a conductor or semiconductor. Distinguishing truth value from causal explanation is the key. Given Data / Assumptions: Electron charge q = −e and mass m. Uniform electric field vector E. Number density of mobile electrons n (m^−3) and drift velocity component v_x. Classical, low-field drift (Ohmic regime), neglecting collisions during the instant of acceleration law statement. Concept / Approach: For a point charge in an electric field, the electric force is F = q * E. Using Newton’s second law, a = F / m = (q/m) * E. For an electron (q = −e), a = −(e/m) * E, which is the assertion. For a carrier ensemble, current density by definition is charge per unit volume times average velocity: J = ρ_v * v. With electron charge −e and number density n, ρ_v = −n e, so J = −n e v; along x, J_x = −n e v_x. This is the reason statement. Step-by-Step Solution: Compute force: F = q * E = −e * E.Apply Newton’s law: a = F/m = −(e/m) * E → A is true.Form current density: J = (charge density) * (drift velocity) = (−n e) * v → R is true.Check explanation linkage: R describes macroscopic current, not why a = −(e/m) * E; it does not causally explain A. Verification / Alternative check: In steady Ohmic conduction, collisions limit average acceleration, giving v_d = μ E with mobility μ; nevertheless, the instantaneous acceleration law and the macroscopic J formula remain consistent. Why Other Options Are Wrong: (a) implies R explains A, but J = −n e v does not yield a = −(e/m) * E. (c) and (d) contradict well-established relations. (e) is false because both statements are correct. Common Pitfalls: Confusing drift velocity with instantaneous acceleration. Dropping the negative sign for electron charge in J. Final Answer: Both A and R are true but R is not correct explanation of A

Question 2

Semiconductor doping — effect of pentavalent (donor) impurities When a pentavalent impurity (for example, phosphorus, arsenic, or antimony) is added to a pure semiconductor crystal, the resulting semiconductor becomes:

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Introduction / Context: Doping controls carrier type and concentration in semiconductors. Pentavalent dopants in group-IV semiconductors provide extra electrons near the conduction band and are central to forming n-type regions in diodes and transistors. Given Data / Assumptions: Host crystal: group-IV (e.g., Si, Ge). Dopant valence: 5 (donor), such as P, As, Sb. Low to moderate doping so the material remains nondegenerate. Concept / Approach: Pentavalent atoms substitute for a host atom and form four covalent bonds, leaving one loosely bound electron. This donor electron is thermally ionized at room temperature, contributing a free electron to the conduction band and creating a positively charged donor ion. Thus electrons become majority carriers and the material is n-type. Step-by-Step Solution: Identify dopant valence: 5 → donor.Ionization releases an electron to the conduction band.Carrier balance shifts: n ≫ p → n-type behavior. Verification / Alternative check: Energy band diagrams show donor levels slightly below the conduction band; at room temperature most donors are ionized, raising electron concentration. Why Other Options Are Wrong: p-type requires acceptors (trivalent B, Al, Ga). “Intrinsic” ignores the dopant. “Neutral with no change” is false; carrier concentration changes markedly. “Degenerate p-type” is unrelated to donor doping. Common Pitfalls: Confusing donor and acceptor terminology. Assuming all dopants create the same carrier type. Final Answer: n-type

Question 3

Solid-state physics — assertion–reason on typical Fermi energy of metals Assertion (A): For most common metals, the Fermi energy E_F is less than about 10 eV. Reason (R): The Fermi level for a free-electron-like metal is approximated by E_F ≈ 3.64 ×…

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Introduction / Context: The Fermi energy sets the top of the occupied electron states at absolute zero in the free-electron model of metals. Typical magnitudes help estimate electron speeds, densities of states, and electronic heat capacities. Given Data / Assumptions: Free-electron approximation: E_F = (h^2 / 2m_e) * (3π^2 n)^(2/3). Representative conduction-electron densities n ≈ 10^28–10^29 m^−3. Energies expressed in electron-volts (eV). Concept / Approach: Inserting typical n values into the free-electron formula yields E_F on the order of a few eV up to roughly 10 eV (e.g., Na ~3 eV, Al ~11.6 eV, Cu ~7 eV). The given empirical form E_F ≈ 3.64 × 10^−19 * n^(2/3) (with n in m^−3) is a convenient numerical version of the theoretical expression after unit conversion to eV. Step-by-Step Solution: Take n = 1 × 10^29 → n^(2/3) ≈ 2.15 × 10^19.Compute E_F ≈ 3.64 × 10^−19 * 2.15 × 10^19 ≈ 7.8 eV.Since many metals have n in this range, E_F typically lies below ~10 eV → A is true and explained by R. Verification / Alternative check: Using the exact formula E_F = (h^2/2m_e)(3π^2 n)^(2/3) and converting to eV reproduces the same magnitude, confirming the numerical coefficient used in R. Why Other Options Are Wrong: (b) denies the explanatory link even though substituting typical n into R’s formula directly supports A. (c) and (d) reject correct statements. (e) is inconsistent with established metal data. Common Pitfalls: Confusing work function with Fermi energy; they are different quantities. Forgetting unit consistency when using empirical constants. Final Answer: Both A and R are true and R is correct explanation of A

