Electronics Glossary

Q: What is the difference between a farad and a microfarad?

A farad (F) is the SI unit of capacitance. It is very large in practice. Electronic capacitors range from picofarads (pF, 10-12 F) for RF circuits to millifarads for power supply filtering. 1 µF = 1000 nF = 1,000,000 pF.

Q: Why does a lower AWG number mean a thicker wire?

AWG numbers reflect the number of wire-drawing passes: more passes produce a thinner wire and a higher AWG number. The relationship is logarithmic: every 6 AWG steps halves cross-sectional area, and every 3 steps changes area by a factor of 2.

Q: What is the relationship between reactance and impedance?

Reactance (X) is frequency-dependent opposition from inductors and capacitors. Resistance (R) is frequency-independent opposition. Impedance magnitude |Z| = sqrt(R² + X²). A purely resistive circuit has Z = R and X = 0.

Q: What forward voltage should I assume if the LED datasheet does not specify it?

Use 2.0 V for red or yellow LEDs and 3.0 V for blue, white or green LEDs. These are midpoints of the typical ranges (red: 1.8 to 2.2 V; blue/white: 2.9 to 3.4 V). Measure Vf at your operating current for precision work.

Q: When should I use E12 rather than E24 resistor values?

Use E12 for general-purpose circuits where 10% resistance spread does not matter. Use E24 when you need a closer match to a calculated value in voltage dividers, filter frequencies, or oscillator timing. E96 is for precision designs.

Plain-English definitions of 27 core electronics terms, from basic quantities (voltage, current, resistance) to component-specific concepts (forward voltage, hFE, capacitor code) and practical standards (AWG, E-series, SMD). Each term links to the relevant calculator where one exists.

