Question 1

In a direct current or alternating current series circuit containing two or more components connected end to end, which statement about the current distribution along the series path is always true?

Accepted Answer

Introduction / Context: In basic circuit theory, a series circuit is one in which components are connected sequentially so that there is only one conducting path. This question tests the fundamental property of current flow in series connections, an idea used throughout electronics and electrical engineering when analyzing resistors, capacitors, inductors, and even complex networks by reduction. Given Data / Assumptions: Components are connected in a single path (true series connection). Wires and connections are ideal unless otherwise stated. The circuit can be powered by DC or AC; the topological rule about current is the same. Concept / Approach: Conservation of charge implies that the rate of charge flow entering any series element must equal the rate of charge flow leaving it. With only one path available, there is no branching where current could split. Hence, the same current must pass through each series element at any instant. Series circuits may have different voltage drops across elements, but the current is identical everywhere in the loop. Step-by-Step Solution: 1) Identify that the network is series: a single path from source through components and back.2) Apply charge continuity: charge cannot accumulate in a passive series element in steady operation.3) Therefore, the instantaneous current entering and leaving each component is the same.4) Conclude: current is equal through all series components, while voltages may differ according to V = I * R, V = I * X_L, or V = I * X_C as applicable. Verification / Alternative check: If two ammeters are placed in series at different locations, both will read the same current. In contrast, voltmeters placed across each element will generally read different voltages that sum (phasor sum for AC) to the source voltage. Why Other Options Are Wrong: Option A: In series, current does not depend on individual resistance at a point; it is identical through all elements. Option B: Summing currents applies to parallel branches, not series elements. Option D: Current equals total voltage divided by total resistance; the statement given multiplies instead of divides. Option E: Incorrect because Option C is correct. Common Pitfalls: Confusing series with parallel circuits and assuming current splits among series components is a frequent mistake. Remember: series means one path for current, parallel means multiple paths for current to split and recombine. Final Answer: The same current flows through every component in a series circuit.

Question 2

In signal analysis, a waveform graph most commonly displays which quantities on the vertical and horizontal axes, respectively?

Accepted Answer

Introduction / Context: Waveforms are foundational visuals in electronics, audio, and control engineering. They show how a signal varies over time. Understanding what is plotted on each axis is essential before learning about frequency content, phase, or modulation. Given Data / Assumptions: We consider a typical oscilloscope or time-domain plot. No special transforms are applied (not a frequency spectrum or spectrogram). The signal could be voltage, current, or any measurable amplitude. Concept / Approach: A time-domain waveform displays the instantaneous value of a signal against time. By convention, the vertical axis represents amplitude (such as volts) and the horizontal axis represents time. Alternative plots like frequency spectra swap time for frequency but those are not called time-domain waveforms. Step-by-Step Solution: 1) Identify plot type: a basic waveform in the time domain.2) Recall convention: vertical axis for signal magnitude, horizontal axis for time progression.3) Therefore, amplitude is mapped to the vertical axis and time to the horizontal axis. Verification / Alternative check: On oscilloscopes, the time base controls the horizontal scale, while volts per division sets the vertical amplitude scale. This directly confirms the axis assignment for standard waveforms. Why Other Options Are Wrong: Option A: That axis choice describes neither a standard time-domain waveform nor a typical spectrum. Option B: Frequency on the horizontal with amplitude vertical defines a frequency spectrum, not a waveform in time. Option D: Swapping axes is nonstandard and would not represent typical time-domain practice. Option E: Incorrect because Option C is correct. Common Pitfalls: Confusing time-domain waveforms with frequency-domain spectra is common. A waveform is amplitude versus time, while a spectrum is amplitude versus frequency. Final Answer: Amplitude on the vertical axis and time on the horizontal axis.

Question 3

In diode characteristics, the knee (cut-in or threshold) voltage is approximately equal to which fundamental diode parameter?

