Question 1

In basic electrochemistry and circuit terminology, a battery is a device composed of one or more chemical cells that convert stored chemical energy into electrical energy—select the most accurate completion.

Accepted Answer

Introduction / Context: Batteries power everything from wristwatches to electric vehicles. Despite the ubiquity of the term “battery,” learners often confuse it with other energy-conversion devices such as solar or piezoelectric elements. This item checks foundational vocabulary in electrical engineering and physics: what a battery is made of and how it produces electrical energy. Given Data / Assumptions: The question asks about the correct constituent unit of a battery. No particular chemistry (lead-acid, Li-ion, NiMH) is assumed. We focus on conventional definitions used in textbooks and standards. Concept / Approach: A single electrochemical “cell” generates electrical energy by redox reactions at two electrodes separated by an electrolyte. A “battery” strictly means one cell or, more commonly, multiple cells connected together to raise voltage, capacity, or both. Therefore, the most precise statement is that a battery is composed of one or more chemical cells. Step-by-Step Solution: 1) Identify the nature of a battery: an electrochemical device.2) Recognize the basic unit: the chemical cell, which provides an electromotive force via redox chemistry.3) Connect terminology: multiple cells in series increase voltage; in parallel they increase available current (capacity).4) Conclude the accurate completion: “chemical cells.” Verification / Alternative check: Open any cell-level spec sheet (e.g., a “18650 Li-ion cell”). Battery packs list configurations such as 3S2P indicating 3 cells in series and 2 in parallel, reaffirming that batteries are assemblies of chemical cells. Why Other Options Are Wrong: Voltage generators: Vague; many devices generate voltage, not specifically what constitutes a battery. Solar cells: Photovoltaic devices convert light to electricity, not chemical energy. Piezoelectric cells: Convert mechanical strain to electricity; not a battery. None of the above: Incorrect because “chemical cells” is correct. Common Pitfalls: Using “battery” for any energy source can blur distinctions among electrochemical, photovoltaic, and electromechanical devices. Precision matters in design and troubleshooting. Final Answer: chemical cells.

Question 2

Instrumentation basics: which instrument is most commonly used to measure the opposition to current flow (i.e., electrical resistance) in a circuit?

Accepted Answer

Introduction / Context: Accurate electrical measurements are fundamental to diagnostics and design. Resistance—the opposition to current flow—is one of the three cornerstone quantities alongside voltage and current. This question checks recognition of the proper instrument used to measure resistance in circuits and components. Given Data / Assumptions: The task is to identify the instrument that measures resistance. We assume standard bench or handheld meters. No unusual measurement setups (e.g., Kelvin connections for very low resistance) are implied. Concept / Approach: An ohmmeter measures resistance directly, usually by sourcing a small known current and measuring the resulting voltage drop, then computing R using R = V / I. Modern digital multimeters integrate ohmmeter, ammeter, and voltmeter functions in one device, but the resistance mode is specifically the ohmmeter function. Step-by-Step Solution: 1) Define the quantity: resistance represents opposition to current, measured in ohms (Ω).2) Recall instrument associations: ammeter → current, voltmeter → voltage, ohmmeter → resistance, wattmeter → power.3) Select the instrument designed to measure resistance directly: the ohmmeter. Verification / Alternative check: Inspect a DMM: the “Ω” setting corresponds to the ohmmeter mode. When activated, the meter injects a test signal and displays the calculated resistance value. Why Other Options Are Wrong: Ammeter: Measures current (A), not resistance. Voltmeter: Measures potential difference (V), not resistance. Wattmeter: Measures power (W), not resistance. None of the above: Incorrect because “Ohmmeter” is correct. Common Pitfalls: Attempting to infer resistance from separate voltage and current readings is valid (R = V / I) but indirect; an ohmmeter provides a direct and safer measurement for passive components. Final Answer: Ohmmeter.

Question 3

Material properties: a dielectric material is best described as which of the following in the context of electrostatics and circuits?

