Question 1

In real-world electronics, fixed resistors can fail after stress or aging. What is the most common failure mode observed for fixed resistors when they are overstressed or reach end-of-life?

Accepted Answer

Introduction / Context: Fixed resistors are widely used as passive current limiters and voltage dividers. Understanding how they typically fail helps technicians diagnose faults quickly and helps designers choose proper power ratings and derating margins to improve reliability. Given Data / Assumptions: We consider common fixed resistor technologies such as carbon film, metal film, metal oxide, and wirewound. The stressors include excessive power dissipation, overvoltage transients, thermal cycling, and environmental factors. We focus on the usual failure mode, not rare corner cases. Concept / Approach: When a resistor is overstressed (P_actual > P_rated or sustained high temperature), the resistive element can crack, burn, or separate from end caps. The typical electrical result is loss of continuity (open circuit), since the film or wire breaks or lead termination fails. Although resistance drift upward can precede failure, the final catastrophic state is usually open rather than short. Step-by-Step Solution: Identify stress: excess power causes temperature rise.Thermal damage: resistive film or wire weakens, micro-cracks form.Separation: conductor path breaks or solder/end-cap joint fails.Electrical symptom: measured resistance goes to extremely high value (open). Verification / Alternative check: In-circuit, the open resistor stops current flow. Out-of-circuit DMM measurement reads very high/OL. Visual inspection often shows discoloration or cracking. Reliability literature and field repair experience consistently report “goes open” as the dominant end state for overheated fixed resistors. Why Other Options Are Wrong: Slowly over time: Descriptive of drift, not the predominant ultimate failure mode. By increasing their value: Upward drift can occur, but the most common failure is open. Increase then open: Possible, but the single most typical mode asked by the question is simply “open.” Common Pitfalls: Confusing temporary thermal coefficient changes with permanent damage. Expecting shorts: shorts are rare in fixed resistors; opens dominate. Final Answer: by becoming an open circuit

Question 2

In resistor networks packaged as SIP (single in-line package) or DIP (dual in-line package) resistor arrays, which technology is most commonly used for the integrated resistors?

Accepted Answer

Introduction / Context: SIP and DIP resistor networks are compact packages that house multiple resistors for pull-ups, pull-downs, and biasing in digital and mixed-signal designs. Knowing the underlying resistor technology helps predict tolerance, noise, and temperature behavior. Given Data / Assumptions: We are discussing typical commercial SIP/DIP resistor arrays used on PCBs. Key tech candidates include thick film, thin/metal film, metal oxide, and wirewound. Goal: identify what is most commonly used in these arrays. Concept / Approach: Thick-film processes screen-print resistive pastes onto ceramic substrates, then laser-trim values. This method is economical, dense, and well-suited to multi-element arrays integrated inside SIP or DIP packages. Wirewound parts are physically larger and better for power applications, not multi-resistor IC-like arrays. Metal film arrays exist but are less common and costlier at the array level compared to thick film solutions. Step-by-Step Solution: Consider package constraints: many resistors on a small ceramic substrate.Match process: thick film enables compact, batch fabrication and trimming.Evaluate cost/performance: thick film strikes favorable balance for pull-ups and general biasing networks.Conclude: thick film is the prevalent technology in SIP/DIP resistor networks. Verification / Alternative check: Vendor datasheets for common resistor arrays consistently list thick-film technology, sometimes specifying resistance tolerance (e.g., 1–5%) and TCR ranges consistent with thick film compositions. Why Other Options Are Wrong: Metal film: Used in precision networks but less common as the mainstream SIP/DIP array technology. Wirewound: Not suitable for dense arrays; used for power and low-inductance applications. Metal oxide: Often used for discrete resistors with good surge behavior, not typical for arrays. Common Pitfalls: Assuming discrete resistor technology directly translates to arrays. Overlooking cost and manufacturability constraints that favor thick film. Final Answer: thick film

Question 3

For standard leaded resistors, how is the wattage (power rating) typically identified or inferred during selection and inspection?