Last updated: May 2026

AWG (American Wire Gauge): A standardized system for specifying the diameter of solid round electrical wire, used primarily in North America. Counterintuitively, a lower AWG number means a thicker wire: AWG 10 has a larger cross-section than AWG 22. Each step of 3 AWG roughly doubles the cross-sectional area. IEC and European wiring standards specify wire size directly in mm² instead. Bench rule: AWG 22 is common for breadboard hookup wire; AWG 12 to 14 is typical for 15 to 20 A household circuits. See also the AWG Ampacity Chart and Voltage Drop Calculator.
Capacitance (C, unit: farad, F): The ability of a component or conductor to store electric charge for a given voltage. 1 farad = 1 coulomb of charge stored per volt of applied potential. A farad is a very large unit in practice: most electronic capacitors are rated in microfarads (µF, 10^-6 F), nanofarads (nF, 10^-9 F) or picofarads (pF, 10^-12 F). Capacitance is distinct from the component: capacitance is the quantity; a capacitor is the component that provides it.
Capacitor: A passive two-terminal component that stores energy in an electric field between two conductive plates separated by an insulator (dielectric). Capacitors block DC and pass AC; they are used for filtering, decoupling power rails, timing circuits, and energy storage. The value printed on a ceramic or SMD capacitor uses a 3-digit code: the first two digits give the value and the third is a multiplier in picofarads. Use the Capacitor Code Calculator to decode any 3-digit code.
Capacitive reactance (X_C, unit: ohm): The opposition that a capacitor presents to alternating current, measured in ohms. Unlike resistance, it varies with frequency: X_C = 1 / (2 × π × f × C). Capacitive reactance decreases as frequency rises, so a capacitor passes high frequencies more easily than low ones. In the impedance convention used here, capacitive reactance is negative (X < 0) to distinguish it from inductive reactance (X > 0). Use the Capacitor Reactance Calculator to solve for any unknown.
Current (I, unit: ampere, A): The flow of electric charge through a conductor, measured in amperes (A). 1 ampere = 1 coulomb of charge passing a point per second. Conventional current flows from positive to negative; electron flow is physically opposite. Current is the quantity you measure with a multimeter in series with a circuit. Smaller currents are expressed in milliamperes (mA, 10^-3 A) or microamperes (µA, 10^-6 A). Current relates to voltage and resistance through Ohm's Law: I = V / R.
E-series (E12, E24, E96): Standardized sets of preferred resistor (and capacitor) values defined by IEC 60063. The number after E gives how many distinct values exist per decade: E12 has 12 values (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82), E24 has 24, and E96 has 96. Resistors are manufactured in these values so that when tolerances overlap, every possible resistance within the decade is covered by at least one value. In practice, E12 covers general-purpose work, E24 covers most precision needs, and E96 is used for tight-tolerance designs. The calculators on this site show the nearest E12 and E24 value for any calculated resistance. See the LED Resistor Calculator and Voltage Divider Calculator.
Forward voltage (V_f, unit: volt): The voltage that must be applied across a diode or LED in the forward direction before significant current flows. Below V_f, the diode blocks current; above it, the diode conducts and the voltage across it stays approximately constant at V_f. Typical values: silicon signal diode 0.6 to 0.7 V; red LED 1.8 to 2.2 V; green or yellow LED 2.0 to 2.4 V; blue or white LED 3.0 to 3.4 V. The exact value depends on current and temperature. V_f is a required input in the LED Resistor Calculator to size the current-limiting resistor correctly.
Frequency (f, unit: hertz, Hz): The number of complete cycles of a periodic signal per second, measured in hertz (Hz). 1 Hz = 1 cycle per second. Mains electricity runs at 50 Hz (Europe, Asia, Africa, most of the world) or 60 Hz (North America, parts of South America and Asia). Audio occupies roughly 20 Hz to 20 kHz. Radio frequencies run from kHz to GHz. Frequency directly affects inductive reactance (rises with frequency) and capacitive reactance (falls with frequency). Use the Inductor Reactance and Capacitor Reactance calculators for frequency-dependent calculations.
Ground (GND, 0 V reference): The reference potential in a circuit, defined as 0 V. All other voltages in the circuit are measured relative to ground. There are two distinct types: Earth ground is a physical connection to the earth (through the building's grounding system) and provides a safety path for fault currents; circuit ground (also called signal ground or return) is simply the return path for current and the voltage reference, and may or may not be connected to Earth. In bench electronics, the negative terminal of the power supply is usually circuit ground. Never assume that circuit ground and Earth ground are the same without checking.
hFE (DC current gain, also beta or β): The DC current gain of a bipolar junction transistor (BJT), defined as β = I_C / I_B, where I_C is the collector current and I_B is the base current. It expresses how many times larger the collector current is than the small base current that controls it. Typical values range from 20 to 500 depending on the transistor type and operating point. hFE is the parameter name used in datasheets (from the h-parameter model); β is the same quantity in circuit analysis notation. Note that hFE is the static (DC) gain; the AC small-signal current gain is called h_fe (lowercase) and can differ from hFE at high frequencies.
Impedance (Z, unit: ohm): The total opposition to alternating current in a circuit, combining resistance (R) and reactance (X) as a complex quantity: Z = R + jX. The magnitude is |Z| = √(R² + X²), measured in ohms. The phase angle θ = arctan(X / R) describes the phase shift between voltage and current. Purely resistive circuits have Z = R and θ = 0°. Inductive circuits have positive X and lagging current; capacitive circuits have negative X and leading current. Use the Impedance Calculator to compute Z and θ from R and X.
Inductance (L, unit: henry, H): The property of a conductor or coil by which a change in current induces a voltage opposing that change (Faraday's law). 1 henry = 1 volt-second per ampere, meaning a current changing at 1 A/s induces 1 V across a 1 H inductor. Practical inductors are rated in millihenries (mH, 10^-3 H), microhenries (µH, 10^-6 H) or nanohenries (nH, 10^-9 H). Inductance is distinct from the component: inductance is the quantity; an inductor is the component that provides it intentionally (though any current-carrying conductor has some stray inductance).
Inductor: A passive two-terminal component, typically a coil of wire wound around a core, that stores energy in a magnetic field when current flows through it. Inductors oppose changes in current: they resist rapidly rising current (like a high-pass filter for current) and release stored energy when current tries to fall. Used in power supplies (switch-mode converters, filters), RF circuits, and EMI suppression. The opposition an inductor presents to AC is called inductive reactance. Use the Inductor Reactance Calculator to find X_L at any frequency.
Inductive reactance (X_L, unit: ohm): The opposition that an inductor presents to alternating current, measured in ohms. X_L = 2 × π × f × L. Inductive reactance rises with frequency, so an inductor passes low frequencies more easily than high ones. In the impedance convention used here, inductive reactance is positive (X > 0) and results in current lagging voltage by up to 90°. Use the Inductor Reactance Calculator to solve for X_L, f or L given any two.