Accepted Answer

Introduction / Context: Dielectrics are central to capacitors, cables, and high-voltage insulation. Understanding what a dielectric is—and is not—helps explain capacitance, breakdown strength, and energy storage in electric fields. This question checks the basic definition used in electronics and physics. Given Data / Assumptions: We focus on idealized behavior of dielectrics under electric fields. No unusual conductive additives or breakdown conditions are assumed. Terminology is standard: conductor vs insulator vs semiconductor. Concept / Approach: A dielectric is an electrical insulator that can be polarized by an electric field. It does not conduct free charge well; instead, bound charges shift slightly, storing energy in the field and increasing capacitance. Thus, the correct description is “a good insulator.” Step-by-Step Solution: 1) Recall: conductors have abundant free carriers; insulators lack them.2) Dielectric behavior involves polarization, not conduction.3) Therefore, the standard description is that a dielectric is a good insulator suitable for sustaining electric fields without significant current. Verification / Alternative check: Capacitor construction places a dielectric between plates to prevent DC conduction while allowing field-dependent charge displacement, raising capacitance by factor k (relative permittivity). Why Other Options Are Wrong: A good conductor: Opposite of a dielectric’s function. A poor conductor of electrostatic fields: Fields are not “conducted”; they permeate materials and polarize insulators. A good conductor of magnetic fields: Magnetic fields are not conducted like currents; materials may guide flux but that is not a dielectric definition. None of the above: Incorrect because “a good insulator” is correct. Common Pitfalls: Confusing dielectric polarization with conduction leads to errors about leakage and breakdown. Remember: ideal dielectrics store energy, they do not carry steady current. Final Answer: a good insulator.

Question 4

Digital and pulse measurements: in a standard definition used by oscilloscopes and signal integrity practice, the rise time of a pulse is the time required for the voltage to move between which two percentages of its final (peak) value?

Accepted Answer

Introduction / Context: Rise time characterizes how quickly a signal transitions from low to high and strongly influences bandwidth, overshoot, and timing margins in digital systems. Industry and academic labs typically adopt a consistent definition so measurements are comparable across instruments and datasheets. Given Data / Assumptions: We consider the common, default rise-time convention. Waveform reaches a stable high (no sustained oscillations). Noise and ringing are small enough not to obscure percent points. Concept / Approach: The widely used convention defines rise time, tr, as the interval for a step response to go from 10% to 90% of its final value. This avoids sensitivity to initial delay and top-flatness while capturing the effective transition speed relevant to bandwidth estimates (e.g., BW ≈ 0.35 / tr for single-pole responses). Step-by-Step Solution: 1) Identify the standard markers for the transition: 10% and 90% of the final amplitude.2) Measure the time difference between crossings at these two percentages on an oscilloscope.3) Record that interval as the rise time. Verification / Alternative check: Instrument manuals (oscilloscopes, TDRs) and high-speed design guides consistently specify the 10%–90% convention for rise time, with 20%–80% also used in some RF contexts. Both are standardized percentile definitions, not zero-to-peak or zero-to-rms intervals. Why Other Options Are Wrong: Zero to rms: RMS relates to power, not edge speed. Zero to peak: Overly sensitive to propagation delay and top flatness. 10% to 70.7%: 70.7% corresponds to 1/√2, a bandwidth marker, not rise-time definition. None of the above: Incorrect because 10%–90% is standard. Common Pitfalls: Confusing rise time with propagation delay or using inconsistent percent points can mislead bandwidth calculations and timing budgets. Final Answer: from 10% of the peak value to 90% of the peak value.

Question 5

Energy conversion devices: which device converts mechanical energy into electrical energy under routine operation?

Accepted Answer

Introduction / Context: Modern systems employ many transducers that convert one form of energy into another. Correctly identifying the direction of energy conversion is essential when selecting power sources or sensors. This question focuses on the classic electromechanical conversion used in power plants and portable generators. Given Data / Assumptions: Normal operating conditions and conventional device definitions. No exotic or hybrid devices considered. Steady conversion from a mechanical prime mover to electricity. Concept / Approach: An electrical generator (dynamo or alternator) converts mechanical energy into electrical energy via electromagnetic induction: motion of conductors in magnetic fields induces an emf. The other listed devices convert non-mechanical energy (light, heat, chemical) to electricity or vice versa. Step-by-Step Solution: 1) Identify the energy source: mechanical rotation, translation, or vibration.2) Apply Faraday’s law qualitatively: changing magnetic flux through a coil induces voltage.3) Conclude that the device intended for mechanical-to-electrical conversion is the generator. Verification / Alternative check: Power stations use turbines (steam, gas, water, wind) to spin alternators, directly demonstrating mechanical-to-electrical conversion at scale. Why Other Options Are Wrong: Solar cell: Converts light (photons) to electricity. Thermocouple: Converts temperature difference to voltage (Seebeck effect). Chemical cell: Converts chemical energy to electricity. None of the above: Incorrect because generator is correct. Common Pitfalls: Confusing a motor with a generator; while many machines can work reversibly, the question asks for mechanical-to-electrical conversion, which is generator mode. Final Answer: generator.