Accepted Answer

Introduction / Context: Resistor power rating (wattage) determines how much heat a resistor can safely dissipate without damage. Correctly recognizing wattage helps prevent overheating and early failures in prototypes and repairs. Given Data / Assumptions: We refer to common through-hole resistors (carbon film, metal film, metal oxide) used on PCBs. No explicit printed wattage on body; color bands usually indicate resistance and tolerance. Physical form factor correlates with wattage rating. Concept / Approach: Manufacturers standardize body lengths and diameters for typical wattage classes (e.g., 1/8 W, 1/4 W, 1/2 W, 1 W). Larger bodies provide more surface area and volume for heat dissipation. Therefore, practitioners generally infer wattage by physical size, often consulting datasheets or sizing charts to match dimensions to rating. Step-by-Step Solution: Identify resistor package dimensions (length/diameter).Compare with standard size-to-wattage tables from vendors.Assign correct wattage class for design or replacement.Verify thermal margins using P = I^2 * R or P = V^2 / R and derating as needed. Verification / Alternative check: Datasheets list exact body dimensions versus wattage. Visual comparison using a kit or calipers confirms the intended rating. For SMD resistors, the package code (e.g., 0603, 1206) and datasheet specify the wattage. Why Other Options Are Wrong: Color code: Indicates resistance and tolerance, not wattage. Types of materials / internal construction: These affect performance but are not a quick external identifier for wattage in most commodity resistors. Common Pitfalls: Mistaking tolerance band color (e.g., gold, silver) for wattage information. Ignoring ambient temperature and airflow; even correct wattage parts need derating. Final Answer: size

Question 4

Using the 4-band resistor color code (1st digit, 2nd digit, multiplier, tolerance not given), what resistance value corresponds to the colors: orange, orange, orange?

Accepted Answer

Introduction / Context: Being able to decode resistor color bands is a fundamental skill for electronics assembly and troubleshooting. This problem uses the common 4-band scheme where the first two bands are significant digits and the third band is the multiplier. Given Data / Assumptions: Color sequence: orange (1st), orange (2nd), orange (multiplier). No tolerance band is specified (often gold for 5% or silver for 10%). Standard digit mapping: black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, gray=8, white=9. Concept / Approach: Translate the first two bands into digits and apply the multiplier from the third band. Finally, convert to a convenient unit (ohms to kilohms) for readability and to match option formatting. Step-by-Step Solution: 1st band (orange) → 3.2nd band (orange) → 3.3rd band (orange) → multiplier 10^3.Resistance R = 33 * 10^3 ohms = 33,000 ohms = 33 kilohms. Verification / Alternative check: Cross-check with a color code chart: orange–orange–orange = 33 with a multiplier of 10^3, confirming 33 kΩ. If a fourth band were gold, tolerance would be ±5%, but tolerance does not change the nominal value. Why Other Options Are Wrong: 22 kilohms: Would correspond to red–red–orange. 3300 ohms: That is 3.3 kΩ, which would be orange–orange–red (10^2 multiplier). 44000 ohms: Would require digits 44, which is not possible in a 2-digit scheme. Common Pitfalls: Misreading the multiplier band or reading the resistor from the wrong end. Confusing red (2) and orange (3), which are visually similar in poor lighting. Final Answer: 33 kilohms

Question 5

For a household lamp rated 100 W and supplied from a 120 V AC source (assume ideal resistive behavior at rated conditions), what steady-state current does it draw?

Accepted Answer

Introduction / Context: Lamp nameplates list rated power and voltage. From these two values, the operating current can be estimated assuming resistive behavior at steady state. This is a straightforward application of the power relation P = V * I. Given Data / Assumptions: Rated power P = 100 W. Supply voltage V ≈ 120 V (RMS). Assume the lamp behaves approximately as a resistive load at operating temperature. Concept / Approach: For a resistive load, power equals the product of RMS voltage and RMS current. Rearranging gives I = P / V. Although incandescent filaments change resistance with temperature when warming up, the steady-state current at rating follows directly from this formula. Step-by-Step Solution: Use I = P / V.I = 100 W / 120 V = 0.833… A.Express in milliamperes: 0.833 A ≈ 833 mA, commonly rounded to 830 mA.Therefore, the lamp draws about 830 mA at 120 V. Verification / Alternative check: Compute equivalent resistance: R = V^2 / P = 120^2 / 100 = 144 Ω. Then I = V / R = 120 / 144 = 0.833 A, consistent with the direct calculation. Why Other Options Are Wrong: 1.2 amps or 12 amps: Would imply P = 144 W or 1440 W at 120 V, not 100 W. 12000 amps: Physically impossible here; a clear distractor. Common Pitfalls: Confusing watts with volt–amperes or using peak instead of RMS voltage. Ignoring warm-up behavior; initial inrush may be higher, but the rating refers to steady state. Final Answer: 830 mA

Question 6

A 10 W lamp operates continuously for one minute. How much energy (in joules) does it dissipate during that time interval?