Accepted Answer

Introduction / Context: The knee voltage, also called cut-in or threshold voltage, marks the point on the forward I–V curve of a diode where current starts rising rapidly with small increases in applied voltage. Recognizing what this knee corresponds to helps interpret diode material types like silicon and germanium in practical circuits. Given Data / Assumptions: We discuss a conventional p-n junction diode under forward bias. Temperature is nominal and effects like series resistance are modest. Typical values: about 0.7 V for silicon and about 0.3 V for germanium at room temperature. Concept / Approach: In forward bias, current remains small until the applied forward voltage approximately equals the built-in barrier potential of the junction. At that point, the potential hill opposing majority carrier flow is effectively overcome and current increases steeply. Thus, the knee corresponds to the barrier potential of the diode junction. Step-by-Step Solution: 1) Consider the diode I–V curve in forward bias.2) Identify the knee region where the slope increases sharply.3) Recognize that this knee appears when the external forward voltage offsets the junction barrier potential.4) Conclude that knee voltage approximates the barrier potential. Verification / Alternative check: Comparing measured forward threshold for silicon diodes near 0.7 V with known barrier potentials supports the equivalence in practical usage. Why Other Options Are Wrong: Applied voltage: Generic term, not a parameter equal to the knee point. Breakdown voltage: Occurs in reverse bias, unrelated to the forward knee. Forward voltage: A broad term; only near the barrier potential does the knee appear. Barrier potential is the precise concept. None of the above: Incorrect because barrier potential is correct. Common Pitfalls: Confusing forward knee with reverse breakdown is a common error. Also, assuming an exact fixed value ignores temperature and current-level dependence; the knee is approximate and context dependent. Final Answer: Barrier potential.

Question 4

For an ideal pure inductor excited by a sinusoidal AC source, what is the phase relationship between current and voltage?

Accepted Answer

Introduction / Context: Understanding the phase relationship between current and voltage in ideal reactive elements is core to AC circuit analysis, power factor correction, and resonance. A pure inductor exhibits a characteristic quadrature phase shift that drives many design choices in filters and power systems. Given Data / Assumptions: Inductor is ideal: zero resistance and no core losses. Source is sinusoidal steady state. No other components are in series with the inductor. Concept / Approach: Voltage across an inductor is v = L * di/dt in time domain. For a sinusoidal current, the derivative shifts phase by +90 degree. Equivalently, with current as reference, voltage leads current by 90 degree. Therefore, current lags voltage by 90 degree in an ideal inductor. This is captured by the inductive reactance X_L = 2 * pi * f * L, which is purely imaginary and positive in phasor form, representing a +90 degree voltage lead. Step-by-Step Solution: 1) Write v = L * di/dt for an inductor.2) For sinusoidal steady state, differentiation introduces a 90 degree phase lead in voltage relative to current.3) Hence, with voltage as the reference, the current must lag by 90 degree. Verification / Alternative check: Using phasors: V = j * X_L * I where j denotes +90 degree. This shows V leads I by 90 degree, so I lags V by 90 degree. Why Other Options Are Wrong: Option A: Reverses the correct lead-lag relation. Option B: In-phase behavior is for resistors, not ideal inductors. Option D: AC is not converted to DC by a pure inductor. Option E: Incorrect because Option C is correct. Common Pitfalls: Mixing up inductor and capacitor phase rules is common. Remember: in an inductor, current lags; in a capacitor, current leads. Final Answer: AC current lags the voltage by 90 degree.

Question 5

For an intrinsic semiconductor at thermal equilibrium, how do the numbers of free electrons and holes compare?

Accepted Answer

Introduction / Context: Carrier balance in intrinsic semiconductors underpins diode and transistor operation. Intrinsic material is chemically pure, without intentional doping. Knowing the equality of electrons and holes is foundational for interpreting conductivity, Fermi level position, and temperature behavior. Given Data / Assumptions: Material is intrinsic (no donor or acceptor impurities added). Thermal equilibrium conditions apply. Temperature is uniform. Concept / Approach: In intrinsic semiconductors, thermal generation creates electron-hole pairs. Each generation event produces one electron and one hole. Recombination removes them in pairs. At equilibrium, the intrinsic carrier concentration n_i satisfies n = p = n_i. Therefore, the number of free electrons equals the number of holes. Step-by-Step Solution: 1) Recognize intrinsic, undoped material.2) Generation produces electron-hole pairs in equal numbers.3) At equilibrium, rates of generation and recombination are equal, keeping n and p equal.4) Conclude n = p, that is, electrons equal holes. Verification / Alternative check: Band theory places the Fermi level near mid-gap for intrinsic material, consistent with symmetric electron and hole populations in the conduction and valence bands at equilibrium. Why Other Options Are Wrong: Less than or greater than: That occurs in doped n-type or p-type materials, not intrinsic. Impossible to say: It is well defined by intrinsic physics. None of the above: Incorrect because equality is correct. Common Pitfalls: Confusing intrinsic with lightly doped behavior leads to wrong conclusions. Always check whether the material is doped or pure when comparing carrier populations. Final Answer: Equals the number of holes.