Question 6

Numerical AC analysis: what is the capacitive reactance Xc of a 0.1 μF capacitor operating at f = 1000 Hz? Provide the closest standard-value option.

Accepted Answer

Introduction / Context: Capacitive reactance quantifies how a capacitor resists changes in voltage in AC circuits. Computing Xc quickly is essential for filter design, impedance matching, and estimating current draw at a given frequency. Given Data / Assumptions: Capacitance C = 0.1 μF = 0.1 * 10^-6 F = 1 * 10^-7 F. Frequency f = 1000 Hz. Ideal capacitor; no series resistance or parasitics considered. Concept / Approach: Use Xc = 1 / (2 * pi * f * C). This relation comes from the frequency-domain impedance of a capacitor, Zc = 1 / (j * 2 * pi * f * C), with magnitude Xc equal to the reciprocal of 2 * pi * f * C. Step-by-Step Solution: Step 1: Compute the product k = 2 * pi * f * C.Step 2: Substitute values: k = 2 * pi * 1000 * (1 * 10^-7) = 2 * pi * 10^-4 ≈ 6.2832 * 10^-4.Step 3: Compute Xc = 1 / k ≈ 1 / (6.2832 * 10^-4) ≈ 1591.55 Ω.Step 4: Round to the nearest listed option: 1590 Ohm. Verification / Alternative check: A quick mental check: doubling frequency halves Xc; at 1 kHz and 0.1 μF, designers often memorize Xc ≈ 1.6 kΩ, matching the computed result. Why Other Options Are Wrong: Less than 1 Ohm: Two orders of magnitude too small. 312 Ohm or 690 Ohm: Do not follow from Xc = 1 / (2 * pi * f * C) with given values. None of the above: Incorrect because 1590 Ohm is correct. Common Pitfalls: Misplacing powers of ten when converting microfarads to farads causes large errors. Always convert 0.1 μF to 1 * 10^-7 F before calculation. Final Answer: 1590 Ohm.

Question 7

Ideal transformer calculation: with a primary voltage of 120 V (rms) and secondary voltage of 25 V (rms), if the secondary current is 1 A (rms), what is the primary current (assume an ideal transformer and negligible losses)?

Accepted Answer

Introduction / Context: Transformers trade voltage for current while conserving apparent power ideally. Being able to move between primary and secondary quantities using ratios is a core skill in power electronics and electrical machines. Given Data / Assumptions: Vp = 120 V rms, Vs = 25 V rms. Is = 1 A rms. Ideal transformer: input VA equals output VA (neglecting losses). Concept / Approach: In an ideal transformer, Vp * Ip = Vs * Is. Therefore Ip = (Vs * Is) / Vp. This relation follows from the turn ratio and energy conservation in the magnetic coupling under sinusoidal steady state. Step-by-Step Solution: Step 1: Write Ip = (Vs * Is) / Vp.Step 2: Substitute values: Ip = (25 * 1) / 120 A.Step 3: Compute Ip = 25 / 120 ≈ 0.2083 A.Step 4: Express in mA: ≈ 208 mA. Verification / Alternative check: Check ratio consistency: the voltage is stepped down by 120/25 ≈ 4.8, so current should step up by ≈ 4.8. 1 A on secondary corresponds to ≈ 0.208 A on primary, matching the calculation. Why Other Options Are Wrong: 7.8 mA: Too small by a factor of ~27. 200 mA or 300 mA: 200 mA is close but not as accurate as 208 mA; 300 mA is too high. None of the above: Incorrect because 208 mA is correct. Common Pitfalls: Forgetting that power (VA) is conserved ideally and incorrectly using turn ratio without considering current inversion leads to errors. Final Answer: 208 mA.

Question 8

Parallel circuit fundamentals: which statement correctly describes how current distributes in a parallel network of branches connected across the same source?