Accepted Answer

Introduction / Context: Power quantifies the rate at which energy is used or converted, while energy measures the total amount consumed over time. Converting between power (watts) and energy (joules) is a fundamental calculation in electrical engineering and physics. Given Data / Assumptions: Lamp power P = 10 W (that is, 10 joules per second). Operating time t = 1 minute = 60 seconds. Assume constant power during the interval. Concept / Approach: Energy E is the time integral of power. For constant power, E = P * t. Units: 1 W = 1 J/s, so multiplying by seconds gives joules directly. This simple relation underpins energy billing (e.g., kilowatt-hours) and battery capacity calculations when adapted to appropriate units. Step-by-Step Solution: Identify P = 10 W and t = 60 s.Compute energy: E = P * t = 10 * 60 = 600 J.Therefore, the lamp dissipates 600 joules in one minute. Verification / Alternative check: Convert to watt-hours for intuition: 10 W for 1 minute is (10 W) * (1/60 h) = 0.1667 Wh. Convert Wh to joules using 1 Wh = 3600 J → 0.1667 * 3600 ≈ 600 J, confirming the result. Why Other Options Are Wrong: 10 joules: That would be the energy after only 1 second at 10 W. 60 joules: Corresponds to 10 W for 6 seconds. 3600 joules: That would be 10 W for 360 seconds (6 minutes) or 1 W for 1 hour. Common Pitfalls: Mistaking minutes for seconds; always convert time to seconds when using SI units. Confusing power (rate) with energy (total amount). Final Answer: 600 joules

Question 7

From the perspective of a power source feeding a complex network of interconnected components, what does the source effectively “sense” at its output terminals?

Accepted Answer

Introduction / Context: Regardless of how complicated a circuit is internally, the source connects to only two terminals: its positive and negative outputs. The source delivers current and voltage based on the total equivalent impedance seen between these terminals. Given Data / Assumptions: The internal network can include many series, parallel, and reactive elements. The question is conceptual: what the source “sees” is the net effect, not the internal details. We assume linear conditions for intuition, though the concept holds generally. Concept / Approach: When reduced by circuit analysis, any two-terminal network is equivalent to a single impedance (or a Thevenin/Norton equivalent). Thus the source responds to the total load, not to individual components independently. For DC resistive analysis, that total reduces to a single equivalent resistance at the terminals. Step-by-Step Solution: Combine internal elements by series/parallel reduction or via Thevenin/Norton.Obtain an equivalent impedance Z_eq seen by the source.For purely resistive networks, Z_eq → R_eq, appearing as a single resistive connection.Therefore, the source “senses” one equivalent connection rather than individual parts. Verification / Alternative check: Measure source current I and terminal voltage V; the ratio V/I defines the terminal impedance. Different internal configurations that yield the same Z_eq will appear identical to the source. Why Other Options Are Wrong: Open circuit components: Internals may include opens, but the source responds to the resulting Z_eq, not specifically to “opens.” When voltages need to be increased / when complex currents are needed: The source does not “decide” this; it simply drives according to its characteristic and the terminal load. Common Pitfalls: Thinking a source “sees” each component; it sees only the combined effect. Ignoring reactive elements; at AC the source sees a complex impedance, but in resistive terms the concept is analogous. Final Answer: only a single resistive connection

Question 8

Considering the construction and typical use of a rheostat (a variable resistor used as a 2-terminal adjustable resistance), how many external connections does it normally employ?

