ASVAB Electronics Information — Digital Logic, Semiconductors, and Communication Systems

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.

Electricity and magnetism are unified phenomena — electromagnetism. Key relationships: (1) ELECTRIC CURRENT creates a MAGNETIC FIELD around the wire (Oersted's discovery, 1820). Direction follows the right-hand rule: thumb in direction of current, fingers curl in direction of magnetic field. Field strength = μ₀I/(2πr) for a straight wire, where r is distance. Coiling the wire (solenoid) concentrates the field — basis for electromagnets. (2) CHANGING MAGNETIC FIELD creates an ELECTRIC FIELD that drives current in conductors (Faraday's discovery, 1831). Basis for generators, transformers, inductors, motors. (3) UNIFICATION (Maxwell's equations, 1860s): four equations describe all classical electromagnetism — Gauss's law (electric), Gauss's law for magnetism, Faraday's law of induction, Ampère's law with Maxwell's correction. They predicted electromagnetic waves traveling at the speed of light, ultimately unifying optics with electromagnetism. Light is an electromagnetic wave. Modern physics treats electricity and magnetism as different aspects of the same field — they appear differently depending on reference frame. Practical implications: (1) ELECTRIC MOTORS — current in a wire within a magnetic field experiences a force (F = IL × B); causes rotation in motor coils; converts electrical to mechanical energy; (2) GENERATORS — rotating coil in magnetic field induces voltage; converts mechanical to electrical; (3) TRANSFORMERS — AC in primary creates changing magnetic field; secondary coil sees changing field; voltage induced; (4) RADIO/COMMUNICATIONS — accelerating electrons (in antennas) create electromagnetic waves; (5) MRI — strong magnetic fields align nuclear spins; radio pulses then perturb them; signals back from the relaxing nuclei produce images; (6) MAGNETIC LEVITATION TRAINS (maglev) — magnetic forces support and propel trains; (7) PARTICLE ACCELERATORS — magnetic fields steer charged particles in beam; (8) HARD DRIVES — magnetic regions on disk encode data; read/write heads detect/change magnetization. Lorentz force: F = qE + qv × B (charged particle in electric and magnetic fields experiences force). Underlies operation of CRTs, particle accelerators, mass spectrometers, plasma physics. ASVAB EI may test basic understanding that current causes magnetic fields and that changing fields induce currents — both crucial to electrical engineering.

Each bulb individually: P = VI, so I = P/V = 60/120 = 0.5 A. In parallel, currents add: total I = 0.5 + 0.5 = 1 A. Verify: total power = 60 + 60 = 120W; P = VI = 120 × 1 = 120W ✓. Parallel circuit principles: (1) SAME VOLTAGE across all branches (each bulb gets full 120V); (2) CURRENTS ADD (total = sum of branch currents); (3) Each branch operates independently — turning one off doesn't affect the other. If the same two bulbs were in SERIES: each would get half the voltage (60V), so each would operate at 1/4 normal brightness (power = V²/R, so half voltage means 1/4 power). Both bulbs would draw the same current. Calculation: each bulb's resistance = V²/P = 120²/60 = 240Ω. In series, total R = 240 + 240 = 480Ω. Current = V/R = 120/480 = 0.25 A. Power dissipated = I²R = 0.0625 × 480 = 30W total (15W per bulb) — far dimmer than rated. This is why household appliances are wired in parallel, not series — each gets full voltage and full power regardless of others. Christmas lights: old-style series-wired light strings each bulb gets a fraction of line voltage (120V/50 bulbs = 2.4V per bulb); when one bulb burns out, the entire string goes out. Modern strings either use parallel wiring or have shunt mechanisms in each bulb that bypass burned-out filaments. LED Christmas lights use LEDs with appropriate series resistors and circuit design. ASVAB EI calculations: practice converting between V, I, R, P using Ohm's Law and Power Law, and applying series/parallel rules. Watts, volts, amps, and ohms are the four fundamental electrical quantities — knowing any two often lets you calculate the others.