Question 6

In a parallel RC circuit excited by AC, which statement correctly describes the relationship between the branch currents and the total current?

Accepted Answer

Introduction / Context: Parallel RC circuits are ubiquitous in filters and timing networks. Because the resistor branch current and capacitor branch current are out of phase, the way they combine is vectorial rather than simple arithmetic. Recognizing this prevents serious errors in current estimation and power factor analysis. Given Data / Assumptions: Parallel connection of a resistor R and capacitor C across the same AC voltage. Steady-state sinusoidal excitation. Ideal components with negligible parasitics for conceptual clarity. Concept / Approach: In a parallel RC, branch currents are I_R = V / R (in phase with V) and I_C = V / X_C (leading V by 90 degree), where X_C = 1 / (2 * pi * f * C). The total current is the phasor sum: I_T = vector sum of I_R and I_C. Since the angle between I_R and I_C is 90 degree, the magnitude is I_T = sqrt( I_R^2 + I_C^2 ), which is strictly less than the arithmetic sum I_R + I_C whenever both are nonzero. Step-by-Step Solution: 1) Compute I_R = V / R, aligned with voltage.2) Compute I_C = V / X_C, leading voltage by 90 degree.3) Combine as perpendicular vectors: I_T = sqrt( I_R^2 + I_C^2 ).4) Conclude I_T is less than I_R + I_C due to quadrature addition. Verification / Alternative check: Phasor diagram shows a right triangle with legs I_R and I_C and hypotenuse I_T. The hypotenuse is less than the sum of the legs, matching the inequality. Why Other Options Are Wrong: Option A: Uses arithmetic sum, which ignores phase difference. Option B: Phases differ between resistor and capacitor branches. Option D: Greater than the sum is impossible for any triangle inequality. Option E: Incorrect because Option C is correct. Common Pitfalls: Adding AC branch currents arithmetically is a frequent mistake. Always combine out-of-phase currents vectorially using phasors or rectangular components. Final Answer: The total current is less than the sum of the currents for the resistance and capacitance.

Question 7

When the current versus voltage characteristic of a device is a straight line through the origin, how is the device classified?

Accepted Answer

Introduction / Context: Device classification by I–V characteristics is a first step in modeling components. A linear device exhibits proportionality between current and voltage, which greatly simplifies analysis using Ohm law and superposition. Given Data / Assumptions: I–V plot is a straight line, ideally through the origin. Small-signal and large-signal behavior align within the observed range. Temperature and parasitics are negligible for the conceptual model. Concept / Approach: For a linear element, V is proportional to I, commonly expressed as V = R * I for resistors. The straight-line I–V graph indicates a constant slope, which is the resistance (or conductance). This proportionality and additivity are the hallmarks of linearity and allow superposition and Thevenin or Norton equivalents to be applied directly. Step-by-Step Solution: 1) Observe straight-line I–V behavior.2) Identify constant slope dV/dI, implying constant resistance.3) Conclude the device obeys linear relations over the range, hence it is linear. Verification / Alternative check: Measure current at multiple voltages; if the ratio V / I remains constant, linearity is confirmed. Deviations indicate nonlinearity or limit effects. Why Other Options Are Wrong: Active: Refers to power gain or controlled sources, not necessarily linearity. Nonlinear: Would show curved I–V characteristics. Bipolar: Describes device type like BJT polarity, not the I–V linearity property. None of the above: Incorrect because linear is correct. Common Pitfalls: Assuming linearity at very high currents or temperatures can be unsafe; many resistors show nonlinearity under stress. The classification here applies within the tested operating region. Final Answer: Linear.

Question 8

In semiconductor physics, how many distinct types of charge flow contribute to conduction inside a semiconductor crystal?