Accepted Answer

Introduction / Context: A rheostat is a variable resistor commonly used to adjust current or set a coarse resistance in power applications. Although many potentiometers have three terminals, a rheostat is used with only two to create an adjustable series resistance. Given Data / Assumptions: Typical device has three terminals physically: two end terminals of the resistive element and a wiper. Rheostat connection convention uses the wiper and one end terminal. Goal: number of connections in normal rheostat usage. Concept / Approach: Using the wiper and one end gives a variable resistance equal to the portion of the track between them. The unused terminal is left floating (or sometimes tied to wiper for noise reduction, but still two external connections are functionally used). Hence, operationally, the rheostat is a two-terminal element. Step-by-Step Solution: Identify terminals: end A, wiper, end B.Choose wiper + one end (A or B) to form the rheostat connections.As the wiper moves, resistance changes from near 0 up to the full track resistance.Therefore, the normal count of connections used is 2. Verification / Alternative check: Datasheets often describe potentiometer usage modes: as voltage divider (3-terminal) or as rheostat (2-terminal). The latter confirms the expected answer. Why Other Options Are Wrong: 1: A single connection cannot form a complete path. 3 or 4: Three terminals are for potentiometer divider mode; rheostat operation uses two. Common Pitfalls: Assuming all potentiometers are always used with three terminals. Confusing physical terminal count with the connected terminal count in a given application. Final Answer: 2

Question 9

When checking a fuse with an ohmmeter (out of circuit), what resistance reading best indicates that the fuse is intact and functioning correctly?

Accepted Answer

Introduction / Context: A fuse is a protective device intended to open (break) a circuit when excessive current flows. Verifying a fuse with an ohmmeter is a quick diagnostic step during troubleshooting of power and control circuits. Given Data / Assumptions: Fuse removed from the circuit for accurate measurement. Digital multimeter set to resistance or continuity mode. Room-temperature test with no current flowing. Concept / Approach: An intact fuse is essentially a short piece of low-resistance metal designed to melt under overcurrent. Therefore, a good fuse should show very low resistance (near zero) and typically will beep in continuity mode. A blown fuse has a melted link and will read infinite or over-range resistance. Step-by-Step Solution: Remove fuse from the circuit to avoid parallel paths.Place meter leads on both ends of the fuse.Reading near 0 Ω (often a fraction of an ohm) indicates continuity → good fuse.Reading OL or very high resistance indicates an open → blown fuse. Verification / Alternative check: Visual inspection can reveal a broken element or discoloration, but some fuses fail invisibly; the ohmmeter provides definitive electrical confirmation. Why Other Options Are Wrong: Medium or high resistance: A fuse is not a resistor; such readings indicate a problem or a measurement error. Infinite resistance: That is the hallmark of a blown fuse. Common Pitfalls: Measuring in-circuit leads to false continuity through alternate paths. Confusing continuity beeper thresholds with absolute resistance; meters typically beep below a few ohms which is normal for a good fuse. Final Answer: zero ohms resistance

Question 10

Rheostat construction and components: In basic electronics, a rheostat (a two-terminal variable resistor used for current control) comprises which fundamental parts?

Accepted Answer

Introduction / Context: A rheostat is a variable resistor typically used to control current in circuits such as dimmers, heaters, and test setups. Understanding its internal parts helps technicians diagnose faults (e.g., noisy adjustment, open track) and choose appropriate components for power handling and resolution. Given Data / Assumptions: The device discussed is a two-terminal variable resistor used primarily for current control (rheostat mode), not a three-terminal potentiometer in divider mode. Standard mechanical construction with a resistive element and a movable contact. No special multi-gang or digital control features assumed. Concept / Approach: A rheostat varies resistance by sliding a movable contact called the wiper along a resistive element (track). The effective resistance between the wiper and one fixed terminal changes with position. In high-power designs, the track is often wirewound on a ceramic former for heat dissipation and robustness. Step-by-Step Solution: Identify the resistive path: a continuous resistor track (carbon composition, cermet, or wirewound).Identify the movable contact: a spring-loaded wiper that maintains pressure on the track.Connection scheme: two terminals are used—one end of the track and the wiper—forming a variable series resistor.Practical effect: rotating or sliding the control changes the length of track in circuit, thereby adjusting resistance and current. Verification / Alternative check: Exploded drawings and datasheets consistently show a resistive element plus a wiper assembly. Potentiometers add a third terminal (the other end of the track) for divider operation, but the physical parts remain wiper + track. Why Other Options Are Wrong: Solenoid and armature: those are electromechanical actuator parts, not rheostat internals. Contact and wire wound: incomplete—missing the essential concept of a wiper sliding on a track (not just “contact”). Center tap and wiper: a center tap is not a standard rheostat feature. Rotor and stator: motor terminology, not applicable here. Common Pitfalls: Confusing potentiometers (three-terminal) with rheostats (two-terminal usage). Assuming all rheostats are carbon; many are wirewound for power. Final Answer: wiper and resistor track

Question 11

Ohm’s law power check: With 1 ampere flowing through a 1 ohm resistor, what is the electrical power dissipated in the resistor?