LED (Light-Emitting Diode): A semiconductor diode that emits light when current flows through it in the forward direction. Like all diodes, an LED has a forward voltage (V_f) that must be exceeded before it conducts. Unlike a resistor, an LED does not limit its own current: without a current-limiting resistor or constant-current driver, excess current will destroy it. The colour of emitted light depends on the semiconductor material and bandgap, not on the applied voltage. Use the LED Resistor Calculator to find the correct series resistor value for your supply voltage and LED specifications.
mAh (milliampere-hour): A unit of electric charge used to express battery capacity. 1 mAh = the charge delivered by 1 mA flowing for 1 hour. 1000 mAh = 1 Ah. A 2000 mAh battery can theoretically supply 100 mA for 20 hours, or 2000 mA for 1 hour, though real-world capacity decreases at high discharge rates. mAh tells you how much charge a battery holds; it says nothing about voltage, so two batteries with the same mAh rating but different voltages store different amounts of energy (energy in watt-hours = capacity in Ah × voltage). Use the Battery Life Calculator to estimate runtime from capacity and load current.
Ohm's Law: The fundamental relationship between voltage, current and resistance in a resistive circuit: V = I × R, where V is in volts, I in amperes and R in ohms. Rearranged: I = V / R and R = V / I. Extended to include power: P = V × I = I² × R = V² / R. Ohm's Law applies strictly to ohmic (linear, resistive) elements at constant temperature; it does not directly apply to non-linear devices such as diodes, transistors or incandescent bulbs (whose resistance changes with temperature). Use the Ohm's Law Calculator to solve any combination of V, I, R and P.
PCB (Printed Circuit Board): A flat board made of insulating material (typically fibreglass laminate, FR4 being the most common) with conductive copper tracks etched or deposited on its surface. Components are mounted on the PCB and connected by the copper tracks, replacing point-to-point wiring. Most boards have two sides; multi-layer boards stack internal copper planes for power distribution and signal routing. The term "printed" refers to the original photographic printing process used to produce the copper pattern, not to ink-jet printing.
Power (P, unit: watt, W): The rate at which energy is converted or transferred, measured in watts (W). 1 watt = 1 joule per second. In a DC circuit: P = V × I = I² × R = V² / R. In an AC circuit with a resistive load, the same formulas apply to RMS values. In an AC circuit with reactive loads, apparent power (V × I, in volt-amperes, VA) differs from real power (W) by the power factor. Power dissipated in a resistor becomes heat: a 1 kΩ resistor carrying 10 mA dissipates P = I² × R = 0.01² × 1000 = 0.1 W. Use the Ohm's Law Calculator and Watts / Volts / Amps Calculator for power calculations.
Reactance (X, unit: ohm): The component of impedance that arises from energy storage (inductance or capacitance) rather than energy dissipation. Reactance is measured in ohms and is frequency-dependent, unlike resistance. Inductive reactance (X_L > 0) rises with frequency; capacitive reactance (X_C < 0) falls with frequency. A pure reactance dissipates no average power: energy oscillates between the source and the reactive element. Total reactance X = X_L + X_C (using signed values), and impedance magnitude |Z| = √(R² + X²). See inductive reactance, capacitive reactance and impedance.
Resistance (R, unit: ohm, Ω): The opposition to the flow of electric current through a material, measured in ohms (Ω). Resistance converts electrical energy to heat. 1 ohm is defined as the resistance of a conductor across which 1 volt produces 1 ampere of current (from Ohm's Law: R = V / I). Resistance is a property of a material and geometry; resistivity (ρ) is the intrinsic material property (copper: 0.0172 Ω·mm²/m). Resistance is frequency-independent in ideal resistors, unlike reactance. The decoder for resistor values is at the Resistor Color Code Calculator.
Resistor: A passive two-terminal component that provides a known, stable resistance. Resistors are manufactured in the E-series preferred values and are marked with a color code (through-hole) or a numerical code (SMD). The key specifications are resistance value (in ohms), tolerance (how close the actual value is to the marked value, typically 1% or 5%), and power rating (maximum wattage before overheating). A resistor's power rating must exceed the actual dissipated power P = I² × R by a comfortable margin, typically at least 2×. Use the Resistor Color Code Calculator to decode any 4-band or 5-band resistor.
RMS (Root Mean Square): The effective value of a time-varying voltage or current, defined as the square root of the mean of the squared instantaneous values. For a sinusoidal signal, V_rms = V_peak / √2 ≈ 0.707 × V_peak. Mains voltage is quoted as RMS: 230 V RMS means the peak is about 325 V. RMS is useful because an AC voltage of V_rms delivers the same average power to a resistive load as a DC voltage of the same numerical value. Measuring AC with a multimeter gives RMS for sinusoidal signals; non-sinusoidal signals require a true-RMS meter.
SMD (Surface Mount Device): A component designed to be soldered directly onto the surface of a PCB, with no leads passing through holes. SMD components are smaller and can be mounted on both sides of the board, enabling higher density and lower cost at volume. Common SMD resistor and capacitor package sizes (metric/imperial): 0402/01005, 0603/0201, 0805/0402, 1206/0603. The first two digits of the imperial code give the length in hundredths of an inch. SMD components are typically hand-soldered with fine-pitch tools or placed by machine. Most modern consumer electronics are entirely SMD. Contrast with through-hole.
Through-hole: A component mounting method where the component's leads pass through drilled holes in the PCB and are soldered on the opposite side. Through-hole components are larger than SMD equivalents, mechanically stronger (the leads are anchored through the board), and easier to hand-solder and replace. Used for connectors, high-power components, electrolytic capacitors, and prototyping. Most breadboard-compatible components are through-hole. Contrast with SMD.
Voltage (V, unit: volt): The electric potential difference between two points in a circuit, measured in volts (V). Voltage is the "pressure" that drives current through a resistance. 1 volt = 1 joule of energy transferred per coulomb of charge. Voltage is always a difference between two points: it is meaningless to state a voltage without specifying the reference (usually ground). A 5 V supply has a 5 V difference between its positive terminal and ground. Voltage is related to current and resistance by Ohm's Law: V = I × R.
Voltage divider: A circuit formed by two resistors in series between a supply voltage and ground. The output voltage is taken from the junction between the two resistors: V_out = V_in × R2 / (R1 + R2). A voltage divider is unloaded (accurate) only when the load connected at V_out has a much higher resistance than R2. If the load resistance is comparable to R2, the effective resistance at V_out changes and the output voltage drops below the formula value. Voltage dividers are used to scale signals, set reference voltages, and bias transistor bases. Use the Voltage Divider Calculator to compute V_out, current and power for any R1/R2 combination.