Question 4

Charge transport — typical order of electron relaxation time in metals In the classical Drude model (room temperature, common metals), the mean relaxation time τ of conduction electrons is typically of the order of:

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Introduction / Context: The relaxation time τ characterizes the average time between momentum-randomizing collisions for conduction electrons. It is a key parameter in Ohmic conduction and appears in mobility, conductivity, and dielectric response models. Given Data / Assumptions: Room temperature metallic conductor. Classical Drude response with τ roughly constant over low frequencies. No strong impurity or phonon anomalies. Concept / Approach: In the Drude picture, conductivity σ is σ = n e^2 τ / m. Using typical σ ≈ 10^7 S/m and n ≈ 10^29 m^−3 gives τ on the order of 10^−14 s. More refined models (Drude–Sommerfeld) adjust numbers slightly but keep the same order of magnitude. Step-by-Step Solution: Assume n ≈ 8 × 10^28 m^−3 and σ ≈ 5.8 × 10^7 S/m (copper).Solve τ = m σ / (n e^2).Insert constants → τ ~ few × 10^−14 s, confirming the order 10^−14 s. Verification / Alternative check: From mean free path l = v_F τ with Fermi velocity v_F ~ 10^6 m/s and l ~ 10^−8 m, τ ~ 10^−14 s again. Why Other Options Are Wrong: 10^−6 s and 10^−2 s are macroscopic timescales, far too long. 10^−10 s is still orders too large. 10^−20 s is unphysically short for electron transport in metals. Common Pitfalls: Confusing scattering time with dielectric relaxation time of materials. Using carrier densities for semiconductors instead of metals. Final Answer: 10^−14 s

Question 5

Permittivity concepts — interpreting ε0 versus εr in terms of material microstructure Which statement best reflects the physical interpretability of absolute permittivity ε0 and relative permittivity εr?

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Introduction / Context: Permittivity describes how electric fields interact with matter. The vacuum permittivity ε0 is a fundamental constant of nature, while the relative permittivity εr captures the additional polarization response due to a material’s microscopic structure. Given Data / Assumptions: Linear, isotropic dielectrics. Microscopic interpretation via dipole polarizability. SI units and conventional definitions: ε = ε0 εr. Concept / Approach: ε0 characterizes empty space and is not derived from specific atomic properties. εr, however, depends on how bound charges within atoms and molecules displace under an applied field; models such as Clausius–Mossotti relate εr to microscopic polarizability. Thus εr, not ε0, is interpretable in terms of the material’s atomic-scale properties. Step-by-Step Solution: Write ε = ε0 εr.Associate εr with polarization P via P = ε0 (εr − 1) E.Relate P to dipole density and polarizability → microstructural interpretation applies to εr. Verification / Alternative check: The Clausius–Mossotti relation (εr − 1)/(εr + 2) = (N α)/(3 ε0) explicitly ties εr to number density N and polarizability α, confirming the statement. Why Other Options Are Wrong: (a) incorrectly attributes medium-dependent atomic interpretation to ε0. (c) reverses roles. (d) denies the well-established significance of εr. (e) dismisses both constants, which is incorrect. Common Pitfalls: Confusing absolute and relative permittivity roles. Assuming ε0 varies with environment; it does not. Final Answer: Only εr can be interpreted in terms of the atomic properties of the medium

Question 6

Low-temperature behavior — what happens to a semiconductor as T → very low? At very low temperatures (carrier freeze-out regime), a semiconductor most closely behaves as:

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Introduction / Context: Carrier concentration in semiconductors is strongly temperature dependent. As temperature decreases, thermal energy becomes insufficient to ionize dopants (in doped material) or excite intrinsic electron–hole pairs, leading to the freeze-out of carriers and a transition toward insulating behavior. Given Data / Assumptions: “Very low temperature” means well below room temperature, approaching cryogenic conditions. No strong optical or impact ionization; equilibrium conditions only. Standard semiconductors (Si, Ge, GaAs) without special impurity band conduction at the temperature considered. Concept / Approach: Carrier density n (and p) depends on temperature through dopant ionization and intrinsic excitation over the band gap. As T decreases, donors and acceptors fail to ionize (freeze-out), and intrinsic excitation is negligible. Conductivity σ = q (n μ_n + p μ_p) therefore drops dramatically, approaching insulating levels. Step-by-Step Solution: Lower T → fewer ionized dopants → n and/or p decrease sharply.Intrinsic n_i also falls exponentially with decreasing T.Hence σ falls; limiting behavior is insulating for most semiconductors. Verification / Alternative check: Conductivity vs. temperature curves show three regions: freeze-out (low T), extrinsic (moderate T), and intrinsic (high T). In the first region, conductivity is smallest, consistent with insulating behavior. Why Other Options Are Wrong: (a) reverses the trend; metals increase resistance slightly with T but remain conductive. (c) is not random; physics dictates freeze-out. (d) ignores the strong temperature dependence. (e) superconductivity is a separate phenomenon not typical for semiconductors. Common Pitfalls: Confusing semiconductor behavior with metallic behavior. Overlooking dopant ionization energy effects at low T. Final Answer: An insulator (carriers frozen out)

Question 7

Assertion–Reason (Chemistry of noble gases) Assertion (A): Helium, Argon, and Neon exist as gases at room temperature. Reason (R): The atoms of Helium, Argon, and Neon are chemically extremely inactive (noble/inert gases).

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Introduction: Assertion–Reason questions test whether you can judge the validity of two statements and also assess causation. Here, we analyze why Helium, Neon, and Argon are gaseous at room temperature and whether their chemical inertness fully explains this physical state. Given Data / Assumptions: Noble gases considered: He, Ne, Ar. Room temperature context (around 25 °C, 1 atm). Atomic (monoatomic) gases with closed-shell electron configurations. Concept / Approach: Noble gases are chemically inert due to filled valence shells that make formation of stable chemical bonds unlikely under ordinary conditions. Their gaseous state at room temperature is primarily a matter of weak intermolecular forces (London dispersion) and very low polarizability, which keep boiling and melting points extremely low. Chemical inactivity and weak intermolecular attractions share an underlying electronic cause (closed shells), but inertness itself is not the immediate explanation for the gaseous state; intermolecular force magnitude is. Step-by-Step Solution: Evaluate Assertion: Helium, Neon, Argon are indeed gases at room temperature → True.Evaluate Reason: They are chemically extremely inactive (noble gases) → True.Causation check: Being inert does not directly cause the gaseous state; rather, very weak dispersion forces and low polarizability keep the phase gaseous → Reason is not the correct explanation. Verification / Alternative check: Boiling points: He (−269 °C), Ne (−246 °C), Ar (−186 °C) are far below room temperature, consistent with weak intermolecular forces, independent of the notion of reactivity in chemical reactions. Why Other Options Are Wrong: Option A: claims correct explanation, which is inaccurate. Option C/D misjudge truth values. Option E is incorrect since both statements are true. Common Pitfalls: Equating chemical reactivity with physical state; ignoring the role of intermolecular forces and polarizability in determining boiling/melting points. Final Answer: Both A and R are true but R is not correct explanation of A

Question 8

Electronic components – tolerance band for a carbon resistor A carbon resistor has the colour bands green–blue–yellow (value bands shown, but no separate tolerance band). What is the tolerance of this resistor?