Accepted Answer

Introduction / Context: Relating current, resistance, voltage, and power is foundational in circuit analysis. When a known current flows through a known resistance, we can compute both voltage drop and power dissipation. This question reinforces the correct unit and formula for power in resistive elements. Given Data / Assumptions: Current I = 1 A through a pure resistor. Resistance R = 1 Ω (ohm). Steady-state DC conditions; no reactive components. Concept / Approach: Ohm’s law: V = I * R. Power relations for resistors: P = V * I = I^2 * R = V^2 / R. Using any of these forms yields the same result if variables are consistent. Units: watt (W) is joule per second, the SI unit of power (rate of energy transfer). Step-by-Step Solution: Compute voltage: V = I * R = 1 A * 1 Ω = 1 V.Compute power: P = I^2 * R = (1 A)^2 * 1 Ω = 1 W.Alternative check: P = V * I = 1 V * 1 A = 1 W (same result). Verification / Alternative check: Dimensional analysis confirms that ampere^2 * ohm simplifies to watt. Measuring with a wattmeter or by monitoring temperature rise (calorimetry) also validates dissipation in practical setups. Why Other Options Are Wrong: 1 horsepower and 1 Btu: units of power and energy respectively but not equal to this electrical case; their magnitudes differ widely. 1 joule: a unit of energy, not power (energy per unit time). 1 volt: a voltage value (which is present) but the question asks specifically for power. Common Pitfalls: Confusing power (watts) with energy (joules). Assuming “1 A through 1 Ω” implies only a voltage answer; power requires P relations. Final Answer: 1 watt

Question 12

Power dissipation from voltage and resistance: What power is dissipated by a 1.2 kΩ resistor when 12 V is applied across it? (Assume a purely resistive load.)

Accepted Answer

Introduction / Context: Computing resistor power is essential for selecting safe component wattage ratings and avoiding overheating. Using Ohm’s law with power relations ensures correct sizing in practical circuits like voltage dividers and bias networks. Given Data / Assumptions: Resistance R = 1.2 kΩ = 1200 Ω. Applied voltage V = 12 V (DC or RMS AC for a resistor). Ideal resistor, no reactive effects. Concept / Approach: The relevant formula is P = V^2 / R for a fixed resistance with known voltage. Alternative equivalent forms include P = I^2 * R and P = V * I after computing current I via I = V / R. Step-by-Step Solution: Convert resistance: R = 1.2 kΩ = 1200 Ω.Apply formula: P = V^2 / R = 12^2 / 1200.Compute: 12^2 = 144; 144 / 1200 = 0.12.Units: 0.12 W = 120 mW. Verification / Alternative check: Compute current first: I = V / R = 12 / 1200 = 0.01 A = 10 mA. Then P = V * I = 12 V * 0.01 A = 0.12 W, confirming the result. Why Other Options Are Wrong: 12 W / 1.2 W / 0.48 W: too large given the modest current of 10 mA. 12 mW: too small by a factor of 10. Common Pitfalls: Forgetting to convert kilohms to ohms. Mixing up V^2/R with I^2*R leading to arithmetic slips; both yield the same when applied correctly. Final Answer: 0.12 W

Question 13

Effect of doubling source voltage: For a fixed resistor obeying Ohm’s law, if the applied voltage doubles, what happens to the circuit current and the resistance value?

Accepted Answer

Introduction / Context: Ohm’s law describes linear resistive behavior, allowing predictions when voltage or current changes. Many troubleshooting tasks hinge on recognizing that fixed resistors do not change value merely because the applied voltage changes within ratings. Given Data / Assumptions: Ideal, linear, temperature-stable resistor. Initial conditions follow Ohm’s law V = I * R. Voltage is doubled; no thermal or nonlinear effects considered. Concept / Approach: For a constant R, current I is proportional to voltage V. Thus, doubling V doubles I. The resistor’s value is a property of the component and remains unchanged (ignoring temperature coefficients and damage limits). Step-by-Step Solution: Start with V = I * R.If V becomes 2V and R is constant, then new current I′ satisfies 2V = I′ * R.Therefore I′ = 2V / R = 2 * (V / R) = 2I.Resistance remains R (component property), so it is unchanged. Verification / Alternative check: Numerical example: R = 100 Ω, V = 10 V → I = 0.1 A. Double V to 20 V → I = 0.2 A (doubles), R stays 100 Ω. Why Other Options Are Wrong: Resistance does not double or halve simply because voltage changes. Current does not remain the same if voltage changes across a fixed resistor. Current halving would occur if voltage halved, not doubled, for constant R. Common Pitfalls: Overlooking resistor heating; extreme voltages can change resistance via temperature coefficients or cause failure, but that is outside ideal assumptions. Final Answer: Current doubles and resistance remains the same.

