Question 1

Dielectric Losses in Liquids and Glasses – Assertion–Reason Assertion (A): Dielectric losses occur in liquids and glassy (amorphous) substances under alternating electric fields. Reason (R): Orientation polarization in many substances is frequency d…

Accepted Answer

Introduction / Context: Dielectric loss is the conversion of electrical energy into heat within an insulating material when subjected to alternating fields. Liquids and glasses often exhibit significant losses due to molecular orientation dynamics and relaxation processes that cannot follow rapid field reversals without phase lag. Given Data / Assumptions: Alternating electric field applied to polar or partially polar media. Orientation (dipolar) polarization present for molecules with permanent dipole moments. Viscous media where rotational diffusion is finite (non-instantaneous). Concept / Approach: Orientation polarization responds to the applied field with a characteristic relaxation time τ. When the excitation frequency is comparable to 1/τ, the dipoles lag behind the field, introducing a quadrature component of polarization and thus energy dissipation per cycle (dielectric loss). Liquids and glasses, with their molecular mobility constraints and distribution of relaxation times, show pronounced frequency-dependent loss tangents. Step-by-Step Solution: Recognize that P(t) in a polar medium cannot follow E(t) instantly at higher f.A phase lag between P and E causes non-zero loss per cycle.Therefore, A is true; and R identifies the frequency dependence causing the loss, which explains A. Verification / Alternative check: Dielectric spectroscopy reveals relaxation peaks (Debye or non-Debye) in loss vs frequency plots for many liquids and glasses, confirming the explanation. Why Other Options Are Wrong: R false: contradicted by extensive experimental data. R not explanatory: it precisely explains the mechanism (frequency-dependent orientation). Common Pitfalls: Confusing electronic/atomic polarization (nearly instantaneous, low loss) with orientation polarization; neglecting temperature dependence of τ which shifts loss peaks. Final Answer: Both A and R are true and R is correct explanation of A

Question 2

Polarization–Field Relation for an Insulator For a linear, isotropic insulating material with relative permittivity εr, what is the correct expression for polarization P (dipole moment per unit volume) as a function of electric field E?

Accepted Answer

Introduction / Context: In linear dielectrics, polarization is proportional to the applied electric field. The proportionality involves permittivity and susceptibility, relating the microscopic dipole alignment to macroscopic field variables. The correct algebra prevents double-counting εr when switching between D, E, and P relations. Given Data / Assumptions: Linear, isotropic, homogeneous dielectric. Constitutive relations: D = ε E, with ε = εr ε0. Polarization defined by D = ε0 E + P. Concept / Approach: Combine D = ε E with D = ε0 E + P to get εr ε0 E = ε0 E + P ⇒ P = (εr − 1) ε0 E. This is equivalent to P = χe ε0 E with χe = εr − 1. Using this prevents common mistakes like setting P equal to ε E, which would misstate units and physical meaning. Step-by-Step Solution: Start: D = ε E = εr ε0 E.Also: D = ε0 E + P.Subtract: P = (εr − 1) ε0 E. Verification / Alternative check: In vacuum, εr = 1 ⇒ P = 0 as expected; in strong dielectrics, εr ≫ 1 ⇒ P grows proportionally, matching physical intuition. Why Other Options Are Wrong: Options A and D incorrectly multiply by εr without subtracting the vacuum part. Option B is the vacuum displacement term, not polarization. Option E contradicts observed behavior of real dielectrics. Common Pitfalls: Confusing D, E, and P; forgetting that P measures the excess over vacuum response. Final Answer: P = ε0 * (εr − 1) * E

Question 3

Charge Transport in Semiconductors In a semiconductor under typical operating conditions, the electrical current is carried by which species?

Accepted Answer

Introduction / Context: Semiconductor devices rely on two kinds of mobile charge carriers: electrons (negative charge) and holes (effective positive charge). Their relative contributions depend on doping, injection, illumination, and bias. Understanding that both carrier types can contribute simultaneously is fundamental to analyzing p–n junctions, BJTs, and CMOS circuits. Given Data / Assumptions: Crystalline semiconductor (e.g., Si, Ge, GaAs). No extreme conditions like ionic conduction in electrolytes. Standard temperatures and fields where electronic transport prevails. Concept / Approach: In an intrinsic semiconductor, n = p and both species carry comparable current. In doped (extrinsic) material, majority carriers dominate, but minority carriers still contribute, especially across junctions and under forward bias where injection occurs. Thus, total current density J = q (n μn E + p μp E) + diffusion terms, showing additive electron and hole contributions. Step-by-Step Solution: Identify typical carriers in solids: electrons and holes.Write drift components: J_drift = q (n μn + p μp) E.Acknowledge diffusion: J_diff = q (D_n ∇n − D_p ∇p).Conclude that current arises from both electrons and holes. Verification / Alternative check: Hall-effect measurements and diode I–V characteristics require both carrier types to explain observed behavior (e.g., minority carrier storage, transistor action). Why Other Options Are Wrong: “Only” electrons or holes ignores minority transport and intrinsic cases. Significant ionic conduction occurs in electrolytes/ionic solids, not standard semiconductors. Common Pitfalls: Assuming minority carriers are negligible in all situations; overlooking diffusion components in non-uniform doping or under gradients. Final Answer: Both holes and electrons

