Question 1

Charge–current calculation in basic electricity: If 6 A flows through a lamp filament for 1.75 s, how many coulombs of electric charge pass through the filament?

Accepted Answer

Introduction / Context: Relating current, charge, and time is a core skill in circuit analysis. This relationship is useful for understanding capacitor charging, pulse currents, and energy transfer over time in basic electronics and power systems. Given Data / Assumptions: Current I = 6 A (amperes). Time t = 1.75 s (seconds). We are asked to find the total charge Q that flows. Concept / Approach: The fundamental relation is Q = I * t, where Q is in coulombs, I in amperes, and t in seconds. One ampere equals one coulomb per second, so multiplying current by time yields charge directly. Step-by-Step Solution: Use Q = I * t.Substitute known values: Q = 6 * 1.75.Compute: Q = 10.5 coulombs. Verification / Alternative check: Check units: A * s = C, which is dimensionally correct. A quick mental estimate: 6 * 2 = 12 C; since time is slightly less than 2 s (1.75 s), 10.5 C is reasonable and correct. Why Other Options Are Wrong: 105 C and 34 C: Each results from using an incorrect time multiplier or arithmetic error. 3.4 C: Much too small; would correspond to a significantly shorter time or smaller current. Common Pitfalls: Mistaking milliseconds or minutes for seconds. Always convert time to seconds before applying Q = I * t. Dropping a decimal (e.g., 1.75 vs 17.5) leading to a tenfold error. Final Answer: 10.5 C

Question 2

Component identification in circuits: The sliding contact used to adjust resistance in a variable resistor is called the wiper of which device?

Accepted Answer

Introduction / Context: Variable resistors are ubiquitous in electronics for adjusting levels such as volume, bias, or reference voltages. Recognizing the parts of these devices helps in wiring and troubleshooting. The movable contact that changes the division of resistance is universally called a 'wiper.' Given Data / Assumptions: A 'sliding contact' is mentioned, which implies mechanical adjustment across a resistive track. Concept / Approach: A potentiometer is a three-terminal variable resistor. The end terminals connect across the entire resistive element; the center terminal is the wiper, which slides over the resistive track to tap a variable fraction of the total resistance, creating an adjustable output voltage or resistance depending on configuration (voltage divider or rheostat mode). Step-by-Step Reasoning: Identify device with a sliding contact: potentiometer.Understand function: the wiper chooses the division point on the resistive track.Confirm: switches are discrete on/off contacts; thermistors vary resistance with temperature; photoconductive cells vary with light—neither uses a mechanical 'wiper.' Verification / Alternative check: Datasheets and schematics label the middle pin of a pot as the wiper. Rotational (knob-type) or linear sliders both use the wiper terminology consistently across manufacturers. Why Other Options Are Wrong: Switch: Changes connectivity, not via a sliding resistive contact. Photoconductive cell: Light-dependent resistor; no mechanical slide. Thermistor: Temperature-dependent resistor; no wiper mechanism. Common Pitfalls: Confusing a two-terminal variable resistor (rheostat connection) with a fixed resistor; the presence of a wiper indicates potentiometer-style construction. Final Answer: potentiometer

Question 3

Electrostatics concept check: When two positively charged materials are brought close together, what interaction occurs between them?

Accepted Answer

Introduction / Context: Electrostatic forces are fundamental to understanding charge behavior in insulators, capacitors, and dielectric phenomena. The interaction between like and unlike charges explains many practical effects, from static cling to the operation of electrostatic precipitators. Given Data / Assumptions: Two objects each carry a net positive charge. They are placed close to one another with no conductive path to share charge. Concept / Approach: The basic electrostatic law states: like charges repel, unlike charges attract. The force magnitude follows Coulomb's law (inversely proportional to distance squared), but for this conceptual question the direction (repulsion vs attraction) is key. Step-by-Step Reasoning: Identify charge polarity: both positive → like charges.Apply rule: like charges repel, unlike attract.Conclusion: the objects repel each other. Verification / Alternative check: In experiments with pith balls or electroscopes, giving both bodies a positive charge causes visible separation due to repulsive electrostatic forces. This is a standard demonstration in physics labs. Why Other Options Are Wrong: Attract: Only opposite charges attract. Become neutral / become negative: Without a charge exchange path, their charges do not spontaneously change sign or cancel. Common Pitfalls: Forgetting that induced charges can cause attraction in nearby neutrals; but between two already positive bodies, the net interaction is repulsive. Final Answer: repel

Question 4

Instrument identification in measurements: An ohmmeter is specifically used to measure which electrical quantity in a circuit or component?

Accepted Answer

Introduction / Context: Selecting the correct instrument for measurement prevents errors and damage. An ohmmeter (often part of a multimeter) measures resistance, which is vital for checking components, continuity, and diagnosing open or short faults in wiring and PCB traces. Given Data / Assumptions: Ohmmeters are used on de-energized circuits for accurate resistance readings. Concept / Approach: Ohm's law and resistance measurement rely on applying a small known current or voltage and observing the corresponding voltage or current to calculate resistance: R = V / I. A dedicated ohmmeter automates this and displays the resistance directly in ohms (Ω), kilohms (kΩ), or megohms (MΩ). Step-by-Step Reasoning: Identify the instrument: ohmmeter function on a DMM.Recognize what it measures: the opposition to current flow, i.e., resistance.Best practices: measure with power off and components isolated to avoid parallel paths that skew readings. Verification / Alternative check: Device manuals and symbols: the Ω symbol on a multimeter denotes the resistance function. Continuity mode is related, sounding a tone for very low resistance paths. Why Other Options Are Wrong: Current and voltage require ammeter and voltmeter settings, respectively. Wattage (power) is not measured directly by an ohmmeter; power is computed from voltage and current (P = V * I). Common Pitfalls: Measuring resistance on a live circuit can damage the meter or produce false readings due to external sources. Final Answer: resistance

Question 5

Fundamental units in electricity: What is the SI unit of electric charge used in circuit calculations and component specifications?

Accepted Answer

Introduction / Context: Correct units are essential when applying equations in electronics. The SI unit of electric charge appears in capacitor sizing, current calculations, and timing circuits. Confusing units leads to errors in design and interpretation of datasheets and measurements. Given Data / Assumptions: The question asks for the fundamental SI unit of charge. Concept / Approach: The coulomb (C) is the SI unit of electric charge. One coulomb is the quantity of electricity transported in one second by a current of one ampere, linking directly to Q = I * t. Other units listed measure different quantities: volt (potential), ampere (current), and joule (energy). Step-by-Step Reasoning: Identify target quantity: electric charge.Recall SI base/derived unit: coulomb (C).Confirm relationships: 1 A = 1 C/s; energy in joules relates via E = V * Q. Verification / Alternative check: Capacitance definition C_cap = Q / V shows Q in coulombs for a given voltage, reinforcing the standard unit usage in component ratings (e.g., microcoulombs, millicoulombs in pulse circuits). Why Other Options Are Wrong: Volt: unit of electric potential difference. Ampere: unit of electric current (rate of charge flow). Joule: unit of energy or work. Common Pitfalls: Mixing up current (A) with charge (C); remember current is charge per unit time. Final Answer: coulomb

Voltage, Current and Resistance Questions