Question 14

Definition refresher — what is electrical power? Choose the most accurate statement describing power in physics/electronics.

Accepted Answer

Introduction / Context: Power underpins sizing of supplies, resistors, motors, and thermal design. It connects directly to energy consumption and operating cost, and determines heat dissipation in components. Clarity on the definition avoids confusion between power (rate) and energy (quantity). Given Data / Assumptions: Conventional SI usage: power in watts (W), energy in joules (J). Electrical context: P = V * I = I^2 * R = V^2 / R for resistors. Time dimension matters: power is energy per unit time. Concept / Approach: Power is defined as the rate of doing work or the rate of energy transfer/consumption. One watt equals one joule per second. Many processes both “use” and “generate” energy, but the core definition emphasizes the rate. Choosing wording that universalizes across loads and sources helps retain the concept. Step-by-Step Solution: Define: Power P = dE/dt (energy rate).Link to circuits: P = V * I in any two-terminal element (positive or negative depending on passive sign convention).Interpret units: 1 W = 1 J/s (joule per second).Therefore, “rate at which energy is used” (or converted/transferred) captures the essence. Verification / Alternative check: For a 60 W lamp operating 1 hour, energy consumed is 60 W * 3600 s = 216 kJ, demonstrating power × time gives energy. Why Other Options Are Wrong: “watts”: a unit name, not the definition. “energy”: quantity, not rate. “rate at which energy is generated”: could be true for sources, but the more universal expression is energy transfer/use per unit time; option A is clearer and context-neutral. “force over a distance without time”: that defines work (energy), not power. Common Pitfalls: Confusing energy (kWh) with power (kW) on utility bills. Assuming constant power when voltage or current varies with time; average power may differ from instantaneous power. Final Answer: the rate at which energy is used

Question 15

American Wire Gauge (AWG): What is the diameter in mils (thousandths of an inch) of 22-AWG copper wire?

Accepted Answer

Introduction / Context: Technicians frequently convert between American Wire Gauge (AWG) and physical diameter for connector selection, current-carrying capacity, and estimating resistance per length. Mils (0.001 inch) provide a convenient imperial measure of small diameters. Given Data / Assumptions: Standard AWG system (Brown & Sharpe). Requested size: 22 AWG copper wire. 1 mil = 0.001 inch. Concept / Approach: AWG sizes follow a logarithmic relation: diameter d(n) in inches = 0.005 inch * 92^((36 − n)/39). While this formula is exact, many practitioners memorize common sizes (e.g., 20 AWG ≈ 0.0320 in, 22 AWG ≈ 0.0253 in, 24 AWG ≈ 0.0201 in). Converting inches to mils multiplies by 1000. Step-by-Step Solution: Recall diameter for 22 AWG ≈ 0.0253 inch.Convert to mils: 0.0253 inch × 1000 = 25.3 mils.Express to two decimals: 25.30 mils. Verification / Alternative check: Use the AWG formula: d(22) = 0.005 × 92^((36 − 22)/39) ≈ 0.0253 in. Tables in handbooks corroborate 22 AWG ≈ 0.0253 in (25.3 mils), cross-sectional area ≈ 642 circular mils. Why Other Options Are Wrong: 10.03 mils: far too small (closer to ~30 AWG). 22.35 mils: somewhat smaller than true; not the standard value. 45.26 and 71.96 mils: much larger; correspond to far lower gauge numbers (thicker wires). Common Pitfalls: Confusing circular mil area with diameter in mils; they are different quantities. Mixing SWG (Standard Wire Gauge) with AWG, which yields different diameters. Final Answer: 25.30 mils

Question 16

Meaning of “work” in physics/electronics: In practical terms, when we say that “work” has been done, what does that imply?

