ASVAB Electronics Information — AC vs DC and Power Calculations

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).

Ohm's Law: V = IR (Voltage = Current × Resistance). Rearranged: I = V/R, or R = V/I. With V = 12, R = 4: I = 12/4 = 3 A. Master these three forms: (1) V = IR (find voltage given current and resistance); (2) I = V/R (find current given voltage and resistance); (3) R = V/I (find resistance given voltage and current). Power equations (often combined with Ohm's Law): P = VI; P = I²R (since V = IR); P = V²/R (since I = V/R). Choose based on what's known. Example: 12V circuit, 4Ω resistance. Current = 3 A (just calculated). Power = VI = 12 × 3 = 36 W; or I²R = 9 × 4 = 36 W; or V²/R = 144/4 = 36 W — all give the same answer. Common ASVAB EI calculations: (1) Find current in a circuit; (2) Find voltage across a resistor; (3) Find resistance needed for desired current; (4) Find power consumption; (5) Total resistance in series/parallel circuits. SERIES circuit: R_total = R₁ + R₂ + R₃ + ... (resistances add); same current through all; voltages add up to source. PARALLEL circuit: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ... (reciprocals add); same voltage across all branches; currents add up to source current. For two parallel resistors: R_total = (R₁ × R₂) / (R₁ + R₂). Parallel resistance is always LESS than the smallest individual resistance.

In a series circuit, total resistance = sum of individual resistances. R_total = 10 + 20 + 30 = 60Ω. Series circuit characteristics: (1) Only ONE PATH for current; (2) Same current through every component; (3) Voltage divides among components (V_total = V₁ + V₂ + V₃); (4) Total resistance = sum of resistances; (5) Failure of one component (e.g., one bulb burning out) breaks the entire circuit; (6) Adding more components increases total resistance, reducing current. Classic example: old Christmas lights — series-wired, one bulb burns out, all go dark. Modern lights are usually parallel. Voltage divider: a common application of series resistors. Two resistors R₁ and R₂ in series across a voltage source V_in. The voltage across R₂ = V_in × R₂/(R₁ + R₂). This 'divides' the voltage proportionally — useful for getting a specific intermediate voltage from a power source. Example: 12V source with R₁ = 1000Ω and R₂ = 2000Ω. Voltage across R₂ = 12 × 2000/3000 = 8V. PARALLEL circuit by contrast: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ = 1/10 + 1/20 + 1/30 = 6/60 + 3/60 + 2/60 = 11/60. R_total = 60/11 ≈ 5.45Ω. Note this is LESS than the smallest single resistor (10Ω) — that's always true for parallel. ASVAB EI tests both calculations and conceptual differences between series and parallel.

Common electronic components and their functions: (1) RESISTOR — limits current and drops voltage; converts electrical energy to heat; color-coded with bands indicating resistance value and tolerance; measured in Ohms; (2) CAPACITOR — stores electrical charge temporarily; passes AC, blocks DC; used for smoothing power supplies, filtering signals, timing circuits; measured in Farads (typically μF or pF); (3) INDUCTOR — stores energy in magnetic field when current flows; resists changes in current; passes DC, restricts AC at high frequencies; measured in Henries; (4) DIODE — allows current in one direction only, blocks reverse direction; used for rectification (converting AC to DC), protection circuits; symbol: triangle pointing to a line; (5) LED (Light Emitting Diode) — diode that emits light when current flows; uses much less energy than incandescent bulbs; long-lasting; specific forward voltage needed; (6) TRANSISTOR — amplifier or switch; controls a large current with a small current/voltage; foundation of all modern electronics; BJT (bipolar junction transistor) and FET (field-effect transistor) types; (7) TRANSFORMER — transfers electrical energy between circuits via magnetic field; can step voltage up or down; works only with AC; primary and secondary windings; (8) FUSE — protective device that breaks the circuit when current exceeds a rated value (the metal element melts); single-use, must be replaced; (9) CIRCUIT BREAKER — like a fuse but resettable mechanically; common in residential breaker panels; (10) SWITCH — manually opens/closes circuit; (11) RELAY — electrically-controlled switch; small electrical signal controls a larger circuit; (12) BATTERY — DC voltage source. Resistor color codes (heavily tested on ASVAB EI): Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Four bands: first two are digits, third is multiplier (10^x), fourth is tolerance (Gold=5%, Silver=10%).