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Introduction: Resistor colour coding conveys both nominal resistance and tolerance. When a dedicated tolerance band (usually gold, silver, or none) is absent, the default tolerance must be inferred. This question checks recognition of the standard E-series coding convention for carbon resistors. Given Data / Assumptions: Three visible bands: green, blue, yellow (digits and multiplier). No fourth band (tolerance) explicitly present. Standard 4-band code conventions apply; if the tolerance band is missing, default tolerance is used. Concept / Approach: In the 4-band scheme: Band 1 = first digit, Band 2 = second digit, Band 3 = multiplier, Band 4 = tolerance. If the tolerance band is missing (i.e., only the first three bands are given), the resistor is taken to have the default tolerance of ± 20% for carbon composition/carbon film types, as per conventional coding used widely in legacy components and many educational contexts. Step-by-Step Solution: Recognize that green–blue–yellow provide only value, not tolerance.Absence of a tolerance band implies the default tolerance.Default tolerance (none) → ± 20%. Verification / Alternative check: By contrast, a gold band indicates ± 5%, silver indicates ± 10%. Their absence defaults to ± 20%, confirming the choice. Why Other Options Are Wrong: ± 5% and ± 10% require gold or silver bands; ± 30% is nonstandard in the typical modern code; ± 1% would require brown tolerance band in a 5-band or special series resistor. Common Pitfalls: Reading the third value band as tolerance; assuming every resistor must show a tolerance band explicitly. Final Answer: ± 20%

Question 9

Quantum relation – frequency of emitted/absorbed radiation during an electronic transition For an electron transitioning between energy levels W1 and W2, the electromagnetic radiation involved has frequency f satisfying which relation?

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Introduction: Atomic and solid-state transitions emit or absorb photons whose energy equals the difference between initial and final energy levels. This fundamental quantum result, introduced by Planck and applied by Einstein and Bohr, underpins spectroscopy, lasers, LEDs, and semiconductor optoelectronics. Given Data / Assumptions: Two discrete energy levels: W1 and W2. A single-photon process (emission or absorption). Planck’s constant h relates photon energy to frequency f. Concept / Approach: The photon energy equals the magnitude of the energy change: E_photon = |W2 − W1|. Planck’s relation states E_photon = h * f. Combining gives h * f = |W2 − W1|. The absolute value ensures positivity regardless of emission (W2 < W1) or absorption (W2 > W1). Step-by-Step Solution: Write energy conservation: E_photon = |ΔW| = |W2 − W1|.Use Planck relation: E_photon = h * f.Combine to obtain: h * f = |W1 − W2|. Verification / Alternative check: Wavelength can be found from c = f * λ; thus λ = h * c / |ΔW|, consistent with spectroscopy formulas. Why Other Options Are Wrong: Options B, C, D, and E rearrange or distort the relation; only a linear equality with h * f is correct. Common Pitfalls: Forgetting the absolute value; mixing frequency with angular frequency (ℏω) where ℏ = h / (2π). Final Answer: hf = |W1 - W2|

Question 10

Electrical insulation classes – impregnated paper Impregnated paper (e.g., paper with suitable insulating oil/varnish impregnation) belongs to which standard insulation class?

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Introduction: Insulation classes categorize materials by their maximum permissible temperature rise and operating temperature, guiding selection in transformers, motors, and other electrical apparatus. The class determines thermal endurance and safe design margins for long-term reliability. Given Data / Assumptions: Impregnated paper as a composite of cellulose and impregnating agent. Conventional/classical temperature classes: A, E, B, F, H, etc. Standardized temperature limits per class. Concept / Approach: Cellulose-based materials such as cotton, silk, and paper with suitable impregnation are traditionally placed in Class A insulation. Class A typically corresponds to a maximum permissible temperature of about 105 °C (sum of ambient plus allowable rise and hot-spot). Higher classes (B ≈ 130 °C, F ≈ 155 °C, H ≈ 180 °C) are for more thermally robust materials like mica, glass, and certain synthetic films/resins. Step-by-Step Solution: Identify material family: cellulose (paper) with impregnation.Map to insulation class tables: cellulose-based → Class A.State class: A (about 105 °C rating). Verification / Alternative check: Transformer design references consistently list paper–oil systems under Class A, confirming the classification. Why Other Options Are Wrong: Classes B, F, H imply higher thermal endurance materials; Class C historically referred to mica/ceramics with very high temperature capability. Common Pitfalls: Assuming all impregnated materials jump to higher classes; ignoring that the base cellulose still limits thermal endurance. Final Answer: A

Question 11

Crystal structures – body-centred cubic (BCC) occupancy In a body-centred cubic crystal structure, the eight corners and the cube centre are occupied by identical atoms. Is this statement correct?