Accepted Answer

Introduction / Context: Work and energy are central to electrical and mechanical systems. In electronics, electrical work occurs when charges move through a potential difference, transferring energy to components (for example, heating a resistor or charging a capacitor). Knowing the language clarifies specifications and lab reports. Given Data / Assumptions: Classical definitions apply (no quantum nuances required). Work W and energy E share the same unit (joule) in SI. Power P relates to work by time: P = dW/dt. Concept / Approach: Work is the process of energy transfer. In mechanics, W = force × displacement in the direction of force. In circuits, electrical work equals charge moved times voltage (W = Q * V). Whenever work is done, energy moves from a source to a load or between forms (electrical, thermal, mechanical). Step-by-Step Solution: Recognize equivalence: Work and energy have identical dimensions (joule).In circuits: moving charge Q through potential V transfers energy W = Q * V.In mechanics: applying force over a distance transfers energy to or from an object.Conclusion: saying “work has been done” means energy transfer has occurred. Verification / Alternative check: Example: A 5 V source moves 2 C of charge through a resistor → W = 2 * 5 = 10 J transferred (and dissipated as heat), demonstrating the concept. Why Other Options Are Wrong: Inverse relation/“not related”: incorrect; they are directly linked. “no energy transferred”: contradicts the definition. “power has increased regardless of time”: power is rate; work concerns total energy transfer, not necessarily implying increased power. Common Pitfalls: Confusing power (rate) with work/energy (quantity). Assuming zero work if net displacement is zero, overlooking cases like cyclic processes where energy can be exchanged. Final Answer: energy has been transferred

Question 17

Incandescent lamp resistance behavior: Which statement best describes the resistance characteristics of a typical incandescent filament lamp?

Accepted Answer

Introduction / Context: Filament lamps are strongly temperature-dependent resistors. Understanding their cold versus hot resistance helps in surge analysis, dimmer design, and failure diagnosis (inrush currents at switch-on can be many times the steady current). Given Data / Assumptions: Tungsten filament with positive temperature coefficient of resistance. Supply is within rated voltage so filament reaches normal operating temperature. No series ballast assumed unless specified. Concept / Approach: At room temperature (cold), filament resistance is low. When energized, current heats the filament to thousands of kelvin; resistance increases several-fold (hot resistance). Therefore, the device exhibits both a cold resistance (initial) and a hot resistance (operating), and these values are markedly different. Step-by-Step Solution: Initial switch-on: low cold resistance allows high inrush current.Self-heating raises temperature → resistance increases (positive tempco).Steady state: hot resistance limits current to rated value and sets light output.Conclusion: the lamp possesses both cold and hot resistance values. Verification / Alternative check: Measure resistance with an ohmmeter (cold) and compute operating resistance from V/I at rated operation (hot). Hot resistance often measures ~10× the cold resistance, confirming the dual behavior. Why Other Options Are Wrong: “cold resistance” or “hot resistance” alone ignores the other operating condition. “ballast resistance”: applies to fluorescent/LED drivers, not incandescent filaments. “zero resistance”: no practical conductor has zero resistance; filament certainly does not. Common Pitfalls: Underestimating inrush current and inadequately rating switches or dimmers. Using cold ohmmeter readings to predict running current without accounting for heating. Final Answer: both cold and hot resistance

Question 18

Spot the imposter — forms of Ohm’s law: Which of the following expressions is NOT a correct rearrangement of Ohm’s law for resistive circuits?

Accepted Answer

Introduction / Context: Ohm’s law relates voltage, current, and resistance for ohmic conductors. Rearranging it correctly is a frequent algebraic step in design and troubleshooting. Identifying incorrect forms prevents calculation errors and misinterpretation of measurements. Given Data / Assumptions: Ohm’s law base form: V = I * R. Linear, time-invariant, resistive element. SI units: volts (V), amperes (A), ohms (Ω). Concept / Approach: Starting from V = I * R, valid rearrangements are obtained by dividing both sides by I or R. Multiplying I and V to obtain resistance is dimensionally incorrect because I * V yields power (watts), not resistance (ohms). Step-by-Step Solution: Base: V = I * R.Divide both sides by R → I = V / R (valid).Divide both sides by I → R = V / I (valid).Check R = I * V → units become A * V = W (power), not Ω → invalid. Verification / Alternative check: Dimensional check: Ω = V/A. Compare to I * V = A * V = W. Since Ω ≠ W, the expression “R = I * V” must be wrong. Why Other Options Are Wrong: The other listed forms are algebraically consistent with V = I * R. Option E restates D; both are valid. Common Pitfalls: Confusing P = I * V (power) with V = I * R (Ohm’s law). Plugging values into a wrong formula that happens to produce a number with wrong units. Final Answer: R = I * V

Question 19

Properties of good electrical insulators: Select the characteristic that best defines a good insulating material used between conductors.