Question 4

Ionic Dielectrics – Components of Polarization (Assertion–Reason) Assertion (A): In ionic dielectrics, the total polarization can be written as P = Pe + Pi (sum of electronic and ionic polarization). Reason (R): In equilibrium, an ideal ionic crysta…

Accepted Answer

Introduction / Context: Polarization mechanisms in solids include electronic polarization (displacement of electron clouds relative to nuclei), ionic polarization (relative displacement of positive and negative ions), and orientation polarization (alignment of permanent molecular dipoles). Ionic crystals like NaCl do not possess permanent dipoles in their centrosymmetric equilibrium structures, so their polarization under field arises from induced mechanisms only. Given Data / Assumptions: Ideal stoichiometric ionic crystal with centrosymmetric unit cell. No pre-existing permanent molecular dipoles in equilibrium. Linear response regime under applied electric field. Concept / Approach: Because orientation polarization requires permanent dipoles, an ionic crystal contributes primarily Pe (electronic) and Pi (ionic) when an external field is applied. The electronic part arises from deformation of electron clouds; the ionic part from small relative shifts of sublattices of cations and anions. Since there is no intrinsic permanent dipole moment in equilibrium (by symmetry), orientation polarization is absent, justifying the decomposition P = Pe + Pi. Step-by-Step Solution: State the components: P_total = Pe + Pi (+ possibly Por if permanent dipoles exist).For ionic crystals: Por = 0 in equilibrium because the unit cell has no permanent dipole.Therefore, P = Pe + Pi for ionic dielectrics, and R explains why orientation polarization is not included. Verification / Alternative check: Infrared/optical spectroscopy shows optic phonon modes associated with ionic displacement (Pi) and electronic polarizability (Pe); absence of permanent dipole alignment confirms R. Why Other Options Are Wrong: If R were false, one would expect an orientation term; typical ionic crystals lack it due to symmetry. Declaring A false contradicts standard dielectric theory for ionic solids. Common Pitfalls: Confusing ferroelectric ionic crystals (where symmetry is broken and spontaneous polarization exists) with ordinary ionic dielectrics; mixing up induced and permanent dipoles. Final Answer: Both A and R are true and R is correct explanation of A

Question 5

Atomic core charge in a neutral copper (Cu) atom In solid-state physics, the 'ion core' (or atomic core) is defined as the nucleus plus all core (non-valence) electrons. For a neutral copper atom (atomic number 29), where the single 4s electron is c…

Accepted Answer

Introduction / Context: In materials science and solid-state physics, we often separate a neutral atom into a positively charged “ion core” (nucleus plus core electrons) and the outer valence electrons that participate in bonding and conduction. This partition helps in building pseudopotentials and explaining electron behavior in metals and semiconductors. For copper (Cu), recognizing the charge state of the core is a key first step. Given Data / Assumptions: Copper atomic number Z = 29. Neutral atom has total electron charge −29 e and nuclear charge +29 e. Valence configuration of Cu is typically taken as 4s^1 (one valence electron). “Core” = nucleus (+29 e) + 28 core electrons (−28 e) = ion core used in many models. Concept / Approach: The ion-core concept removes the valence electron(s) from the neutral atom and groups the remainder into an effective core. The core's net charge is the nuclear charge minus the charge of all core electrons. Because a neutral Cu atom has one valence electron outside the core, the core must be net positive by +1 e to balance the missing valence charge when reconstructing neutrality. Step-by-Step Solution: Start with neutrality: (+29 e) + (−29 e) = 0.Identify the valence electron: one 4s electron (charge −1 e) is excluded from the core.Core now contains +29 e (nucleus) and −28 e (core electrons) → net core charge = +1 e. Verification / Alternative check: If we recombine the +1 e core with the one valence electron (−1 e), total returns to zero, confirming neutrality. This convention is standard in pseudopotential and nearly-free-electron treatments for Cu-based metals and alloys. Why Other Options Are Wrong: 0 would mean no net core charge, contradicting the definition that removes one valence electron. −1 or −2 would imply the core is negative, which cannot balance neutrality when the valence electron is also negative. +2 would correspond to removing two valence electrons, not appropriate for elemental Cu with one 4s valence electron in this model. Common Pitfalls: Confusing “ion core” with an ionic oxidation state in chemistry; here it is a modeling construct, not the chemical Cu^+ ion in a compound. Final Answer: +1