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Introduction: Crystallography describes how atoms are arranged in repeating unit cells. Recognizing simple structures such as simple cubic, body-centred cubic (BCC), and face-centred cubic (FCC) is fundamental for interpreting material properties like packing efficiency, slip systems, and mechanical behaviour. Given Data / Assumptions: Ideal, perfect crystal without defects. Atoms identical at lattice points for an elemental BCC solid (e.g., α-iron at room temperature). Unit cell description according to conventional crystallography. Concept / Approach: In BCC, lattice points are at the eight cube corners and at the body centre. When atoms occupy these lattice points, the conventional picture is “corners plus centre” occupied by identical atoms for pure elements. Effective atom count per cell is 2 (8 corners × 1/8 + 1 centre × 1 = 2). This contrasts with FCC, which has atoms at corners and face centres (effective count 4). Step-by-Step Solution: Identify structure: BCC → lattice points at corners and body centre.Occupancy: identical atoms in pure BCC metals (e.g., Fe, Cr, W).Conclude the statement is correct. Verification / Alternative check: X-ray diffraction and crystallographic databases confirm BCC lattice for α-Fe, showing the body-centred atom. Why Other Options Are Wrong: “False” contradicts the BCC definition; the qualifier “only at 0 K” is unnecessary; centre is not empty in the occupied BCC lattice of pure metals. Common Pitfalls: Confusing BCC with simple cubic or FCC; miscounting atoms per unit cell. Final Answer: True

Question 12

Magnetic materials – variation of relative permeability with flux density An iron specimen has relative permeability µr = 1000 at flux density B = 0.30 T. When B increases to 0.35 T, how will µr typically change?

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Introduction: The B–H characteristics of ferromagnetic materials are nonlinear. Relative permeability µr varies with operating point and generally peaks near the knee of the magnetization curve before declining as the material moves toward saturation. Designers must understand this trend to avoid excessive magnetizing current and core losses. Given Data / Assumptions: Material: iron specimen (ferromagnet). Initial operating point: B = 0.30 T with µr = 1000. New operating point: B = 0.35 T (higher flux density). Concept / Approach: As B increases beyond the region of maximum incremental permeability, domains become increasingly aligned; the remaining unaligned domains contribute less to further increases in B per unit H, reducing incremental µ. Therefore, moving to a higher B (0.35 T) from 0.30 T typically results in a decrease in µr, unless the initial point was far below the knee (which is uncommon for iron near these B values). Step-by-Step Solution: Recognize nonlinearity: µr is not constant.Increasing B toward saturation → incremental µ falls.Therefore µr at 0.35 T < µr at 0.30 T → less than 1000. Verification / Alternative check: Typical catalog B–H curves for electrical steels show µr peaking at modest B (often 0.1–0.3 T), then declining toward higher B, confirming the qualitative change. Why Other Options Are Wrong: Constant or increased µr contradicts standard ferromagnetic behaviour near saturation; “may be more or less” is non-committal and usually untrue for this B range; zero is impossible for a ferromagnet in this region. Common Pitfalls: Assuming µr is constant like in linear dielectrics; ignoring that engineering “µr” quoted is operating-point dependent. Final Answer: will be less than 1000

Question 13

Solid-state physics – mean free path relation If v denotes the average electron speed between collisions and t_c is the mean time between collisions, then the mean free path λ is given by λ = v * t_c. Is this statement correct?

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Introduction: Transport in metals and semiconductors is often modeled using the Drude or semiclassical framework. Mean free path λ indicates the average distance a charge carrier travels between successive scattering events, crucial for mobility, conductivity, and nanoscale device analysis. Given Data / Assumptions: Average carrier speed v between collisions is well defined (e.g., drift-averaged or thermal-averaged appropriate to the model). Mean free time between collisions t_c is defined. Steady-state conditions with many scattering events. Concept / Approach: By definition, mean free path equals mean speed times the mean time between scattering events: λ = v * t_c. In a purely drift picture, v would be the average velocity; in thermal motion, a thermal speed can be used for order-of-magnitude estimates. The identity captures the intuitive notion that faster carriers or longer times between collisions yield longer paths between collisions. Step-by-Step Solution: Define λ as the ensemble-averaged distance between collisions.Distance = speed * time for each free-flight segment.Averaging over many events gives λ = v * t_c. Verification / Alternative check: Drude conductivity σ = n * e^2 * t_c / m relates to mobility µ = e * t_c / m; combining with typical thermal speeds allows cross-checks of λ scales in metals and semiconductors. Why Other Options Are Wrong: λ = v / t_c has wrong dimensions; restricting to 0 K or to holes only is unjustified. Common Pitfalls: Confusing drift velocity with thermal velocity; misusing instantaneous velocity instead of an appropriate average. Final Answer: True

Question 14

Resistor colour code – compute value with tolerance A carbon resistor shows the bands red–red–black–silver. Determine its resistance value (with tolerance).