Accepted Answer

Introduction / Context: Conduction in semiconductors differs from metals because two kinds of mobile charge carriers participate. Recognizing both carrier types is critical for understanding p–n junctions, bipolar transistors, and diode I–V behavior. Given Data / Assumptions: Material may be intrinsic or doped. We consider standard band theory without exotic effects. Thermal equilibrium or steady-state bias with carriers present. Concept / Approach: There are two carrier types: electrons in the conduction band and holes in the valence band. Electrons carry negative charge, while holes act as positive charge carriers representing the absence of an electron. Both contribute to current flow under electric fields and concentration gradients, leading to drift and diffusion components in device equations. Step-by-Step Solution: 1) Identify available mobile carriers: electrons and holes.2) Note that both can move under an applied electric field, producing current.3) Conclude there are two types of flow contributing to conduction. Verification / Alternative check: In p–n junction forward bias, both electron injection into p-side and hole injection into n-side produce the total current, directly demonstrating two-carrier conduction. Why Other Options Are Wrong: 0 or 1: Metals have effectively one type of carrier; semiconductors typically have two. 3: There are not three fundamental carrier types in standard semiconductor physics. None of the above: Incorrect because the correct count is two. Common Pitfalls: Assuming holes are merely mathematical is misleading; hole motion and recombination are physically observable through device behavior and measured currents. Final Answer: 2.

Question 9

Which statement is a direct expression of Ohm law for a passive resistor?

Accepted Answer

Introduction / Context: Ohm law is the cornerstone of resistive circuit analysis. It relates voltage, current, and resistance in a simple proportional law used in nearly every calculation from bias networks to power estimation. Given Data / Assumptions: Passive, linear resistor behavior in the considered operating range. Temperature and self-heating effects are negligible. DC or AC steady-state conditions are acceptable for the proportional relation. Concept / Approach: Ohm law states V = I * R. Therefore, the voltage drop across a resistance is directly proportional to current and proportional to resistance. This is a direct statement of the law. The law enables simple series-parallel reductions and power calculations with P = V * I = I^2 * R = V^2 / R. Step-by-Step Solution: 1) Recall the relation V = I * R.2) Interpret: for a given R, voltage varies linearly with current; for a given I, voltage scales with R.3) Select the statement that explicitly expresses this proportionality. Verification / Alternative check: Plotting V versus I for a resistor yields a straight line with slope R, confirming the proportional relationship. Why Other Options Are Wrong: Option B: True property of parallels but not Ohm law. Option C: This is Kirchhoff Voltage Law, not Ohm law. Option D: This is Kirchhoff Current Law, not Ohm law. Option E: Incorrect because Option A is correct. Common Pitfalls: Confusing Ohm law with Kirchhoff laws or network theorems is common. Always tie Ohm law to the element relation of a resistor only. Final Answer: The voltage drop across a resistance is proportional to the value of resistance and the amount of current flowing through it.

Question 10

For an intrinsic semiconductor, how does temperature affect the number of free electrons and holes present?

Accepted Answer

Introduction / Context: Temperature strongly influences carrier concentration in semiconductors. For intrinsic materials, thermal energy controls how many electrons are promoted from the valence band to the conduction band, simultaneously creating holes. Understanding this trend is essential for sensor design and for predicting leakage currents in devices. Given Data / Assumptions: Intrinsic semiconductor with no intentional doping. Uniform temperature across the sample. Thermal equilibrium conditions. Concept / Approach: The intrinsic carrier concentration n_i increases rapidly with temperature due to the exponential dependence on bandgap and thermal energy. When an electron gains sufficient energy to cross the bandgap, it becomes a free electron in the conduction band and leaves behind a hole in the valence band. Thus, both electrons and holes increase with temperature, keeping n = p = n_i at equilibrium. Step-by-Step Solution: 1) Consider thermal generation of electron-hole pairs.2) As temperature rises, more pairs are generated per unit time.3) Therefore, the concentrations of both electrons and holes increase together in intrinsic material. Verification / Alternative check: Experimental I–V measurements of diodes show increased reverse saturation current with temperature, reflecting higher intrinsic carrier concentration. This aligns with the theoretical temperature dependence of n_i. Why Other Options Are Wrong: Decreases: Opposite of the true trend for intrinsic materials. Stays the same: Only approximately valid over very narrow ranges, not generally true. 0oC: This is not a trend description and is irrelevant as an option. None of the above: Incorrect because the correct trend is an increase. Common Pitfalls: Confusing intrinsic behavior with doped behavior can lead to mistakes. While mobility may decrease with temperature, the dominant effect on intrinsic conductivity is the steep increase in carrier concentration. Final Answer: Increases.