Question 6

Parallel-plate capacitor with a thin floating metal plate inserted A parallel-plate capacitor has plate area A, separation d, and capacitance C. A thin, perfectly conducting plate P of area A and negligible thickness is inserted midway between the p…

Accepted Answer

Introduction / Context: Understanding how inserting conductors or dielectrics between capacitor plates alters the equivalent capacitance is crucial in sensor design, shielding, and high-voltage insulation. A floating (unconnected) metal sheet partitions the electric field but does not change the net energy storage in a way that alters the overall capacitance, as long as geometry is idealized and fringe effects are neglected. Given Data / Assumptions: Original capacitor: area A, separation d, capacitance C = ε A / d (uniform field, no fringing). A thin, perfectly conducting plate of area A is inserted at mid-plane and remains electrically floating (isolated). No dielectric other than vacuum/air; negligible fringing. Concept / Approach: The floating conductor divides the single capacitor into two capacitors in series, each with gap d/2. Each sub-capacitance is C1 = ε A / (d/2) = 2C. Two equal capacitors 2C in series combine to give C_eq via 1/C_eq = 1/(2C) + 1/(2C) = 1/C → C_eq = C. Thus, insertion of a floating ideal metal sheet leaves the overall capacitance unchanged in this ideal model. Step-by-Step Solution: Compute each half-gap capacitance: C1 = ε A / (d/2) = 2C.Series combination: 1/C_eq = 1/C1 + 1/C1 = 2/(2C) = 1/C.Therefore C_eq = C (no net change). Verification / Alternative check: Energy approach: for a fixed voltage V, the energy in each of the two sub-capacitors sums to U_total = 1/2 * C_eq * V^2. With equal partition and equal potential drop, the computed U_total corresponds to C_eq = C, consistent with the series calculation. Why Other Options Are Wrong: 2C would occur if the two halves were in parallel (not the case). 0.5 C or 0.25 C would require increasing separation or placing dielectrics reducing effective ε; none applies. C/3 is arbitrary and not derivable from the geometry. Common Pitfalls: Confusing a floating insert with a plate connected to one terminal (which would change boundary conditions) or ignoring that two identical series capacitors simply revert to the original C. Final Answer: C

Question 7

Intrinsic semiconductor drift current expression and intrinsic concentration formula (assertion–reason) Assertion (A): In an intrinsic semiconductor, the drift current density is J = (μn × μp) * e * ni. Reason (R): The intrinsic carrier concentratio…

Accepted Answer

Introduction / Context: Two cornerstone relations in semiconductor physics are the drift current formula and the temperature dependence of intrinsic carrier concentration. Getting the exact forms right is essential for device modeling and for understanding how conductivity changes with electric field and temperature in intrinsic materials like ultrapure silicon and germanium. Given Data / Assumptions: Intrinsic semiconductor (n = p = ni). Uniform electric field E is the driving force for drift current. Mobility symbols: μn (electrons) and μp (holes); e is elementary charge. Temperature dependence: bandgap Eg0, Boltzmann constant k, material constant A0. Concept / Approach: The correct drift current density in an intrinsic semiconductor is J = e * ni * (μn + μp) * E. It is additive in mobilities, not multiplicative, and it is proportional to E. Therefore, the assertion J = (μn × μp) e ni (with no E and product of mobilities) is dimensionally and physically incorrect. On the other hand, the relation ni^2 = A0 * T^3 * exp(−Eg0/(kT)) is the standard expression (up to material-specific constants) for intrinsic concentration in many semiconductors, capturing the strong exponential increase with temperature. Step-by-Step Solution: Write correct drift current: J = e ni (μn + μp) E.Compare with asserted form: product μn μp and no E are wrong → Assertion is false.Check Reason: ni^2 ∝ T^3 exp(−Eg0/(kT)) → widely accepted relation → Reason is true. Verification / Alternative check: Dimensional analysis confirms that without E, the asserted J cannot have units of A/m^2. Textbook derivations from band theory and effective density of states lead to the stated ni^2 expression. Why Other Options Are Wrong: Any option claiming Assertion true contradicts both physics and units; any option denying the standard ni relation is likewise incorrect. Common Pitfalls: Mixing drift with diffusion current or forgetting the additive contribution of electron and hole mobilities in intrinsic material. Final Answer: A is false but R is true

Question 8

Characteristics of a good dielectric material (engineering insulation) Which combination best describes the desired properties of an engineering dielectric intended for capacitors, cables, and high-voltage insulation?