Accepted Answer

Introduction / Context: Parallel circuits provide multiple paths for current. Understanding how current splits among branches is vital for design (sizing resistors, fuses) and for analysis (Kirchhoff current law, equivalent resistance). Given Data / Assumptions: All branches share the same applied voltage. Branch resistances can differ. Steady-state conditions; components are ideal. Concept / Approach: Ohm law at the branch level gives I_k = V / R_k for branch k. Because V is common across branches, currents generally differ in proportion to 1 / R_k. The source current equals the sum of branch currents: I_total = Σ I_k. Therefore, it is correct that current through each branch can be different depending on its resistance (and the applied voltage). Step-by-Step Solution: 1) Recognize the defining property of parallel networks: equal voltage across each branch.2) Apply Ohm law per branch: I_k = V / R_k.3) Sum currents to get source current: I_total = Σ (V / R_k).4) Conclude that branch currents vary with branch resistances. Verification / Alternative check: Measure with ammeters in each branch; readings differ when R values differ, while a voltmeter shows identical voltage across branches, confirming the principle. Why Other Options Are Wrong: The same current everywhere: That is a series-circuit property, not parallel. Total current = total voltage * total resistance: Dimensionally incorrect; correct is I = V / R_eq. Total current less than the smallest branch current: False; total is the sum, hence ≥ the largest single branch current. None of the above: Incorrect because the first statement is true. Common Pitfalls: Confusing series and parallel rules is common. Remember: series → same current, parallel → same voltage. Final Answer: The amount of current flow through each branch of a parallel circuit can be different, depending on the resistance of each branch part and the amount of voltage applied to it.

Question 9

Zener diode small-signal behavior: a zener has breakdown voltage 10 V and zener (dynamic) resistance r_z = 8.5 Ω. When the current is 20 mA, what is the additional voltage drop contributed by r_z (beyond the nominal 10 V breakdown)?

Accepted Answer

Introduction / Context: Real zener diodes do not exhibit a perfectly flat I–V curve in breakdown. The slope is represented by a small dynamic resistance r_z, which adds an incremental voltage proportional to current changes. Designers must account for this when creating references or clamps. Given Data / Assumptions: Nominal breakdown V_z0 = 10 V (at or near a test current). Dynamic resistance r_z = 8.5 Ω. Operating current I = 20 mA = 0.02 A. Concept / Approach: The additional voltage due to dynamic resistance is ΔV = I * r_z for the operating point (relative to the intercept). Therefore compute ΔV directly from given I and r_z. If the total zener voltage were requested, it would be V_total ≈ V_z0 + ΔV. Step-by-Step Solution: Step 1: Convert current: 20 mA = 0.02 A.Step 2: Compute ΔV = I * r_z = 0.02 * 8.5 = 0.17 V.Step 3: Because the question asks for the additional voltage, report 0.17 V (not the sum). Verification / Alternative check: If asked for the total, V_total ≈ 10 V + 0.17 V = 10.17 V; this corresponds to option 10.17 V, but the stem explicitly requests the additional voltage only. Why Other Options Are Wrong: 10.17 V: This is the total voltage, not the increment. 20 V and 18.8 V: Unrelated numerically to r_z * I. None of the above: Incorrect because 0.17 V is correct. Common Pitfalls: Mixing up “incremental” with “total” voltage leads to choosing 10.17 V. Always read whether the problem asks for ΔV or V_total. Final Answer: 0.17 V.

Question 10

Filter fundamentals: what is the defining action of a high-pass filter on signal frequencies?

Accepted Answer

Introduction / Context: Filters shape spectra to meet bandwidth and noise objectives. High-pass filters are ubiquitous in audio coupling, sensor signal conditioning, and communications front ends. Recognizing their basic pass/reject behavior is foundational before tackling cutoff frequency and order. Given Data / Assumptions: Linear time-invariant filter behavior. “Pass” means relatively low attenuation; “reduce” means attenuate strongly. No special equalization or active gain stages considered. Concept / Approach: A high-pass filter attenuates frequencies below its cutoff while allowing higher frequencies to pass with less attenuation. The simplest RC high-pass uses a series capacitor and shunt resistor; at low frequencies, the capacitor’s reactance is large (blocking), and at high frequencies, it is small (passing). Step-by-Step Solution: 1) Recall RC high-pass impedance: Xc = 1 / (2 * pi * f * C).2) At low f, Xc is high, so output is reduced (attenuated).3) At high f, Xc is low, so the signal passes with minimal loss relative to the passband.4) Therefore, the correct description is “passes higher-frequency signals and reduces lower-frequency signals.” Verification / Alternative check: Bode magnitude of a first-order high-pass shows +20 dB/decade slope below cutoff and ~0 dB flat passband above the corner f_c = 1 / (2 * pi * R * C). Why Other Options Are Wrong: Passes low and reduces high: That is a low-pass filter. No effect on AC: False; filters are defined by frequency-dependent effects. Increases all frequencies: Describes a broadband amplifier, not a filter. None of the above: Incorrect because the high-pass behavior is well defined. Common Pitfalls: Confusing high-pass with low-pass due to series vs shunt placement of components is common. Always check the behavior of Xc versus frequency. Final Answer: passes higher-frequency signals and reduces lower-frequency signals.