Question 11

A flashlight battery has an internal resistance of 1 Ω. For what range of load resistance will the source effectively behave as a stiff (low-drop) voltage source?

Accepted Answer

Introduction / Context: A real battery has an internal resistance r that causes a voltage drop under load. A source is called 'stiff' if its terminal voltage hardly sags when the load current changes. This question checks whether you can relate the stiffness idea to the ratio of load resistance RL to internal resistance r. Given Data / Assumptions: Internal resistance r = 1 Ω. We want RL values for which the terminal voltage is close to the ideal emf. Assume a simple Thevenin model: source E in series with r feeding RL. Concept / Approach: Voltage division: Vout = E * RL / (r + RL). Stiff source criterion (rule-of-thumb): RL ≥ 10 * r keeps drop across r ≤ ~10% of E. Larger RL relative to r → smaller current → smaller Ir drop in r. Step-by-Step Solution: Use RL ≥ 10 * r as a practical stiffness threshold.Given r = 1 Ω ⇒ RL ≥ 10 Ω meets minimum; RL much larger makes it stiffer.Among options, '100 Ω or more' clearly indicates a regime far beyond 10r, giving very small sag. Verification / Alternative check: If RL = 100 Ω: Vout = E * 100 / (1 + 100) ≈ 0.990 * E (≈1% drop) which is very stiff. Why Other Options Are Wrong: 60 Ω and 50 Ω: Better than 10r, but '100 Ω or more' is the most clearly stiff region offered. 10 Ω: Barely at the 10r edge; still ~9% drop, not strongly 'stiff'. None of the above: Incorrect because 100 Ω or more is suitable. Common Pitfalls: Confusing maximum power transfer (RL = r) with stiffness; RL = r gives 50% voltage drop, not stiff. Ignoring that 'stiff' is about small percentage sag, not absolute resistance values. Final Answer: 100 Ω or more

Question 12

Magnetic materials: which type exhibits the lowest permeability (i.e., slightly less than the permeability of free space)?

Accepted Answer

Introduction / Context: Permeability measures how easily a material supports the formation of magnetic flux. Understanding how different classes of materials respond to an applied magnetic field is fundamental in electromagnetics and transformer/core design. This item asks which class has the lowest permeability relative to free space (μ0). Given Data / Assumptions: Classes considered: diamagnetic, paramagnetic, ferromagnetic. Relative permeability μr = μ / μ0. We compare typical trends, not rare edge cases or extreme temperatures. Concept / Approach: Diamagnetic: weakly repelled by magnetic fields; induced magnetization opposes applied field → μr slightly less than 1. Paramagnetic: weak attraction; μr slightly greater than 1. Ferromagnetic: strong attraction and domain alignment; μr » 1, often hundreds to thousands. Step-by-Step Reasoning: Identify trend: μr(diamagnetic) < 1 < μr(paramagnetic) < μr(ferromagnetic).Therefore the lowest permeability occurs in diamagnetic materials because their response reduces net flux relative to free space.Examples: bismuth, copper, gold, and water are weakly diamagnetic; iron and steel are ferromagnetic with very high μ. Verification / Alternative check: Check with sorting/induction experiments: diamagnetic samples are weakly expelled from strong field regions, consistent with μr < 1. Why Other Options Are Wrong: A paramagnetic material: μr slightly > 1, not the lowest. A ferromagnetic material: μr » 1, the highest class among the three. All of the above / None of the above: Contradict the established ordering of μr. Common Pitfalls: Confusing susceptibility sign (χ < 0 for diamagnetism) with magnitude; the key is μr relative to 1. Assuming 'weak' always means 'lowest' across all classes without recalling μr positions. Final Answer: A diamagnetic material

Question 13

An electromagnet has 1000 turns (N = 1000). What current is required to produce an mmf of 100 ampere-turns (NI = 100 At)?