Accepted Answer

Introduction / Context: Dielectrics are used wherever electric fields must be sustained with minimal loss and without breakdown — from power capacitors and cables to printed circuit boards and sensor dielectrics. A “good” dielectric should meet multiple criteria simultaneously, not just a high permittivity value. This question consolidates the key performance attributes designers look for in practice. Given Data / Assumptions: Operating in AC or DC fields with acceptable temperature rise. Materials compared at similar thickness and environmental conditions. Focus on bulk properties important for reliability and efficiency. Concept / Approach: (a) Low dielectric loss (low loss tangent tan δ) minimizes I^2R-like heating and improves efficiency and stability; it is crucial for RF, high-frequency, and precision applications. (b) Good thermal conductivity helps to remove whatever heat is generated, preventing thermal runaway and extending lifetime; modern dielectrics may be filled with ceramic particles to enhance heat conduction. (c) High intrinsic dielectric strength allows the material to withstand high electric fields before breakdown, enabling compact, high-voltage designs. The best dielectrics balance these traits to meet mechanical, chemical, and cost constraints as well. Step-by-Step Solution: Identify key requirement 1: low loss → reduces self-heating and energy waste.Identify key requirement 2: good heat conduction → dissipates any generated heat.Identify key requirement 3: high strength → resists breakdown at required field levels. Verification / Alternative check: Standards such as IEC and ASTM tests on tan δ, thermal conductivity, and breakdown strength collectively determine suitability in capacitors, cables, and HV bushings. Why Other Options Are Wrong: Picking only one or two properties ignores the multidisciplinary constraints. Real insulation systems fail if any of these is inadequate. Common Pitfalls: Overemphasizing dielectric constant εr while neglecting thermal and loss performance, or vice versa. Final Answer: All of the above

Question 9

Temperature coefficient of resistivity for semiconductors How does the resistivity of intrinsic or lightly doped semiconductors vary with temperature at ordinary temperatures?

Accepted Answer

Introduction / Context: Unlike metals, semiconductors show strong temperature-dependent conductivity due to changes in carrier concentration and mobility. Recognizing the sign of the temperature coefficient is foundational for selecting sensors (thermistors), designing bias networks, and predicting device behavior across ambient conditions. Given Data / Assumptions: Intrinsic or lightly doped semiconductor (e.g., Si, Ge) near room temperature. Dominant effect with rising temperature is an increase in thermally generated carriers. Moderate fields; no impact ionization or self-heating anomalies. Concept / Approach: The conductivity σ = e (n μn + p μp). In intrinsic material, n = p = ni, and ni increases steeply with temperature as ni ∝ T^(3/2) exp(−Eg/(2 k T)). Although carrier mobilities μ typically decrease with temperature due to enhanced phonon scattering, the exponential increase in ni dominates, causing σ to increase and thus resistivity ρ = 1/σ to decrease with T. Hence the temperature coefficient of resistivity is negative in the normal operating range. Step-by-Step Solution: Note ni(T) increases rapidly with T.σ(T) ∝ ni(T) despite μ(T) declining somewhat.Therefore ρ(T) decreases with T → negative temperature coefficient. Verification / Alternative check: NTC thermistors exploit this behavior: resistance drops markedly as temperature rises, enabling temperature sensing and inrush current limiting (with proper design). Why Other Options Are Wrong: (a) describes metallic behavior; (c), (d), and (e) are not representative of semiconductor physics at ordinary temperatures under DC bias. Common Pitfalls: Confusing doped semiconductor trends at extreme doping levels or very low temperatures with the intrinsic/lightly doped regime. Final Answer: Negative temperature coefficient (resistivity decreases with T)

Question 10

Highest dielectric strength among common insulating materials Among the following materials, which typically exhibits the highest dielectric strength (breakdown field): mica, air, cotton, or rubber?