Question 11

Basic capacitor design principle (electronics): Which action increases the capacitance of a parallel-plate capacitor, all else being equal (same plate area and dielectric unless noted)?

Accepted Answer

Introduction / Context: Capacitance measures a capacitor’s ability to store charge per unit voltage. For a parallel-plate capacitor, it depends on plate area, separation, and the dielectric constant. Understanding how geometry affects capacitance is fundamental in analog design, filters, and energy storage applications. Given Data / Assumptions: Parallel-plate capacitor model. Plate area A and dielectric constant epsilon are fixed unless explicitly changed. We consider geometric separation d between plates. Concept / Approach: The ideal formula is C = epsilon * A / d, where epsilon = epsilon0 * epsilon_r. Thus, for fixed A and epsilon, the only geometric variable is distance d. Reducing d increases C proportionally; increasing d reduces C. Applied voltage does not change capacitance in the linear region (it changes charge Q = C * V, not C itself). Step-by-Step Solution: Start with C = epsilon * A / d.Identify how to increase C: decrease d or increase A or epsilon_r.Among the given choices, only “move plates closer” decreases d, increasing C. Verification / Alternative check: Consider a numeric example: let epsilon * A = k. If d = 2 mm, C = k/2. Halve the distance to 1 mm → C doubles to k/1. This matches the inverse relationship. Why Other Options Are Wrong: (a) Increases d → reduces C. (c) and (d) alter V, which affects Q but not C for linear dielectrics. (e) is invalid because a correct action is listed. Common Pitfalls: Confusing stored charge with capacitance; thinking that a higher voltage increases capacitance. Capacitance is a geometry and material property, not a function of applied V in ideal capacitors. Final Answer: Moving the plates closer together.

Question 12

Circuit theorems — source transformations: The Norton current IN is also known by which descriptive name in network analysis?

Accepted Answer

Introduction / Context: Norton’s and Thevenin’s theorems let you replace a linear network with an equivalent source and impedance. Understanding the physical meaning of Norton current helps when measuring or calculating equivalents and when interconverting between Norton and Thevenin forms. Given Data / Assumptions: Norton form: a current source IN in parallel with RN. IN equals the short-circuit current seen at the network’s terminals. RN equals the Thevenin/Norton equivalent resistance. Concept / Approach: By definition, the Norton current IN is the current that would flow if the output terminals were shorted together (load replaced by 0 ohms). Thus, IN is also called the short-circuit current, sometimes phrased as the shorted-load current. This contrasts with open-circuit voltage used in Thevenin’s form. Step-by-Step Solution: Define IN: current through a short across the output.Relate to measurement: set load to 0 Ω → measure current → IN.Choose the name that matches: “Shorted-load current.” Verification / Alternative check: The Thevenin equivalent uses VOC, the open-circuit voltage. Converting between forms uses IN = VOC / RTH and RN = RTH, confirming the short-circuit interpretation of IN. Why Other Options Are Wrong: (b) Open-load current would be zero for a linear passive network. (c) There is no standard “Thevenin current” term; Thevenin uses voltage. (d) Thevenin voltage refers to VOC, not a current. Common Pitfalls: Mixing up open-circuit vs short-circuit measurements; forgetting that current sources and voltage sources swap roles between Norton and Thevenin forms. Final Answer: Shorted-load current.

Question 13

BJT bias point (Q-point) intuition: A transistor is biased at the middle of the load line. If the base resistance RB is increased (with other conditions unchanged), what happens to the Q-point on the load line?