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Introduction: Four-band resistor codes use two significant digits, a multiplier, and a tolerance band. Correctly reading the colours prevents order-of-magnitude mistakes in circuit assembly and troubleshooting. Given Data / Assumptions: Bands: 1st = red, 2nd = red, 3rd (multiplier) = black, 4th (tolerance) = silver. Standard 4-band colour code chart. Ohmic value sought in Ω with tolerance. Concept / Approach: Digits: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Multiplier: black = 10^0 = 1. Tolerance: silver = ± 10%. Thus the first two bands form the number 22, multiplied by 1, and tolerance from silver is ± 10%. Step-by-Step Solution: Read significant digits: red (2), red (2) → 22.Apply multiplier: black → × 10^0 = × 1 → 22 Ω.Tolerance: silver → ± 10%.Final: 22 ± 10% Ω. Verification / Alternative check: If the multiplier were brown, value would be 220 Ω; if gold were the tolerance band, it would be ± 5%. The provided colours uniquely determine 22 ± 10% Ω. Why Other Options Are Wrong: 2200 Ω and 22000 Ω require brown or red multipliers; ± 5% needs gold; 2.2 kΩ would need red–red–red–silver or equivalent. Common Pitfalls: Reading the bands from the wrong end; confusing black multiplier (×1) with digit 0 in the first position (first band is never black). Final Answer: 22 ± 10% Ω

Question 15

Dielectrics – orientation polarization dependence Orientation polarization in a polar dielectric depends on temperature and the permanent dipole moment p. Which statements are correct?  directly proportional to temperature  inversely proportional to…

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Introduction: Polar dielectrics possess permanent dipole moments. Under an applied electric field, these dipoles tend to align, producing orientation polarization. Thermal agitation resists alignment, and the magnitude of the permanent dipole determines the strength of alignment. Understanding this dependence is important in capacitor dielectrics and microwave materials. Given Data / Assumptions: Low-field, linear response regime. Classical Debye model for orientation polarization. Uniform temperature and isotropic material. Concept / Approach: The Debye expression for orientation polarization P_o is proportional to (n * p^2 / (3 * k * T)) * E in the linear limit. Hence, P_o ∝ p^2 and P_o ∝ 1/T. Larger permanent dipole moments give stronger polarization; higher temperature increases randomization, reducing polarization for a given field. Step-by-Step Solution: Identify temperature dependence: P_o ∝ 1 / T → inversely proportional (statement 2 true).Identify dipole dependence: P_o ∝ p^2 → directly proportional to the square (statement 4 true).Statements 1 and 3 contradict Debye relation → false. Verification / Alternative check: Dielectric constant ε′ for polar liquids decreases with rising temperature (orientation part diminishes), consistent with P_o ∝ 1/T; materials with larger molecular dipoles show larger orientation contributions, consistent with p^2 dependence. Why Other Options Are Wrong: Options A, B, C include incorrect statements 1 or 3; only D includes both correct statements (2 and 4). Common Pitfalls: Assuming linear proportionality to p rather than p^2; overlooking thermal agitation’s inverse role. Final Answer: 2 and 4 only

Question 16

Magnetic circuits – necessity of magnetomotive force (mmf) To establish magnetic flux in a magnetic circuit or air gap, is a magnetomotive force (mmf) necessary?