Inductor: a coil of wire (often around a magnetic core) that stores energy in a magnetic field. Energy stored: E = ½LI², where L is inductance (Henries) and I is current. Key properties: (1) Resists CHANGES in current — voltage across inductor = L × (di/dt); when current is steady, voltage drop is ~0 (just the wire resistance); rapid changes in current produce large voltage spikes; (2) Passes DC easily — once current is steady, inductor looks like a wire (just its DC resistance); (3) Opposes AC — impedance increases with frequency: X_L = 2πfL (reactance, in ohms); at high frequencies, inductors become like 'open circuits'; (4) Stores energy in magnetic field — opposite of capacitor which stores in electric field. Applications: (1) FILTERS — inductors in series block high frequencies (low-pass filter); used in audio crossovers, power supply filtering; (2) TRANSFORMERS — magnetic coupling between two coils; (3) MOTORS, GENERATORS, SOLENOIDS — convert between electrical and mechanical energy via magnetic field; (4) RELAYS — coil energizes to operate switch contacts; (5) IGNITION COILS — automotive; pulse the primary, get high-voltage spark from secondary; (6) RADIO TUNING — LC circuit (inductor + capacitor) resonates at specific frequency; (7) ENERGY STORAGE — buck/boost switching converters store and release energy in inductor; (8) CHOKES — block AC noise from power lines while passing DC. Construction: wire wound around an air or magnetic core. Core materials: AIR (low inductance, used at high frequencies); FERRITE (intermediate, common in switching power supplies); IRON or LAMINATED STEEL (high inductance, used in transformers and 60 Hz applications). Inductance increases with: more turns, larger cross-sectional area, magnetic core material, closer winding. Symbol: looped or coiled line; sometimes with bars indicating iron core. Units: HENRY (H), millihenry (mH), microhenry (μH), nanohenry (nH). Typical values: μH range for RF circuits, mH for audio crossovers, H for power applications. Inductor and capacitor behave oppositely: capacitor — passes AC, blocks DC; inductor — passes DC, blocks AC (high frequency). LC circuits combining both can resonate at a specific frequency: f = 1/(2π√LC) — fundamental to radio receivers, oscillators, filters.

Human hearing range: roughly 20 Hz to 20,000 Hz (20 kHz). The upper limit declines with age (presbycusis) — most adults can't hear above 15-16 kHz by age 40-50. Infants and children have the full range; teenagers can typically hear ringtones around 17-19 kHz that adults cannot. Sound frequency relates to PITCH: low frequencies (low pitch — bass) at 20-250 Hz; mid (voice fundamental, instruments) 250-4000 Hz; high (cymbals, high voice, hi-hat) 4000-20000 Hz. Specific examples: deep bass guitar fundamental ~40-50 Hz; piano ranges 27-4186 Hz (its 88 keys); human speech fundamentals 85-255 Hz (men ~85-180, women ~165-255), with overtones extending much higher; soprano voices reach ~1000+ Hz on top notes; violin ~196-3000+ Hz; piccolo ~600-5000+ Hz. SOUND INTENSITY measured in decibels (dB) — logarithmic scale: 0 dB = threshold of hearing; 20 dB = whisper; 60 dB = conversation; 80 dB = busy traffic; 100 dB = lawn mower; 120 dB = rock concert (pain threshold approached); 140 dB = jet engine (immediate hearing damage). Each 10 dB increase = 10x sound INTENSITY (but only ~2x perceived loudness because of how hearing works). Hearing damage: prolonged exposure above 85 dB causes gradual hearing loss; brief exposure above 140 dB can cause immediate damage. Hearing protection (earplugs, earmuffs) is essential in noisy environments. Sound waves: longitudinal pressure waves in air (or other media). Travel at ~340 m/s in air (varies with temperature, humidity, altitude); 1500 m/s in water; ~5000 m/s in steel. WAVELENGTH = speed / frequency. 1000 Hz sound has wavelength 0.34 m in air. ULTRASOUND: frequencies above 20 kHz, inaudible to humans but heard by some animals (dogs hear to ~45 kHz, bats use 14-100+ kHz for echolocation); medical ultrasound 2-15 MHz; cleaning ultrasound 20-40 kHz. INFRASOUND: frequencies below 20 Hz; produced by earthquakes, large machinery, some animals (elephants and whales communicate with infrasound over long distances).

In a series circuit, resistances add: R_total = 10 + 20 + 30 = 60 Ω.

Ohm's Law: I = V/R = 12/4 = 3 amperes.

A diode is a semiconductor device that allows current to flow in one direction only — it acts as a one-way valve for electrical current.

Power = Voltage × Current = 110 × 5 = 550 watts.