Accepted Answer

Introduction / Context: Biasing a BJT at the middle of the load line aims for symmetric signal swing. The base network (often RB or a divider) sets base current and thus collector current for a given transistor. Understanding how RB affects the Q-point is critical for stability and distortion control. Given Data / Assumptions: Transistor initially biased at mid-load-line (roughly half of VCE across transistor). We increase RB while other elements remain unchanged. Assume a fixed supply and temperature; small-signal beta variations ignored. Concept / Approach: Base current IB is approximately (VBIAS − VBE) / RB (for a simple bias). Increasing RB reduces IB. Since IC ≈ beta * IB in the active region, a smaller IB reduces IC. On the load line IC decreases, moving the Q-point downward (toward cutoff), and VCE rises accordingly. Step-by-Step Solution: Original IB sets IC(mid) on the load line.Increase RB → IB decreases.IC decreases → Q-point moves “down” (smaller IC) along the load line.VCE increases as the collector drop RC*IC reduces. Verification / Alternative check: Graphically, with the same load line (fixed RC and supply), decreasing IC shifts the operating point downward (lower IC axis value). SPICE simulations of RB sweep reproduce this movement. Why Other Options Are Wrong: (b) “Up” would require increased IC (opposite effect). (c) A parameter change must move the Q-point somewhere. (d) It stays on the same load line unless parameters like RC or supply change drastically. Common Pitfalls: Confusing the vertical (IC) direction with horizontal (VCE) on the plot; forgetting that less base drive reduces collector current. Final Answer: down.

Question 14

Transformers — qualitative comparison: In a step-down transformer operating under normal conditions, which voltage is larger?

Accepted Answer

Introduction / Context: Transformers change AC voltage levels based on the turns ratio between primary and secondary windings. Recognizing which side is higher for a step-down device is essential in power supplies and isolation stages. Given Data / Assumptions: A step-down transformer is specified (turns ratio Np:Ns > 1). We compare RMS voltages under nominal operation. Losses and regulation are ignored for this qualitative question. Concept / Approach: The ideal transformer relation is Vp/Vs = Np/Ns. For step-down, Np > Ns, so Vp > Vs. Therefore, the primary voltage is larger than the secondary voltage by the turns ratio factor. Step-by-Step Solution: Identify: “step-down” implies Np/Ns > 1.Apply relation: Vp/Vs = Np/Ns → Vp > Vs.Select “Primary voltage.” Verification / Alternative check: Example: Np:Ns = 10:1. If Vp = 230 V RMS, then Vs ≈ 23 V RMS (ideal), confirming primary > secondary. Why Other Options Are Wrong: (b) Secondary is lower in a step-down. (c) Equality would be an isolation (1:1) transformer. (d) Frequency does not reverse step-down vs step-up; it matters for core design, not the basic ratio. Common Pitfalls: Confusing “step-down” with “step-up” or mixing current and voltage relations (current increases on the step-down side). Final Answer: Primary voltage.

Question 15

Basic electrical units: Identify the SI base unit for electric current used in circuit analysis and measurements.

Accepted Answer

Introduction / Context: Electrical engineering relies on a small set of SI units. Recognizing which quantity each unit measures helps when interpreting datasheets and equations such as Ohm’s law and power relations. Given Data / Assumptions: The question asks specifically for the SI base unit of electric current. Common electrical units: volt (V), watt (W), ohm (Ω), and ampere (A). We distinguish base vs derived units. Concept / Approach: The ampere (A) is the SI base unit for electric current. By contrast, the volt is a derived unit (J/C), the ohm is derived (V/A), and the watt is derived (J/s = V*A). Thus, only “ampere” correctly answers the prompt. Step-by-Step Solution: List candidates: volt, watt, ampere, ohm.Identify base vs derived: ampere is base; others are derived.Select “ampere.” Verification / Alternative check: Ohm’s law I = V/R uses ampere directly for current. Power P = V * I shows watt as derived from volt and ampere. Why Other Options Are Wrong: (a) Volt measures electric potential difference. (b) Watt measures power. (d) Ohm measures resistance. None are base units for current. Common Pitfalls: Confusing commonly seen units with base units; many electrical units are derived from A, s, kg, m, and K. Final Answer: ampere.

Question 16

BJT current gain (β) vs collector current (IC): From typical device graphs, how does the DC current gain generally behave as IC varies over the normal operating range?