Accepted Answer

Introduction / Context: Dielectric strength indicates how much electric field a material can withstand before electrical breakdown. It is a key selection parameter for capacitors, slot liners, and high-voltage insulation. Different materials span orders of magnitude in breakdown field, influenced by purity, microstructure, and moisture content. Given Data / Assumptions: Representative values at room temperature, dry conditions, and standard test thicknesses. Comparative, order-of-magnitude reasoning; exact values vary by grade and test method. Concept / Approach: Mica is a laminated, layered silicate with very high dielectric strength, typically on the order of 100–200 kV/mm (or higher for special grades). Rubber (and many polymers) are strong insulators but usually in the 20–40 kV/mm range. Dry air breaks down near 3 kV/mm under uniform fields. Cotton, being a fibrous organic material that readily absorbs moisture, has low and inconsistent dielectric strength. Therefore, among the listed choices, mica clearly provides the highest breakdown field and is widely used as a premium insulation and capacitor dielectric. Step-by-Step Solution: Air ≈ 3 kV/mm → low.Rubber ≈ 20–40 kV/mm → moderate.Cotton → low and moisture-sensitive.Mica ≈ 100–200 kV/mm → highest among given options. Verification / Alternative check: Capacitor industries employ mica sheets in high-voltage, high-reliability applications, confirming its superior breakdown properties compared to common polymers and air gaps. Why Other Options Are Wrong: Air and cotton are weak dielectrics; rubber, while decent, does not match mica’s breakdown field. Common Pitfalls: Overlooking moisture effects (cotton and even rubber can degrade substantially when wet). Test geometry and defects also lower apparent dielectric strength. Final Answer: Mica

Question 11

Dipole model of polarization in a dielectric under electric field When a homogeneous dielectric is subjected to an external electric field E, can each infinitesimal volume element be modeled as possessing an electric dipole moment (leading to a pola…

Accepted Answer

Introduction / Context: Macroscopic polarization is a central concept in dielectrics. It describes how bound charges in a material shift slightly under an applied electric field, producing an average dipole moment density. Modeling each differential volume as carrying a dipole helps link microscopic charge displacements to macroscopic field equations (Maxwell’s equations with polarization and bound charge densities). Given Data / Assumptions: Linear, isotropic dielectric for simplicity (though the idea extends more generally). Small fields such that response is linear: P = ε0 χe E. No free charge present within the dielectric volume under consideration. Concept / Approach: The polarization vector P is defined as electric dipole moment per unit volume, P = dp/dV. In the continuum viewpoint, any sufficiently small volume element ΔV within a polarized medium possesses a net dipole moment p = P ΔV. This does not require permanent dipoles; even in nonpolar materials, electronic and ionic displacements create induced dipoles proportional to E. This local dipole model leads to bound charge densities ρb = −∇·P and surface bound charge σb = P·n̂ at interfaces, which correctly predict field distributions in composite dielectrics. Step-by-Step Solution: Define P as dipole moment per unit volume.Associate each small element ΔV with dipole p = P ΔV.Use P to compute bound charges and interface conditions. Verification / Alternative check: Microscopic models (electronic/ionic/orientational polarization) average to the same macroscopic P. Measurements of capacitance and displacement current confirm the continuum description. Why Other Options Are Wrong: Restriction to ionic crystals or gases is unnecessary; permanent dipoles are not required for induced polarization. Common Pitfalls: Confusing permanent molecular dipoles with induced dipoles; both are encompassed by the macroscopic P field. Final Answer: True

Question 12

Extrinsic semiconductor doping level and effect (assertion–reason) Assertion (A): Doping levels in extrinsic semiconductors are extremely small fractions of the host atoms. Reason (R): Adding impurity atoms at a ratio of about 1 in 10^8 parts can in…

Accepted Answer

Introduction / Context: Doping transforms intrinsic semiconductors into n- or p-type extrinsic materials by introducing donors or acceptors at extremely low atomic fractions. Even minute dopant concentrations radically change carrier density and conductivity. This assertion–reason item probes both the qualitative smallness of doping and the quantitative impact on carriers. Given Data / Assumptions: Host atomic density for Si ≈ 5×10^22 atoms/cm^3 (order of magnitude). Intrinsic carrier concentration at 300 K: ni ≈ 10^10 cm^−3 for Si. Typical dopings: 10^13–10^18 cm^−3, i.e., atomic fractions as low as 10^−10 to 10^−4. Concept / Approach: Assertion: True. The fraction of dopant atoms is tiny relative to host atoms, yet it dominates electrical properties. Reason: False as stated. A ratio of 1 in 10^8 corresponds to about 5×10^14 cm^−3 in Si, which exceeds ni by roughly 10^4–10^5, not merely “about 20”. Thus the numerical claim understates the effect by many orders of magnitude. Although the spirit that “very little dopant makes a big difference” is correct, the stated factor of ~20 is inaccurate. Step-by-Step Solution: Compute dopant density: ND ≈ 5×10^22 / 10^8 = 5×10^14 cm^−3.Compare with ni ≈ 10^10 cm^−3 → ratio ≈ 5×10^4.Therefore R is quantitatively false; A remains true. Verification / Alternative check: Device textbooks show conductivity changing by many orders of magnitude when moving from intrinsic to doped silicon even at ppm–ppb dopant levels. Why Other Options Are Wrong: Any option treating R as true or as a correct explanation of A conflicts with the realistic numbers for silicon at room temperature. Common Pitfalls: Confusing fractional dopant level with fractional change in carriers; the former can be tiny while the latter is huge. Final Answer: A is true but R is false

Question 13

Automatic control of street lights – suitable sensor component Which component is commonly used to sense ambient light for automatic switching of street lights (dusk-to-dawn control)?