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Introduction: Magnetic circuits are often analyzed by analogy to electric circuits. Magnetomotive force (mmf) is the driving “cause” of flux, just as electromotive force drives electric current. Recognizing the role of mmf helps in sizing windings, currents, and gaps in devices like transformers, relays, and motors. Given Data / Assumptions: Magnetic circuit with ferromagnetic path and/or air gaps. Steady or slowly varying fields (magnetostatics/quasi-static). mmf provided by current in turns (N * I) or by permanent magnets (intrinsic mmf). Concept / Approach: Flux Φ in a magnetic circuit follows Φ = F / R_m, where F is mmf and R_m is magnetic reluctance. Nonzero flux requires nonzero net mmf, whether supplied by a coil (N * I) or by the magnetization of a permanent magnet (equivalent mmf). Even with high-permeability cores, some mmf is required, particularly to overcome air-gap reluctance. Step-by-Step Solution: Define mmf: F = N * I (for a current-driven coil) or an equivalent for a permanent magnet.Relate to flux: Φ = F / R_m → no mmf implies no flux in a passive circuit.Conclude: To produce flux, mmf is necessary. Verification / Alternative check: Devices with permanent magnets demonstrate that magnetization provides mmf without external current, yet mmf is still present in the magnetic circuit description. Why Other Options Are Wrong: Claiming mmf is unnecessary contradicts the fundamental relation; limiting necessity to air gaps or AC is incorrect, as DC magnetization also needs mmf. Common Pitfalls: Confusing permeability with absence of required mmf; neglecting air gap dominance in mmf budgeting. Final Answer: True

Question 17

Resistor Colour Code – Meaning of a Silver Third Band In the colour-coding scheme for carbon (composition/film) resistors, if the third band (the multiplier band) is silver, what does it indicate for the resistance value?

Accepted Answer

Introduction / Context: The resistor colour code is a compact way to print resistance values and tolerances on small components. In the common 4-band code, the first two bands give the significant figures, the third band is the multiplier, and the fourth band is the tolerance. Understanding the special meanings of gold and silver in the multiplier position is essential, because unlike other colours (which multiply by powers of 10^+n), these indicate fractional multipliers (10^-1 and 10^-2). Given Data / Assumptions: Carbon resistor using the standard colour code. Third band (multiplier band) is silver. We are asked to interpret the resistance value effect, not tolerance (unless the fourth band is specified). Concept / Approach: In the 4-band code: band 1 = first significant digit, band 2 = second significant digit, band 3 = multiplier, band 4 = tolerance. Multipliers map colours to powers of ten. Notably, gold corresponds to 10^-1 (0.1) and silver corresponds to 10^-2 (0.01). Thus a silver multiplier reduces the two-digit number by a factor of 100. Tolerance is read from the fourth band (e.g., gold ±5%, silver ±10%); the third band never indicates tolerance in a 4-band code. Step-by-Step Solution: Identify the role of the third band: multiplier.Recall mapping: silver → 10^-2.Therefore, the indicated resistance is (two-digit number) * 0.01 Ω.Hence, a silver third band means the multiplier is 0.01. Verification / Alternative check: Compare with gold in the multiplier position: gold → 10^-1 (0.1). This confirms that precious-metal colours in the multiplier band represent fractional powers of ten, with silver being 10^-2. Why Other Options Are Wrong: Tolerance ±5% refers to a gold fourth band, not a multiplier. Multiplier 0.1 corresponds to gold, not silver. “Three zeros appended” corresponds to orange as multiplier (10^3), not silver. Common Pitfalls: Confusing the third and fourth bands; assuming silver always means ±10% (only true when silver is the tolerance band, i.e., the fourth band). Final Answer: Multiplier is 0.01

Question 18

Ferromagnetism vs Temperature – Curie Point Behaviour (Assertion–Reason) Assertion (A): Ferromagnetic materials exhibit ferromagnetic properties only when the temperature is below the (ferromagnetic) Curie temperature θf. Reason (R): For a ferromagn…

Accepted Answer

Introduction / Context: Ferromagnets show spontaneous magnetization due to exchange coupling among atomic moments. This long-range order is temperature sensitive. Above a critical temperature (Curie point), thermal agitation disrupts alignment and the material transitions to paramagnetic behavior with no spontaneous magnetization. The Assertion–Reason pair explores this fundamental thermal transition. Given Data / Assumptions: Ferromagnetic material with Curie temperature θf (also written Tc). Spontaneous magnetization present only below θf. Classical/quantum statistical descriptions (Weiss/Heisenberg models) apply. Concept / Approach: Below θf, exchange interactions dominate over thermal disorder, producing spontaneous domain alignment (ferromagnetism). At T ≥ θf, thermal energy is sufficient to overcome cooperative ordering; the susceptibility follows the Curie–Weiss law, χ ∝ 1/(T − θ), and the material behaves paramagnetically without remanence or coercivity in the zero-field limit. Hence, both statements are true and the Reason directly explains the Assertion. Step-by-Step Solution: Identify phase: T < θf → ferromagnetic order present.Increase temperature beyond θf → order vanishes; becomes paramagnetic.Therefore, A is true; R correctly states the mechanism/condition causing A. Verification / Alternative check: Experimentally, magnetization–temperature curves show spontaneous magnetization dropping to zero at θf, with susceptibility changing to Curie–Weiss behavior above θf. Why Other Options Are Wrong: A and R not independent; R explicitly explains A by stating the high-temperature phase. Claiming R is false contradicts established magnetic phase transitions. Common Pitfalls: Confusing the “paramagnetic Curie temperature” sign conventions; forgetting that antiferromagnets have a Néel temperature, not a Curie point. Final Answer: Both A and R are true and R is correct explanation of A

