Electricity Basics: Practice Questions & Explanations

Electrical quantities and units to memorize: CURRENT (I) = rate of charge flow = Amperes (A); VOLTAGE (V) = electrical potential difference, the 'push' that drives current = Volts (V); RESISTANCE (R) = opposition to current = Ohms (Ω); POWER (P) = rate of energy use = Watts (W); CHARGE (Q) = quantity of electricity = Coulombs (C); ENERGY = Joules (J) or kilowatt-hours (kWh) for electric utility. Mnemonic 'A Very Old Witch': Amp = A, Volt = V, Ohm = Ω, Watt = W. Plumbing analogy: voltage = water pressure, current = flow rate (gallons/second), resistance = pipe narrowness, power = total energy delivered. A garden hose with high pressure (V) and wide pipe (low R) delivers high flow (I). Definitions: 1 Ampere = 1 Coulomb of charge flowing past a point per second; 1 Volt = 1 Joule of energy per Coulomb of charge; 1 Ohm = the resistance allowing 1 Amp to flow under 1 Volt; 1 Watt = 1 Joule per second. Practical contexts: household outlets in US deliver 120V; appliances draw varying current (typical lamp ~0.5 A, hair dryer ~15 A, vacuum ~10 A); circuits have 15-20 A breakers; the wire size limits current capacity. AC vs DC: ALTERNATING CURRENT (AC) reverses direction periodically (60 Hz in US household wiring); DIRECT CURRENT (DC) flows one direction only (batteries, solar cells, USB power). Transformers work only with AC; electronics typically use DC internally (power supplies convert AC to DC).

Best electrical conductors (in order, from best): SILVER > COPPER > GOLD > ALUMINUM > tungsten > brass > iron > steel. Silver is the best conductor of all elements, but COPPER is most commonly used in electrical wiring because: silver is too expensive for bulk wire; copper is 95% as conductive as silver, far cheaper, and abundant. GOLD is used where corrosion resistance matters more than ultimate conductivity (connector plating, integrated circuits). ALUMINUM is lighter and cheaper than copper, used in high-voltage transmission lines and some house wiring (though aluminum house wiring has had issues with connector heating). Why metals conduct: free electrons in the outer shell of metal atoms are loosely bound and move freely under an applied voltage. INSULATORS (poor conductors): rubber, glass, plastic, wood (dry), air, distilled water, ceramic, mica. Electrons are tightly bound — current cannot flow easily. Used for wire insulation, switch housings, circuit boards. SEMICONDUCTORS (intermediate): silicon, germanium, gallium arsenide. Conductivity is between conductors and insulators; can be modified by doping with impurities; foundation of all modern electronics (transistors, diodes, ICs, solar cells, LEDs). Resistivity (intrinsic property of material, inverse of conductivity): silver 1.6×10⁻⁸ Ω·m, copper 1.7×10⁻⁸, aluminum 2.8×10⁻⁸, iron 10×10⁻⁸, rubber 10¹³ Ω·m. Wire RESISTANCE depends on resistivity × length / cross-sectional area. Thicker wires (larger area) have less resistance — handle more current without overheating. Long wires have more resistance — voltage drop over distance. Temperature affects conductivity: most metals' resistance INCREASES with temperature; semiconductors' resistance DECREASES with temperature (more electrons gain energy to conduct). Superconductors: certain materials at very low temperatures have ZERO resistance — used in MRI magnets, particle accelerators, some power transmission.

Grounding (Earth bonding): connecting electrical systems to the earth provides safety and a reference point. Purposes: (1) SAFETY — if a fault occurs (hot wire contacts metal casing of appliance), the grounded casing provides a low-resistance path back to the panel through the ground wire, causing a high fault current that immediately trips the breaker. Without grounding, the casing would remain energized at line voltage; a person touching it while standing on damp ground could become the current path — potentially fatal shock; (2) REFERENCE — ground provides a 0V reference for voltage measurements; (3) STATIC DISCHARGE — accumulated static charge dissipates through ground connection; (4) LIGHTNING PROTECTION — lightning rods provide a path to ground; (5) RADIO/COMMUNICATIONS — ground plane reference for antennas. Modern household wiring (US): three wires — HOT (black, red, or blue — carries current to load), NEUTRAL (white — returns current to source), GROUND (green or bare copper — safety only, normally no current). At the main panel: neutral and ground bond together AND connect to ground rod driven into earth. Outlets: hot on right (smaller slot), neutral on left (larger slot), ground on round bottom hole. Two-prong outlets (older): lack ground; GFCI outlets can provide some protection. GFCI (Ground-Fault Circuit Interrupter): detects current imbalance between hot and neutral; trips quickly (within milliseconds) if as little as 4-6 mA leaks to ground; required in bathrooms, kitchens, garages, outdoors, near sinks; prevents electrocution. Cars: ground is typically the metal chassis/frame (NEGATIVE terminal of battery connected to chassis in modern vehicles); 'positive ground' systems existed in older British cars. Electronics ground: ground plane on circuit board provides low-impedance reference; analog and digital grounds may be separated to prevent noise coupling. NEVER work on electrical circuits live; ALWAYS verify de-energized; PROPER lockout-tagout for industrial work.