Accepted Answer

Introduction / Context: Automatic street lighting requires a reliable, inexpensive way to detect ambient illumination so that lamps switch on at dusk and off at dawn. Several passive and active components exist, but not all are suited to sensing light directly. The most common low-cost choice is an LDR (light-dependent resistor), also called a photoconductor cell. Given Data / Assumptions: Goal: detect ambient light level with a simple threshold circuit. Cost and ruggedness matter for outdoor installations. We need a component whose resistance changes strongly with light intensity. Concept / Approach: Photoconductors (e.g., CdS or CdSe LDRs) have resistance that drops dramatically when illuminated and rises in darkness. A simple voltage divider feeding a comparator or transistor can switch a relay or a triac to control the lamp. Thermistors respond to temperature, not light. Transistors require a preceding photosensor or phototransistor/photodiode variant to detect light. Varistors respond to voltage surges and are used for protection, not sensing, while Zener diodes provide references or clamps, not light sensing. Step-by-Step Solution: Select component with R = f(light): LDR fits.Build divider: LDR + fixed resistor → voltage tied to light level.Set threshold via comparator or transistor base bias → drives relay for lamp. Verification / Alternative check: Commercial dusk-to-dawn switches widely use LDRs or photodiodes; LDR-based designs are ubiquitous for cost-sensitive applications. Why Other Options Are Wrong: Thermistors sense temperature; varistors clamp surges; a plain transistor needs a light-sensitive front end; Zeners regulate voltage. Common Pitfalls: Ignoring ambient temperature effects on LDR resistance; proper hysteresis and weatherproofing improve reliability. Final Answer: Photoconductor / LDR (light-dependent resistor)

Question 14

Lossy (leaky) dielectrics and current components A leaky dielectric has complex relative permittivity εr* = εr′ − j εr″. When an electric field E is applied, is the conduction-like current component that is in phase with E proportional to εr″?

Accepted Answer

Introduction / Context: In AC fields, dielectrics exhibit both displacement (reactive) and loss (resistive) currents. The complex permittivity εr* = εr′ − j εr″ compactly represents this behavior. Understanding which part controls the in-phase (dissipative) current is vital for estimating dielectric heating and specifying materials for RF and power applications. Given Data / Assumptions: Time-harmonic electric field with angular frequency ω. Linear, isotropic material with small to moderate losses. Engineering sign convention: negative imaginary part indicates loss. Concept / Approach: The current density in a linear dielectric can be written as J = ∂D/∂t = j ω ε0 εr* E = j ω ε0 (εr′ − j εr″) E. Separating components: J = j ω ε0 εr′ E + ω ε0 εr″ E. The first term (with j) is 90° out of phase with E (purely reactive, displacement current), while the second term is in phase with E and represents power dissipation. Hence, the in-phase current component magnitude is proportional to εr″ (and to ω and ε0). Power loss density is p_loss = Re{E · J*} = ω ε0 εr″ |E|^2, also showing linear proportionality to εr″. Step-by-Step Solution: Write J = j ω ε0 εr* E.Expand: J = j ω ε0 εr′ E + ω ε0 εr″ E.Identify in-phase component: ω ε0 εr″ E → proportional to εr″. Verification / Alternative check: Equivalent-circuit modeling (capacitor with a parallel resistance) yields identical results: the resistive branch current is in phase with voltage and corresponds to εr″ in the permittivity representation. Why Other Options Are Wrong: (b) contradicts the algebra; (c) proportionality holds at all frequencies where linear material behavior is valid; (d) “perfect insulators” have εr″ → 0, thus negligible in-phase current; (e) εr′ governs reactive displacement, not dissipative in-phase current. Common Pitfalls: Mixing sign conventions or confusing εr″ with the loss tangent tan δ = εr″/εr′; both relate to loss but play different roles in formulas. Final Answer: True

Question 15

Carbon resistor colour code — compute the resistance value A carbon resistor has colour bands: yellow–violet–orange (first band = yellow, second band = violet, third band = multiplier). Determine the nominal resistance value in ohms (ignore toleranc…