Accepted Answer

Introduction / Context: Magnetomotive force (mmf) quantifies the 'driving force' for magnetic flux in a magnetic circuit, analogous to emf in electrical circuits. The basic relation is mmf = N * I (ampere-turns). This question reinforces direct proportionality among mmf, turns, and current. Given Data / Assumptions: Number of turns N = 1000. Required mmf NI = 100 ampere-turns. Core properties and reluctance are irrelevant for computing current from mmf. Concept / Approach: Use mmf formula: NI = N * I. Solve for I: I = NI / N. Step-by-Step Solution: Write formula: NI = N * I.Substitute values: 100 = 1000 * I.Compute: I = 100 / 1000 = 0.1 A. Verification / Alternative check: Scale reasoning: If 1000 turns produce 100 At at 0.1 A, then 1 A would give 1000 At; linear scaling confirms correctness. Why Other Options Are Wrong: 10 A and 100 A: Would produce 10,000 At and 100,000 At respectively, far above 100 At. 1000 A: Physically unrealistic for small coils; would give 1,000,000 At. None of the above: Incorrect because 0.1 A satisfies the requirement. Common Pitfalls: Confusing ampere-turns with amperes; mmf depends on both current and turns. Overlooking simple division when rearranging NI = N * I. Final Answer: 0.1 A

Question 14

For a pure sine wave, the rms (root-mean-square) value is related to the peak amplitude how?

Accepted Answer

Introduction / Context: The rms value of a periodic signal represents the DC-equivalent heating value in a resistive load. For sinusoidal waveforms, there is a fixed relationship between the peak value and the rms value that is used extensively in AC circuit analysis and instrumentation. Given Data / Assumptions: Signal is a pure sine: v(t) = Vm * sin(ωt). We compare Vrms to Vm (peak amplitude). No distortion or DC offset. Concept / Approach: Definition: Vrms = sqrt( (1/T) * ∫ v(t)^2 dt over one period ). For a sine: Vrms = Vm / √2 ≈ 0.707 * Vm. Step-by-Step Solution: Start with Vrms = Vm / √2.Compute numeric factor: 1 / √2 ≈ 0.707.Conclude: Vrms is about 70.7% of the peak amplitude. Verification / Alternative check: Given Vp-p = 2 * Vm, one can convert between rms and peak using Vrms = Vm / √2 and Vm = Vrms * √2 ≈ 1.414 * Vrms. Why Other Options Are Wrong: Number of cycles in one second: That is frequency (Hz), not rms. Time for one cycle: That is period T, not rms. 1.41 times peak: Inverse relation; peak ≈ 1.414 * Vrms, not rms ≈ 1.41 * peak. None of the above: Incorrect because 0.707 * peak is correct. Common Pitfalls: Confusing peak, peak-to-peak, and rms values. Applying the sine relation to non-sinusoidal waveforms where the factor differs. Final Answer: equal to 0.707 times the peak amplitude

Question 15

A BJT has current gain β = 125 and base current I_B = 30 µA. Calculate the collector current I_C.

Accepted Answer

Introduction / Context: In a bipolar junction transistor operating in active region, the collector current is approximately β times the base current (neglecting I_CBO). This quick calculation is central to biasing and small-signal design estimates. Given Data / Assumptions: β (DC current gain) = 125. I_B = 30 µA. Assume active region and negligible leakage currents. Concept / Approach: Relation: I_C ≈ β * I_B. Convert microamps to milliamps for the final expression. Step-by-Step Solution: Compute product: I_C = 125 * 30 µA.Multiply: 125 * 30 = 3750 µA.Convert units: 3750 µA = 3.75 mA. Verification / Alternative check: Rough check: 100 * 30 µA = 3.0 mA; extra 25 * 30 µA = 0.75 mA; total 3.75 mA. Why Other Options Are Wrong: 3.2 mA: Underestimates β * I_B. 0.05 mA: Off by orders of magnitude. 13 mA: Too high for given β and I_B. None of the above: Incorrect because 3.75 mA is correct. Common Pitfalls: Mismatching units (µA vs mA). Using α (common-base gain) instead of β; here β relation is explicit. Final Answer: 3.75 mA

Question 16

Zener regulator design: with Vin ranging 15–20 V, Vz = 6.8 V, and load current 5–20 mA, choose the series resistor value R for regulation.