Electrical power: P = V × I (DC, or AC with RMS values). Units: WATTS (W) = volts × amperes. P = V × I × cos(φ) for AC with phase shift (φ = phase angle between V and I); cos(φ) is the POWER FACTOR. Combined with Ohm's Law (V = IR): P = I²R (current-squared times resistance) and P = V²/R (voltage-squared divided by resistance). Choose the formula based on what's known. Examples: (1) 120V × 10A = 1200W (typical microwave); (2) 12V × 5A = 60W (laptop power supply); (3) 5V × 2A = 10W (phone charger); (4) 240V × 30A = 7200W (electric dryer). Power consumption examples: LED bulb 9-15W; incandescent equivalent 60W (LEDs 4-6x more efficient); laptop 50-100W; refrigerator 100-400W average; washer/dryer 1000-5000W; air conditioner 1000-3500W; electric oven 2000-5000W; electric vehicle charging 6-22 kW for Level 2, 50-350 kW for fast charging. Energy = Power × Time. Common energy unit: kWh (kilowatt-hour) = 1000W × 1hour = 3,600,000 J. Electric bills typically quoted in kWh. Cost = kWh × rate (US average ~$0.16/kWh). A 100W bulb running 10 hours = 1 kWh; runs every day = 365 kWh/year ≈ $58/year. LED equivalent (15W) saves ~$50/year per bulb. RMS values for AC: 120V AC household is RMS — peak is about 170V; 240V AC is RMS, peak 340V. RMS is used because it gives the equivalent DC voltage that delivers the same power; P = V_RMS × I_RMS for resistive loads. POWER FACTOR: for AC loads with inductive or capacitive components (motors, transformers, fluorescent lights), current is OUT OF PHASE with voltage; real power is less than apparent power (VA). Power factor = real power / apparent power. Resistive loads (heaters, incandescent lights) have PF = 1.0. Motors and electronics may have PF 0.5-0.95. Utilities charge industrial customers based on apparent power; power factor correction (capacitors) reduces costs.

Multimeter: combination instrument measuring multiple electrical quantities. Functions: (1) DC VOLTAGE — typical ranges 200mV to 1000V; connect probes in PARALLEL across the points being measured (DON'T break the circuit); high input impedance prevents disrupting the circuit; (2) AC VOLTAGE — for measuring household power, transformers; (3) DC CURRENT (Amperes) — typical ranges μA to 10A; connect probes in SERIES (break the circuit and route current through meter); very low resistance shunt; HIGH current ranges have separate input jack; using current setting across voltage = SHORT CIRCUIT, can blow fuse and damage meter; (4) AC CURRENT — less common in basic meters; clamp meters measure without breaking circuit by sensing magnetic field around wire; (5) RESISTANCE — typical ranges 200Ω to 200MΩ; meter applies small voltage and measures resulting current; MUST measure with power off (live circuits give wrong readings or damage meter); (6) CONTINUITY — beeps if resistance is very low (typically <50Ω); used to verify wire connections, switch closures, fuses intact; (7) DIODE TEST — checks forward voltage drop and reverse blocking; (8) CAPACITANCE — measures capacitor value; better meters; (9) FREQUENCY — measures Hz of AC signal; (10) TEMPERATURE — with thermocouple probe; (11) DUTY CYCLE — percentage of time signal is high in a PWM signal. Types: ANALOG — needle deflects; less common today but useful for trends; DIGITAL — digital readout; most common; AUTORANGING — automatically selects appropriate range. Safety: rated voltage categories (CAT I, II, III, IV) — higher categories handle higher transient voltages; always check rating matches application; use insulated probes; don't exceed meter's max voltage/current; replace fuses with same rating. Common errors: (1) Trying to measure resistance with circuit powered; (2) Using current range across voltage source (short circuit); (3) Exceeding meter ratings; (4) Wrong probe placement. Quality matters: cheap meters may give wrong readings or fail dangerously; reputable brands (Fluke, Klein, Amprobe, Triplett) have proper safety design. ASVAB EI may show multimeter readings and ask about interpretation.

Conductors are materials that allow electric current to flow easily because they have free electrons — examples include copper, aluminum, silver, and gold. Insulators resist or block current flow because their electrons are tightly bound — examples include rubber, glass, plastic, and dry wood. This is why wires use copper (conductor) wrapped in plastic or rubber (insulator). Knowing which materials conduct and which insulate is fundamental electronics content. Semiconductors, like silicon, fall between the two and conduct under certain conditions, forming the basis of modern electronic components.

Direct current (DC) flows in one constant direction, like the current from a battery. Alternating current (AC) periodically reverses direction, switching back and forth many times per second (60 times per second, or 60 Hz, in North American household power). AC is used for power distribution because it can be easily transformed to different voltages for efficient long-distance transmission. DC is common in batteries and electronic devices. Many devices use a power adapter to convert AC from the wall into the DC the electronics need. Knowing the AC/DC distinction is fundamental.

Voltage is the electrical potential difference between two points — often described as the 'pressure' that pushes electric charge (current) through a circuit. It is measured in volts. A helpful water analogy: voltage is like the water pressure in a pipe, current is the rate of water flow, and resistance is the narrowing of the pipe that opposes flow. Without a voltage difference, no current flows. Distinguishing voltage (the push), current (the flow), and resistance (the opposition) — and how Ohm's law links them — is the foundation of the Electronics subtest.