Accepted Answer

Introduction / Context: This item checks your mastery of the resistor colour code used in electronics. Interpreting the digit bands and the multiplier band quickly and accurately is a core lab skill for circuit assembly and troubleshooting. Given Data / Assumptions: Band 1 (most significant digit): yellow. Band 2 (second digit): violet. Band 3 (multiplier): orange. Tolerance band is not considered. Concept / Approach: The first two bands provide the significant figures; the third band gives the power-of-ten multiplier. Standard mapping: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. Multipliers use the same exponents: orange means 10^3. Step-by-Step Solution: First digit (yellow) = 4.Second digit (violet) = 7.Multiplier (orange) = 10^3.Resistance = 47 * 10^3 Ω = 47000 Ω = 47 kΩ. Verification / Alternative check: Many resistor charts list yellow–violet–orange explicitly as 47 kΩ. A quick digital multimeter measurement on a typical E12/E24 resistor marked with these bands will read close to 47 kΩ within tolerance. Why Other Options Are Wrong: 17000 Ω, 27000 Ω, and 37000 Ω use incorrect digit or multiplier interpretations. 56000 Ω corresponds to green–blue–orange, not yellow–violet–orange. Common Pitfalls: Reading the bands from the wrong end; confusing violet (7) with blue (6); misapplying the multiplier colour. Final Answer: 47000 Ω

Question 16

Isotropic scattering probability — normalization condition An electron in a metal suffers a collision. If the probability of it being scattered into the solid angle between polar angles θ and θ + dθ (integrated over azimuth) is given by 2π P(θ) sinθ…

Accepted Answer

Introduction / Context: In transport theory for metals and plasmas, scattering probabilities over directions must integrate to unity. The function P(θ) describes how likely a carrier is deflected by an angle θ; the factor sinθ dθ accounts for the spherical surface element, and 2π integrates out the azimuthal angle for axisymmetric scattering. Given Data / Assumptions: Scattering distribution depends only on polar angle θ, not on azimuth. Element of solid angle: dΩ = sinθ dθ dφ. Azimuth φ runs from 0 to 2π. Concept / Approach: Any probability density over solid angle must integrate to 1 over the full sphere. Since azimuthal symmetry is assumed, integration over φ yields a factor 2π, leaving the polar integral with sinθ dθ. The normalization condition imposes a single scalar equation on P(θ). Step-by-Step Solution: Start with total probability = ∫ dΩ P(θ) = 1.Write dΩ = sinθ dθ dφ, integrate φ from 0 to 2π → factor 2π.Hence, ∫₀^π 2π P(θ) sinθ dθ = 1.This ensures the distribution is properly normalized. Verification / Alternative check: For isotropic scattering, P(θ) is constant. Setting P(θ) = C gives 2π C ∫₀^π sinθ dθ = 2π C * 2 = 4π C = 1 ⇒ C = 1/(4π), which is the familiar uniform distribution over a sphere. Why Other Options Are Wrong: Equalities without the integral are meaningless for a differential element. Values 0, 2, or −1 violate probability axioms. The last option incorrectly integrates θ over φ’s range. Common Pitfalls: Forgetting the sinθ Jacobian; mixing up the roles of θ and φ; omitting limits. Final Answer: ∫₀^π 2π P(θ) sinθ dθ = 1

Question 17

Fourier’s law in one dimension — sign convention Relate the heat-flux density Q (W/m^2), thermal conductivity K (W/m·°C), and temperature gradient dT/dx (°C per metre) for one-dimensional steady conduction, using the proper sign convention.

Accepted Answer

Introduction / Context: Fourier’s law is the cornerstone of heat conduction analysis. The sign convention indicates that heat flows from higher to lower temperature, opposite to the direction of increasing temperature. Given Data / Assumptions: One-dimensional steady conduction along x. Q is defined positive in the +x direction. K is positive for physical materials. Concept / Approach: The physical statement “heat flows down the temperature gradient” is captured mathematically by a negative sign. If temperature decreases with x (dT/dx < 0), then Q should be positive in +x, which requires Q = -K dT/dx. Step-by-Step Solution: Write Fourier’s law: Q = - K * (dT/dx).Check sign with a linear profile: T(x) = T0 - ax with a > 0 ⇒ dT/dx = -a.Then Q = -K(-a) = K a > 0, i.e., heat flows toward +x where T is lower. Verification / Alternative check: Dimensional consistency: K has units W/m·°C, dT/dx has °C/m, product gives W/m^2, which matches Q. Why Other Options Are Wrong: Option (a) misses the sign; (c), (d), and (e) are algebraically or dimensionally incorrect as written. Common Pitfalls: Dropping the minus sign; mixing up heat rate (W) with heat flux (W/m^2); confusing K’s units. Final Answer: Q = - K (dT/dx)