Resistor color code system: 4-band resistor — first two bands are digits, third band is multiplier (number of zeros), fourth band is tolerance. Color values: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Violet=7, Grey=8, White=9. Tolerance: Gold=5%, Silver=10%, None=20%. For Red-Red-Orange-Gold: Red=2, Red=2 (two digits = 22), Orange=3 (multiplier = 10³ = 1000), so value = 22 × 1000 = 22,000Ω = 22kΩ. Tolerance Gold = ±5%, meaning actual value is between 20,900Ω and 23,100Ω. Mnemonic for color order: 'Bad Boys Race Our Young Girls But Violet Generally Wins.' Or 'Better Be Right Or Your Great Big Venture Goes West.' Memorize the order — heavily tested. Some examples: Brown-Black-Red-Gold = 1, 0, ×100, 5% = 1,000Ω = 1kΩ ± 5%; Yellow-Violet-Yellow-Gold = 4, 7, ×10,000, 5% = 470,000Ω = 470kΩ ± 5%; Brown-Black-Black-Gold = 1, 0, ×1, 5% = 10Ω ± 5%. 5-BAND resistors (precision): three digit bands, multiplier, tolerance. 6-BAND adds temperature coefficient. Modern surface-mount resistors use printed numbers (e.g., '472' means 47 × 10² = 4700Ω). ASVAB EI definitely tests color codes — practice converting between colors and values both ways. Standard resistor values follow the E-series (E12, E24, etc.) — not arbitrary numbers but standardized increments.

Capacitor structure: two conductive plates separated by an insulating material (the 'dielectric' — air, ceramic, plastic, electrolyte). When voltage is applied, positive charge accumulates on one plate and negative on the other; energy is stored in the electric field between them. Behavior in circuits: (1) DC: capacitor initially conducts as it charges, but once fully charged it blocks DC; like an open circuit at steady state; (2) AC: capacitor continuously charges and discharges as voltage alternates; appears as a frequency-dependent impedance (Xc = 1/(2πfC)); lower impedance at higher frequencies. Applications: (1) POWER SUPPLY FILTERING — large capacitors smooth ripple from rectified AC, providing cleaner DC; (2) DECOUPLING — small capacitors near IC power pins filter noise; (3) TIMING — capacitor charges through a resistor at a predictable rate (RC time constant = R × C); used in oscillators and timers (555 timer is famous); (4) AC COUPLING — passes AC signal between stages while blocking DC bias differences; (5) TUNING — variable capacitors in radio receivers select frequencies; (6) ENERGY STORAGE — large capacitors in flash photography, defibrillators, electric vehicles. Units: Farad (F) — large; typical practical values are microfarad (μF = 10⁻⁶ F), nanofarad (nF = 10⁻⁹), picofarad (pF = 10⁻¹²). Types: ceramic (small, stable), electrolytic (large capacitance, polarized — must be installed correctly), tantalum (compact, polarized), film (high quality, audio), supercapacitor (very large, energy storage). Safety: large capacitors store dangerous charge even after circuit is unplugged; discharge before working on equipment. CAUTION: electrolytic capacitors are polarized — backwards installation can cause failure, leakage, or explosion.

Diode: semiconductor device that allows current in one direction (FORWARD) and blocks it in the reverse direction. Terminals: ANODE (+) and CATHODE (-). Current flows from anode to cathode when forward-biased (anode positive relative to cathode); blocked when reverse-biased. Voltage drop: silicon diodes have ~0.6-0.7V forward voltage drop when conducting; germanium ~0.3V; Schottky ~0.2V; LEDs vary 1.5V (red) to 3.5V (white/blue). Symbol: triangle pointing to a vertical line; current direction follows the triangle's point. Stripe on diode body marks cathode end. Applications: (1) RECTIFICATION — convert AC to pulsing DC: HALF-WAVE rectifier uses one diode (allows only half of AC cycle); FULL-WAVE uses two diodes with center-tapped transformer; BRIDGE rectifier uses four diodes for full-wave from any AC source (most common); (2) PROTECTION — protect circuits from reverse polarity; (3) FREE-WHEELING — protect against voltage spikes from inductive loads (motors, relays) — diode connected across inductor allows the current to circulate safely when switched off; (4) VOLTAGE REGULATION — ZENER DIODES are designed to operate in reverse breakdown at a precise voltage; used for voltage references; (5) SIGNAL DEMODULATION — extracting audio from radio carrier waves; (6) LEDs — special diodes that emit light; used for indicators, displays, lighting (highly efficient compared to incandescent); (7) PHOTODIODES — convert light to current (opposite of LEDs); (8) LASER DIODES — emit coherent light; used in laser pointers, optical disc readers, fiber optic communications. ASVAB EI may show schematic symbols — recognize the diode triangle-and-line symbol with current flow direction.