Accepted Answer

Introduction / Context: In bipolar junction transistors (BJTs), the DC current gain β (or hFE) is not a perfect constant. Datasheets include plots of β versus collector current IC and temperature to guide biasing. Understanding the qualitative behavior helps choose operating points with stable gain and low distortion. Given Data / Assumptions: We consider a typical small-signal BJT over its recommended IC range. Temperature is assumed nominal and fixed. We focus on qualitative variation, not exact numerical β. Concept / Approach: For most BJTs, β rises from very low IC, reaches a broad maximum across a mid-current region, then declines at higher IC due to high-level injection and other effects. Over the practical mid-range used for linear amplification, β changes modestly—often treated as “approximately constant,” but in reality it varies slightly with IC and temperature. Step-by-Step Solution: Examine typical β–IC curves: low IC → lower β; mid IC → higher β; high IC → β roll-off.Interpretation: not constant, but not wildly changing within the normal region.Therefore, the best qualitative description is “varies slightly.” Verification / Alternative check: Datasheet plots (e.g., for 2N3904/BC547 families) show β variation across decades of IC, with relatively mild change within a recommended operating band. Why Other Options Are Wrong: (a) “Enormous” variation is not representative of the mid-range. (b) Exactly constant is idealized and untrue. (d) Although β = IC/IB is true as a definition, it does not answer how β behaves with IC; it is not a behavior description. Common Pitfalls: Assuming β is a fixed number regardless of bias; ignoring temperature and device spread. Designers often include emitter degeneration to desensitize circuits to β variations. Final Answer: varies slightly.

Question 17

In AC circuit theory, consider a parallel R–C (resistor–capacitor) network connected to a single AC source. Which statement best describes the voltage distribution and phase in such a parallel RC circuit?

Accepted Answer

Introduction / Context: A key property of parallel AC networks is that all components connected across the same two nodes share exactly the same voltage waveform. In a parallel R–C circuit, the resistor current is in phase with the voltage, while the capacitor current leads the voltage by 90 degrees. This question probes whether you remember that voltage in parallel is common, whereas current divides with different phase angles. Given Data / Assumptions: Ideal sinusoidal steady-state conditions. Ideal components (R and C) connected in parallel to a single AC source. No parasitic wiring impedance considered. Concept / Approach: By definition of parallel circuits, each branch is connected to the same pair of nodes; therefore, the branch voltages are equal in amplitude and phase to the source voltage. The branch currents differ in amplitude and phase due to the elements’ impedances: I_R is in phase with V, and I_C leads V by 90 degrees. The vector sum of these branch currents gives the total current, which leads V by some angle between 0 and 90 degrees; however, the voltage itself does not lead the current in this case—rather, it lags the leading current. Step-by-Step Solution: Identify topology: R and C in parallel across the same source nodes.Apply the parallel rule: V_R = V_C = V_source in both amplitude and phase.Determine currents: I_R in phase with V; I_C leads V by 90 degrees.Infer total current: I_total leads V by an angle between 0 and 90 degrees due to capacitive branch.Conclude: Voltage is common to branches; therefore option (a) is correct. Verification / Alternative check: Use phasor analysis: V is the reference phasor. I_R = V/R (angle 0), I_C = V/X_C (angle +90). Since both currents derive from the same V phasor, branch voltages necessarily match the source exactly. Why Other Options Are Wrong: (b) Summing voltages branch-wise is a series rule, not parallel. (c) Reverses the lead/lag: in RC parallel, current leads voltage, not the other way. (d) Implies a series-like relationship, which is inapplicable in parallel networks. Common Pitfalls: Confusing series and parallel rules; assuming voltage division in parallel; mixing up whether voltage or current leads in capacitive circuits. Final Answer: The voltage always has the same amplitude and phase at every parallel branch (same node voltage).

Question 18

Series R–L–C resonance behavior: How does the total impedance of a series RLC circuit vary at the resonant frequency compared to off-resonance?

Accepted Answer

Introduction / Context: Resonance in a series RLC circuit is a foundational AC topic. At resonance, the inductive reactance and capacitive reactance cancel, leaving only the resistive part. Understanding this helps predict current peaks and bandwidth in filters and tuners. Given Data / Assumptions: Series connection of R, L, and C. Sinusoidal steady state. Resonant frequency f_0 where X_L = X_C. Concept / Approach: Total impedance Z for series RLC is Z = R + j(X_L − X_C). At resonance, X_L = X_C, so Z = R. Since R is the smallest possible magnitude of Z (reactances cancel out), the impedance magnitude is minimal and the current is maximal for a given source voltage. Step-by-Step Solution: Write reactances: X_L = 2πfL, X_C = 1/(2πfC).At f = f_0, X_L − X_C = 0.Therefore Z = R (purely real), |Z| = R.Away from resonance, |Z| = sqrt(R^2 + (X_L − X_C)^2) > R.Hence |Z| is minimum at resonance. Verification / Alternative check: Examine current I = V/|Z|. The current peaks at resonance—empirical confirmation of minimal impedance. Why Other Options Are Wrong: (b) and (c) impose monotonic behavior with frequency that is not true; impedance decreases toward resonance and increases away. (d) The maximum occurs far from resonance for large reactance differences, not at resonance. Common Pitfalls: Confusing series with parallel resonance (where behavior differs) and mixing up current/impedance relationships. Final Answer: It is minimum at the resonant frequency.