Accepted Answer

Introduction / Context: A simple shunt zener regulator uses a series resistor R to drop excess voltage and a zener diode to clamp the load voltage at Vz. Choosing R requires considering input voltage variation and load current range so that the zener remains in regulation without reversing current (negative IZ) or dropping out at low Vin. Given Data / Assumptions: Input voltage Vin: 15–20 V. Zener voltage Vz = 6.8 V. Load current IL: 5–20 mA. No zener minimum current specified; design to avoid IZ < 0 at Vin max and to supply IL max at Vin min. Concept / Approach: Current through R: IR = (Vin − Vz) / R. Node equation: IR = IL + IZ, with IZ ≥ 0 for regulation. To avoid IZ < 0 at Vin max with IL min: (Vin_max − Vz) / R ≥ IL_min. To supply IL max at Vin min (just at edge of regulation with IZ ≈ 0): (Vin_min − Vz) / R ≥ IL_max. Step-by-Step Solution: Compute R_max from low Vin, high load: R_max = (15 − 6.8) / 0.020 = 8.2 / 0.020 = 410 Ω.Check at Vin_max and IL_min: IR = (20 − 6.8) / 410 ≈ 13.2 / 410 ≈ 32.2 mA ⇒ IZ ≈ 32.2 − 5 = 27.2 mA ≥ 0, so still regulating.Therefore, R ≈ 410 Ω satisfies both edges; nearest choice is 410 Ω. Verification / Alternative check: At Vin_min = 15 V and IL_max = 20 mA: IR = 8.2 / 410 = 20 mA ⇒ IZ ≈ 0 mA, right at regulation threshold, which is the limiting case. Why Other Options Are Wrong: 310 Ω or 400 Ω: Work but provide unnecessary extra zener current at Vin_max (higher dissipation) and are not the computed limit. 550 Ω: Too large; at Vin_min cannot supply IL_max while maintaining Vz. None of the above: Incorrect because 410 Ω meets the design edge. Common Pitfalls: Ignoring the worst-case corners of Vin and IL. Forgetting to verify IZ ≥ 0 at Vin_max and adequate current at Vin_min. Final Answer: 410 Ω

Question 17

AC inductive reactance: how much current flows through a 0.02 H choke connected to a 12 V (rms), 100 Hz AC source?

Accepted Answer

Introduction / Context: An inductor resists changes in current with a frequency-dependent reactance XL. In steady-state AC, the current magnitude through an ideal inductor is I = V / XL. This problem tests the ability to compute XL and the resulting rms current for given L, f, and V. Given Data / Assumptions: Inductance L = 0.02 H. Frequency f = 100 Hz. Applied voltage (rms) V = 12 V. Ideal inductor; ignore winding resistance. Concept / Approach: Inductive reactance: XL = 2 * π * f * L. Current magnitude: I = V / XL. Step-by-Step Solution: Compute XL = 2 * π * 100 * 0.02 = 2 * π * 2 = 4 * π ≈ 12.566 Ω.Compute I = 12 / 12.566 ≈ 0.955 A (rms).Therefore, the current is approximately 0.955 A. Verification / Alternative check: Sanity check: If L were halved, XL halves and current doubles; numbers scale as expected. Why Other Options Are Wrong: 0.02 A: Would imply XL = 600 Ω, inconsistent with given f and L. 10 A and 2.02 A: Far too high; would require very small XL not supported by L and f. None of the above: Incorrect because 0.955 A matches calculation. Common Pitfalls: Confusing 2 * π * f * L with 2 * π * L / f (incorrect). Mixing peak and rms values; V given is rms. Final Answer: 0.955 A

Question 18

Residual magnetism: which description best defines it in the context of electromagnets and ferromagnetic cores?

Accepted Answer

Introduction / Context: When a ferromagnetic core has been magnetized by coil current, some magnetization persists even after the current is removed. This remanent magnetization is called residual magnetism (or remanence) and is central to understanding hysteresis, memory effects, and core reset procedures in magnetic devices. Given Data / Assumptions: Electromagnet with a ferromagnetic core. After excitation ceases, some flux remains due to domain alignment. We are not describing leakage flux or external field geometries. Concept / Approach: Ferromagnetic domains align under applied field; removing the field does not randomize them fully. The B–H hysteresis loop shows nonzero flux density (Br) at zero applied field, representing residual magnetism. Resetting cores may require reverse magnetizing force to bring B to zero (coercivity Hc). Step-by-Step Reasoning: Apply current to coil → magnetizes core via H = N * I / l.Remove current → H returns to zero but B remains at Br due to hysteresis.This leftover magnetization is residual magnetism. Verification / Alternative check: Examine hysteresis curve: at H = 0, B = Br. Demagnetization requires applying negative H to reach B = 0 at H = −Hc. Why Other Options Are Wrong: Force of attraction between poles: Describes interaction, not remanence. Flux not confined to core: That is leakage flux. Circular magnetic field around a current: That is the field from a straight conductor, unrelated to remanence. None of the above: Incorrect because residual magnetism is accurately defined in option A. Common Pitfalls: Confusing residual magnetism with leakage flux or stray fields. Assuming cores lose all magnetization instantly when current is removed. Final Answer: Magnetism that remains in the core of an electromagnet after the current through the coil is turned off