Question 18

Ionic solids and electrical conductivity — evaluate the assertion–reason Assertion (A): Ionic solids, in general, have no electrical conductivity in the solid state. Reason (R): In ionic solids, electrons are tightly bound to the nuclei and are not …

Accepted Answer

Introduction / Context: Ionic crystals (e.g., NaCl) conduct electricity when molten or dissolved but not typically in the solid state. Distinguishing the correct reason requires recognizing which charge carriers are responsible for conduction in different phases. Given Data / Assumptions: Solid, stoichiometric ionic lattice at ordinary temperature. No significant defect-mediated ionic conduction (fast-ion conductors are special cases). Conduction mechanisms: electrons/holes vs mobile ions. Concept / Approach: In solid ionic lattices, the ions are held rigidly at lattice sites; there are no free charge carriers able to drift under an electric field. Thus bulk solid ionic compounds are insulators. The statement about tightly bound electrons is true but does not explain the lack of conduction in the ionic solid: even if electrons are localized, molten ionic liquids conduct because ions are mobile. The key factor is ion mobility, not electron binding. Step-by-Step Solution: Accept A: solid ionic compounds generally do not conduct.Evaluate R: electrons are indeed localized in ionic bonds (true).Causation: lack of mobile ions in the solid lattice, not just electron localization, explains non-conduction.Therefore, A and R true, but R is not the correct explanation of A. Verification / Alternative check: Experimentally, NaCl(s) is an insulator, NaCl(l) conducts well via ions; this change cannot be explained by electron binding alone. Why Other Options Are Wrong: Declaring R false ignores electronic structure; claiming R as the correct explanation overlooks ion immobility as the real cause. Common Pitfalls: Assuming only electrons can carry current; forgetting ionic conductivity in melts and solutions. Final Answer: Both A and R are true but R is not correct explanation of A

Question 19

Minority carriers in semiconductors In which type of semiconductor are holes the minority carriers under normal doping and thermal equilibrium?

Accepted Answer

Introduction / Context: Carrier types and their majority/minority roles underpin diode and transistor action. Recognizing which carriers dominate in each semiconductor type is essential for predicting current flow and junction behavior. Given Data / Assumptions: Thermal equilibrium, moderate doping (nondegenerate). n-type: donors provide extra electrons. p-type: acceptors create holes. Concept / Approach: In n-type material, electrons are the majority carriers because donor impurities elevate the electron concentration well above the intrinsic level; holes arise from thermal generation and are comparatively few. Conversely, holes are majority carriers in p-type material and electrons are the minority. Step-by-Step Solution: Recall definitions: majority carrier has higher concentration; minority has lower.n-type: n ≫ p ⇒ holes are minority carriers.Therefore, choose “n-type”. Verification / Alternative check: Mass-action law n p = n_i^2 implies if n increases via doping, p must decrease, confirming holes are minority in n-type. Why Other Options Are Wrong: “p-type” reverses roles; “intrinsic” has n ≈ p (no minority); “extrinsic (any type)” is too vague; “degenerate” concerns Fermi level position, not necessarily hole minority. Common Pitfalls: Confusing majority/minority with mobility; majority depends on concentration, not speed. Final Answer: n-type

Question 20

Silicon valence electrons — periodic table recall How many valence electrons does a silicon (Si) atom possess?

Accepted Answer

Introduction / Context: Semiconductor bonding and band structure originate from the valence electron configuration. Silicon, the workhorse of microelectronics, forms covalent bonds using its outer-shell electrons. Given Data / Assumptions: Element: Silicon (Z = 14). Electronic configuration (ground state): 1s² 2s² 2p⁶ 3s² 3p². Concept / Approach: Valence electrons are those in the outermost shell participating in bonding. For Si, the outer shell is n = 3 with 3s² 3p², totaling 4 valence electrons. This tetra-valence enables sp³ covalent bonding in the diamond cubic crystal structure. Step-by-Step Solution: Identify outer shell: n = 3.Count electrons in 3s and 3p: 2 + 2 = 4.Hence, valence electrons = 4. Verification / Alternative check: Group IV (14) elements in the periodic table (C, Si, Ge, Sn) all have 4 valence electrons, consistent with tetrahedral bonding. Why Other Options Are Wrong: 2, 1, or 0 contradict the electron configuration; 6 would suit Group 16 elements, not Group 14. Common Pitfalls: Confusing total electrons with valence electrons; overlooking shell structure. Final Answer: 4

Materials and Components Questions