ASVAB Electronics — Ohm's Law and Circuits

A fuse is a safety device — a thin wire designed to MELT (blow) when current exceeds its rated value, opening the circuit and protecting downstream components from damage. Once blown, the fuse must be REPLACED — single-use device. Types: (1) GLASS TUBE fuses — small, used in electronics, vehicles; (2) BLADE fuses — automotive (typical colors indicate amperage: tan=5A, brown=7.5A, red=10A, blue=15A, yellow=20A, clear=25A, green=30A); (3) AGC/MDL fuses — household; (4) High-voltage / industrial fuses. Ratings: marked with maximum continuous current (e.g., 5A, 10A, 20A); voltage rating (e.g., 250V); response speed (fast-acting vs slow-blow / time-delay — slow-blow tolerates brief startup surges from motors). CIRCUIT BREAKERS are like reusable fuses — they trip when current is too high, but reset by flipping the switch. Used in residential/commercial breaker panels instead of fuses. Modern circuit breakers: thermal (heating element bends), magnetic (electromagnet opens contacts), or combination; GFCI (ground-fault circuit interrupter) detects current imbalance between hot and neutral wires — used in bathrooms, kitchens, outdoors to prevent shocks; AFCI (arc-fault circuit interrupter) detects arcing — used in bedrooms to prevent fires. Replacing fuses: ALWAYS replace with the SAME rating — using a higher-rated fuse defeats the protection and can cause fire or damage; lower-rated fuse will blow repeatedly. Resettable polyfuses (PPTC) in some electronics — increase resistance when hot, recover when cool; reusable but slower response. Surge protectors are different — they protect against voltage spikes, not excess current. NEVER bypass a fuse or breaker — it's there for a reason. Common ASVAB EI safety items address fuse function and proper replacement.

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.

Circuit topologies: SERIES — components connected end-to-end in a single path; only ONE PATH for current; SAME CURRENT through every component; voltage DIVIDES among components (V_total = V₁ + V₂ + V₃...). Resistances ADD (R_total = R₁ + R₂ + R₃). Adding more components increases total resistance, decreases current. Failure of one component (open) breaks the entire circuit. Examples: old Christmas lights (one bulb out = all dark); switches and protective devices in line with loads; voltage divider circuits. PARALLEL — components connected across the same two nodes; MULTIPLE PATHS for current; SAME VOLTAGE across every component; current DIVIDES among branches (I_total = I₁ + I₂ + I₃...). Reciprocal resistances add: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃. Total resistance is LESS than smallest individual resistance. Failure of one component does NOT break others. Examples: household wiring (multiple outlets in parallel — one device off doesn't affect others); modern LED Christmas lights; speakers wired in parallel. OPEN CIRCUIT — break in the circuit; no current can flow; infinite resistance. SHORT CIRCUIT — unintended low-resistance path bypassing the load; very high current; can cause damage, fire, or trip breakers. Why parallel for household wiring: each appliance gets full line voltage (120V) regardless of others; turning off one appliance doesn't affect others; failure of one doesn't affect others. Series-parallel combinations: real circuits often combine both; analyze by reducing parallel groups to equivalent resistances, then summing series elements. Kirchhoff's Laws: (1) VOLTAGE LAW (KVL): sum of voltage changes around any closed loop = 0; (2) CURRENT LAW (KCL): sum of currents into any node = sum of currents out. Used to analyze complex circuits. ASVAB EI commonly tests series vs parallel identification and calculation.

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.

PARALLEL CIRCUIT CURRENT DISTRIBUTION: In a parallel circuit, ALL branches share the SAME VOLTAGE (equal to the supply voltage). Current in each branch is determined by Ohm's Law: I = V/R. The 10Ω branch has the lowest resistance, so with the same voltage applied, it carries the most current (twice as much as each 20Ω branch). This is why fuses and circuit breakers are connected in series — if resistance drops (short circuit), current in that branch increases dramatically, blowing the fuse.

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.

Ohm's law states V = I × R, where V is voltage, I is current, and R is resistance. To find current, rearrange to I = V ÷ R. Here I = 12 volts ÷ 4 ohms = 3 amps. Ohm's law is the single most important relationship in basic electronics, and you should be able to solve for any of the three variables: V = IR, I = V/R, and R = V/I. A common error is multiplying instead of dividing (12 × 4 = 48). The current in this circuit is 3 amperes.

In a series circuit, the components are connected end to end in a single path, so the total resistance is simply the sum of the individual resistances: R_total = R1 + R2 + R3. The same current flows through every component in series, while the voltage divides among them. This contrasts with a parallel circuit, where total resistance is found with a reciprocal formula and is always less than the smallest individual resistor. Recognizing series (add resistances, one path) versus parallel (multiple paths, lower total resistance) is essential circuit knowledge.

In a series circuit there is only one path for current to flow. If one bulb burns out, it breaks the path (creates an open circuit), so current can no longer flow and all the bulbs go out. This is the classic problem with old-style series holiday lights. In a parallel circuit, by contrast, each bulb has its own path, so one burning out does not affect the others — which is why modern wiring and most household circuits use parallel connections. The behavior when one component fails is a key way to tell series from parallel.