Transformer: two coils of wire wound around a common iron core; AC current in the PRIMARY coil creates a changing magnetic field; the changing magnetic field induces an AC voltage in the SECONDARY coil. No direct electrical connection between primary and secondary — energy transfers through magnetic field. Turns ratio: V_secondary / V_primary = N_secondary / N_primary, where N = number of turns. STEP-UP transformer (more turns on secondary): increases voltage, decreases current proportionally. STEP-DOWN transformer (fewer turns on secondary): decreases voltage, increases current proportionally. Power (P = VI) is approximately conserved (minor losses to heat); higher voltage = lower current for the same power. Why transformers matter: ELECTRICAL TRANSMISSION uses very high voltage (250-765 kV) for long distances because higher voltage = lower current = less I²R power loss in wires; transformers step down to lower voltages near use (distribution lines 4-25 kV; household 120/240V US, 230V UK). Transformers ONLY work with AC because they require a CHANGING magnetic field to induce voltage in the secondary — DC creates a constant field that doesn't induce. Common applications: (1) UTILITY POWER GRID — countless transformers from generators (10s of kV) → transmission lines (100s of kV) → distribution (kV) → service drops (240V) → appliances (5-120V); (2) ELECTRONIC POWER SUPPLIES — step down household 120V AC to lower AC for further conversion to DC (older devices); (3) AUDIO — impedance matching, isolation; (4) WELDING — step down to provide high current at low voltage; (5) DOORBELLS — step down to 16-24V for the chime. Losses: COPPER LOSS (I²R in wire), CORE LOSS (eddy currents and hysteresis in iron) — minimized by using laminated cores and good materials. Modern switched-mode power supplies use small high-frequency transformers, more efficient than older 60 Hz transformers.

Electromagnetic induction (discovered by Michael Faraday, 1831): a changing magnetic field through or near a conductor INDUCES a voltage in the conductor. If the conductor is part of a circuit, this induced voltage drives a current. Faraday's Law: EMF = -N × dΦ/dt, where N is number of turns, Φ is magnetic flux, and t is time. Negative sign reflects Lenz's Law: induced current opposes the change causing it. Key principle: the magnetic field must be CHANGING (relative to the conductor) for induction. Three ways to create a changing field: (1) Move the conductor through a stationary field; (2) Keep the conductor still, move the field source; (3) Change the field strength while conductor is stationary (e.g., AC current in a nearby coil). Applications: (1) GENERATORS — rotate a coil through a magnetic field (or rotate a magnet near a coil); produces AC; the foundation of nearly all electricity generation (coal, gas, hydro, nuclear, wind — they all spin a generator); (2) TRANSFORMERS — AC in primary creates changing field; secondary coil sees changing field; voltage induced; (3) ELECTRIC MOTORS — opposite of generators (apply electrical energy to produce rotation); current in coil within magnetic field experiences force; (4) WIRELESS CHARGING — varying magnetic field induces voltage in coil of receiving device; (5) GUITAR PICKUPS — vibrating steel guitar string in magnetic field of pickup coil induces signal in coil; (6) METAL DETECTORS — coils create field; metal disturbs field; second coil detects disturbance; (7) INDUCTION COOKTOPS — varying magnetic field induces current in iron/steel pan, heating it directly (the pan is the conductor); (8) MAGNETIC TAPE/HARD DRIVE READING — moving magnetized surface past a read head induces signals. Magnetic flux is measured in Webers (Wb); flux density in Tesla (T). Magnetic field around a current-carrying wire: right-hand rule predicts direction (thumb in direction of current, fingers curl in direction of magnetic field).

A coil of wire carrying current behaves like a bar magnet — this device is called an ELECTROMAGNET or SOLENOID. The magnetic field strength depends on: (1) CURRENT — more current = stronger field (proportional); (2) NUMBER OF TURNS — more turns concentrate the field; (3) CORE MATERIAL — iron core dramatically increases field strength (typically 1000x) due to high magnetic permeability; (4) COIL GEOMETRY — tighter, denser coils make stronger fields. Right-hand rule for solenoid: curl right-hand fingers in the direction of current flow; thumb points to the NORTH pole of the resulting magnet. Applications: (1) ELECTROMAGNETS — large lifting magnets in scrapyards; door locks; magnetic levitation; (2) RELAYS — small current activates electromagnet, which mechanically closes a switch controlling a larger circuit; (3) SOLENOIDS — coil with a movable iron core; energizing the coil pulls the core in; used in solenoid valves (control flow of fluids/gas), starter motors (engaging starter to flywheel), pinball machines, door locks; (4) MOTORS — coils in rotor and/or stator create magnetic fields; rotational force from interaction of fields; (5) GENERATORS — reverse of motors; (6) LOUDSPEAKERS — varying current in coil within magnetic field moves the coil and attached cone, producing sound; (7) MRI MACHINES — extremely powerful superconducting electromagnets create magnetic fields for imaging. Permanent magnets and electromagnets share the property of magnetic field but: permanent magnets have constant field strength (set by material); electromagnets have variable field controlled by current — turn off the current, field disappears. Magnetic poles: north and south, attract opposite poles, repel like poles. Earth has a magnetic field that compass needles align with — Earth's magnetic NORTH POLE is actually a SOUTH magnetic pole (which attracts compass north needles).