Question 19

Orientation Polarization Decay After Field Removal A polar liquid contains N permanent dipoles of moment μp per unit volume. After a long DC field application, the orientation polarization equals P0. At time t = 0 the field is suddenly removed. What…

Accepted Answer

Introduction / Context: In dielectrics with permanent dipoles (polar liquids), an applied electric field partially aligns the dipoles, producing orientation polarization. When the field is removed, thermal agitation randomizes orientations again. The kinetics of this relaxation are classically described by a first-order (Debye) process with a characteristic relaxation time τ. Given Data / Assumptions: Permanent dipoles of density N and moment μp. Steady orientation polarization P0 reached under a DC field. At t = 0 the field is set to zero; linear, isothermal response. Concept / Approach: Debye theory models rotational diffusion in a viscous medium, leading to exponential relaxation of polarization after a step change in the field. After removal, the driving torque vanishes and randomizing collisions cause P(t) to decay from P0 to 0 with time constant τ that depends on viscosity, temperature, and molecular size. Step-by-Step Solution: Model polarization dynamics as dP/dt = −(P/τ) after field removal.Integrate: P(t) = P0 * exp(−t/τ).As t → ∞, P(t) → 0; at t = 0+, P is continuous (no instantaneous drop to 0). Verification / Alternative check: Dielectric relaxation spectroscopy measures frequency-dependent permittivity; the time-domain equivalent shows exponential decay following a field step, confirming the Debye-type response. Why Other Options Are Wrong: Constant P0 contradicts thermal randomization once the aligning field is removed. Instantaneous zero would imply infinite relaxation rate, unphysical for molecular rotations. Dependence on initial field strength does not change the qualitative exponential decay for linear response. Common Pitfalls: Confusing electronic/atomic polarization (nearly instantaneous) with orientation polarization (viscous, time-dependent); assuming non-causal jumps in macroscopic polarization. Final Answer: It decays toward zero with a characteristic time constant

Question 20

Thermistors – Sign of the Temperature Coefficient of Resistance For a typical thermistor device used in sensing applications, what is the sign of its temperature coefficient of resistance (TCR)?

Accepted Answer

Introduction / Context: Thermistors are temperature-sensitive resistors widely used for measurement, compensation, and protection. Two broad types exist: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). In common sensing roles, NTC thermistors dominate, exhibiting a decrease in resistance with increasing temperature. Given Data / Assumptions: General term “thermistor” in typical instrumentation context. Focus on most widely used sensing elements. Operating over moderate temperature ranges where behavior is well characterized. Concept / Approach: In NTC thermistors (e.g., metal oxide ceramics), carrier concentration increases significantly with temperature due to semiconductor-like behavior, causing resistance to drop; thus dR/dT < 0. PTC thermistors exist (e.g., doped polycrystalline barium titanate), but are more specialized for switching/overcurrent protection. Therefore, asked generally, the expected TCR is negative. Step-by-Step Solution: Identify common thermistor type used for sensing → NTC.Relate semiconductor physics: increasing T raises carrier density → lowers R.Conclude sign: negative temperature coefficient. Verification / Alternative check: Manufacturers’ datasheets show resistance–temperature curves descending with temperature (e.g., 10 kΩ at 25 °C dropping to a few kΩ at elevated temperatures). Why Other Options Are Wrong: Positive/zero: do not describe NTC behavior. “May be positive or negative” is true across all thermistor families, but the typical sensor thermistor is NTC; the question seeks the prevalent characteristic. Common Pitfalls: Assuming all thermistors are NTC in every application; PTC devices are common in resettable fuses and motor protection. Final Answer: Negative

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