Accepted Answer

Introduction / Context: Insulating materials prevent unintended current flow and arcing in cables, capacitors, and high-voltage equipment. Their selection affects safety, reliability, and size. Knowing what makes a good insulator helps in material choice and failure analysis (e.g., corona, tracking, puncture). Given Data / Assumptions: We compare generic properties of dielectrics (glass, ceramics, polymers) versus conductors. Room-temperature, dry conditions unless otherwise specified. Focus on intrinsic material properties, not just geometry. Concept / Approach: Two key traits of insulators are low free-carrier density (high resistivity) and large dielectric strength (ability to withstand strong electric fields without breakdown). Among the options, “large dielectric strength” is the most direct and defining quality for insulation performance under high fields. Step-by-Step Solution: Define dielectric strength: maximum electric field a material can endure without electrical breakdown (units often kV/mm).Relate to application: higher dielectric strength allows thinner insulation for a given voltage rating, reducing size/weight.Link to resistivity: good insulators also have very low free-carrier concentrations → high volume/surface resistivity.Select best descriptor: “large dielectric strength”. Verification / Alternative check: Datasheets for insulators list breakdown field (e.g., PTFE ~ 60–170 kV/mm). High values correspond to excellent insulating capability, validating the chosen characteristic. Why Other Options Are Wrong: Few free charge carriers: generally true, but dielectric strength more directly defines insulation performance under stress; we select the most specific rating attribute. Small breakdown voltage: the opposite of desired. Many free electrons / high DC conductivity: properties of conductors, not insulators. Common Pitfalls: Equating only high resistivity with insulation; high dielectric strength is also essential for high-voltage applications. Ignoring environmental effects (humidity, temperature) that reduce dielectric strength. Final Answer: large dielectric strength (high breakdown field)

Question 20

Incandescent lamp behavior: After switching on a lamp, how does the filament's resistance change as it quickly heats from room temperature to operating temperature?

Accepted Answer

Introduction / Context: An incandescent lamp uses a metal filament (typically tungsten). When you first apply voltage, the filament is at room temperature and draws a large inrush current. As the filament heats up to a white-hot operating temperature, its electrical properties change. Understanding how filament resistance varies with temperature is important for surge analysis, fuse sizing, dimmer design, and lifetime prediction. Given Data / Assumptions: Filament material has a positive temperature coefficient of resistance (typical for metals such as tungsten). Voltage is applied suddenly to a cold filament. Steady-state is reached within a fraction of a second as the filament becomes incandescent. Concept / Approach: For metals, resistance R increases with temperature T according to R ≈ R0 * (1 + α * ΔT), where α is the positive temperature coefficient. At the moment of switch-on, the filament is cold, so R is low and current is high. As temperature rises, the increased lattice vibrations impede electron flow, causing resistance to increase markedly, which in turn limits the current to its normal operating value. Step-by-Step Solution: Start: cold filament → low R_cold.Apply voltage: I_initial = V / R_cold → high inrush current.Rapid heating: temperature rises by hundreds of degrees → R increases.Steady-state: R_hot » R_cold, current settles to nominal operating value. Verification / Alternative check: Practical measurements show cold resistance can be roughly 1/10 to 1/15 of the hot resistance in small lamps. The observable effect is a brief bright surge then stabilization, consistent with an increasing resistance as temperature climbs. Why Other Options Are Wrong: Less resistive: Opposite of metallic behavior under heating. Cooler: Describes temperature, not resistance; in reality it gets hotter. Brighter: Illumination increases but this does not state the electrical resistance change. Common Pitfalls: Confusing semiconductors (often negative temperature coefficient) with metals. Also, assuming constant resistance when calculating startup current leads to underestimating surge stress on switches and power supplies. Final Answer: more resistive

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