Question 19

Units and magnitudes in circuit analysis: What is the correct equivalent value of 1 megaohm (1 MΩ) expressed in ohms?

Accepted Answer

Introduction / Context: Accurate unit conversions are essential in electronics. Confusing kilo (10^3), mega (10^6), and milli (10^−3) leads to major design and measurement errors. This item checks your comfort with SI prefixes as applied to resistance units (ohms). Given Data / Assumptions: Prefix definitions: k = 10^3, M = 10^6, m = 10^−3, μ = 10^−6. Target: convert 1 MΩ to Ω. Context: typical datasheets use these prefixes regularly. Concept / Approach: Apply SI prefix conversion: 1 megaohm = 1 × 10^6 ohms. Compare with common confusions: 1 kΩ = 10^3 Ω; one-millionth of an ohm is micro-ohm (μΩ), not megaohm. Step-by-Step Solution: Identify prefix: M = 10^6.Compute: 1 MΩ = 1 × 10^6 Ω.State in words: one million ohms. Verification / Alternative check: Cross-check with other prefixes: 1 kΩ = 1,000 Ω confirms that mega is three orders of magnitude larger than kilo. Why Other Options Are Wrong: (a) 1 kΩ is 1,000 Ω, not 1 MΩ. (b) 100,000 Ω equals 100 kΩ. (c) One-millionth of an ohm is 1 μΩ, the inverse of mega in exponent sign. Common Pitfalls: Mixing up M (mega) with m (milli) due to case sensitivity; always respect uppercase vs lowercase in SI. Final Answer: 1,000,000 Ω (one million ohms)

Question 20

Current-source stiffness (Norton model intuition): A current source delivers an ideal 10 mA and has an internal resistance of 20 MΩ in parallel. For which load resistance values will the source appear “stiff” (i.e., load current remains very close t…

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Introduction / Context: “Stiffness” for a current source means the delivered load current stays close to the ideal source current regardless of reasonable load changes. A Norton current source has an internal resistance in parallel with the ideal current source. Current divides between the internal resistance and the external load; to keep the load current near the nominal value, we want most current to go through the load, not the internal shunt path. Given Data / Assumptions: Ideal current I_N = 10 mA. Internal (shunt) resistance R_int = 20 MΩ in parallel. Load resistance R_L in parallel with R_int. Concept / Approach: For a Norton source, the load current is I_L = I_N * (R_int / (R_int + R_L)). If R_L ≪ R_int, then R_int/(R_int + R_L) ≈ 1, making I_L ≈ I_N. A practical “stiff” guideline is that R_L should be at least one to two orders of magnitude smaller than R_int so that the current through R_int is negligible. Step-by-Step Solution: Compute order-of-magnitude: R_int = 20 MΩ; choose R_L ≤ 0.01 * R_int ≈ 200 kΩ for ≲1% error.Check current division: If R_L = 200 kΩ, factor = 20 MΩ / (20 MΩ + 200 kΩ) ≈ 20,000 kΩ / 20,200 kΩ ≈ 0.99 → I_L ≈ 9.9 mA.Thus, loads of 200 kΩ or less keep the source behavior close to ideal 10 mA. Verification / Alternative check: Try R_L = 20 kΩ: factor ≈ 20,000 kΩ / 20,020 kΩ ≈ 0.999 → even stiffer. As R_L decreases further, the approximation improves. Why Other Options Are Wrong: (a) “20 kΩ or more” includes large R_L values approaching MΩ, which reduce stiffness. (c) “200 kΩ or more” trends the wrong way for stiffness. (d) “20 km” is a units mistake (distance, not resistance). Common Pitfalls: Confusing Norton (parallel R_int) with Thevenin (series R_th); forgetting that for current sources, small R_L compared to R_int is desirable for “stiffness.” Final Answer: 200 kΩ or less

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