Question 19

Batteries and energy conversion: which form of energy maintains the charge separation (potential difference) between a battery’s terminals?

Accepted Answer

Introduction / Context: Batteries convert stored chemical energy into electrical energy by driving redox reactions that separate charges. Understanding the internal energy source clarifies why batteries have finite capacity, depend on temperature, and can be recharged only for certain chemistries. Given Data / Assumptions: Conventional batteries (primary or secondary) with two electrodes and an electrolyte. Open-circuit voltage arises from electrode potentials determined by chemistry. No external mechanical or solar input during normal discharge. Concept / Approach: Electrochemical cells perform oxidation at the anode and reduction at the cathode. Gibbs free energy change ΔG of the reaction relates to cell emf: E = −ΔG / (n * F). The chemical processes maintain ion/electron flow paths that sustain terminal voltage until reactants are depleted. Step-by-Step Reasoning: Chemical reactions pump charges internally, building an electric potential difference across the electrodes.External circuit connection allows electrons to flow from anode to cathode, performing work while reactions proceed.When reactants/products reach equilibrium or are exhausted, the voltage collapses. Verification / Alternative check: Compare with capacitors: capacitors store electric field energy directly, not chemical energy. Batteries rely on chemical conversion to sustain a quasi-constant voltage. Why Other Options Are Wrong: Electronical energy: Not a standard form; electrical energy is produced, not the stored source. Mechanical energy: Relevant in generators, not ordinary batteries. Solar energy: Powers photovoltaic cells, not chemical batteries by themselves. None of the above: Incorrect because chemical energy is correct. Common Pitfalls: Confusing batteries with capacitors or fuel cells; the underlying storage and conversion differ. Assuming the open-circuit voltage is set only by charge quantity rather than electrode potentials. Final Answer: Chemical energy

Question 20

Capacitance definition: a farad is the capacitance that changes the plate voltage at 1 V per second when 1 A flows (i.e., C = I / (dV/dt)).

Accepted Answer

Introduction / Context: Capacitance measures a capacitor’s ability to store charge per unit voltage. In time-domain terms, current through a capacitor equals C times the rate of change of voltage. This definition leads directly to a practical unit statement for the farad (F). Given Data / Assumptions: Ideal capacitor with capacitance C. Relation i(t) = C * dv(t)/dt. We interpret the SI unit definition in terms of current and voltage rate of change. Concept / Approach: From i = C * dv/dt, rearrange to C = i / (dv/dt). Set i = 1 A and dv/dt = 1 V/s to obtain C = 1 F. This avoids misconceptions about energy or phase limits. Step-by-Step Solution: Start with i = C * dv/dt.Solve: C = i / (dv/dt).Plug i = 1 A, dv/dt = 1 V/s ⇒ C = 1 F. Verification / Alternative check: Energy in a capacitor: W = 0.5 * C * V^2; unit consistency corroborates the farad definition but is not the primary definition. Why Other Options Are Wrong: Dissipating 1 W: Ideal capacitors do not dissipate real power in steady sinusoidal excitation. Phase shift > 90°: For linear time-invariant networks, capacitors alone produce 90°, not > 90°. Storing 1 V for 1 s: Voltage is not 'stored' as a time product; that phrasing is incorrect. None of the above: Incorrect because option D states the standard definition. Common Pitfalls: Confusing current-voltage rate relation with energy or power directly. Misinterpreting '1 farad' as a fixed energy quantity rather than a proportionality between charge and voltage. Final Answer: changing the voltage on the plates at the rate of 1 V per second when 1 A of current is flowing

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