Frequently Asked Questions

What is the difference between a farad and a microfarad?

A farad (F) is the SI unit of capacitance. It is a very large unit: a 1 F capacitor at 5 V stores 5 coulombs of charge, which is enormous for most circuits. In practice, electronic capacitors range from a few picofarads (pF, 10^-12 F, used in RF circuits) to a few millifarads (mF, 10^-3 F, used in power supply filtering) and up to farads for supercapacitors (energy storage). The microfarad (µF = 10^-6 F) is the most common everyday unit: a 100 µF electrolytic capacitor in a 5 V power supply stores 0.5 mC of charge. To convert: 1 µF = 1000 nF = 1,000,000 pF.

Why does a lower AWG number mean a thicker wire?

The AWG numbering system was originally based on the number of drawing passes through progressively smaller dies required to produce a wire of a given diameter. More passes produce a thinner wire, so a higher draw count (higher AWG number) gives a thinner wire. AWG 36 requires more draws than AWG 10. The relationship is logarithmic: every 6 AWG steps halves the cross-sectional area, and every 3 steps changes the area by a factor of 2. IEC wiring codes (mm²) directly state the cross-sectional area, which is why they feel more intuitive to engineers outside North America.

What is the relationship between reactance and impedance?

Reactance (X) is the frequency-dependent opposition to AC from inductors and capacitors. Resistance (R) is the frequency-independent opposition from resistors. Impedance (Z) is the combined total: Z = R + jX, where j is the imaginary unit. The magnitude |Z| = √(R² + X²) is what you measure as ohms in an AC circuit. A purely resistive circuit has Z = R, X = 0. A circuit with only an inductor has Z = jX_L, R = 0. In a real RLC circuit all three terms contribute. The Impedance Calculator computes |Z| and phase angle from R and X.

What forward voltage should I assume if the LED datasheet does not specify it?

Use 2.0 V as a safe starting assumption for red or yellow LEDs, and 3.0 V for blue, white or green LEDs. These are the midpoints of the typical ranges (red: 1.8 to 2.2 V; blue/white: 2.9 to 3.4 V). The LED Resistor Calculator includes standard presets for common LED colours. If you are building anything where the exact current matters (long LED strips, high-brightness applications), measure V_f at your intended operating current with a multimeter, as it varies between batches and with temperature.

When should I use E12 rather than E24 resistor values?

Use E12 for general-purpose circuits where a 10% spread in resistance value does not matter: pull-up resistors, current limiters for indicators, RC time constants where the exact time is not critical. Use E24 (24 values per decade, roughly 5% spacing) when you need a closer match to a calculated value, for example in voltage divider ratios, filter frequencies, or oscillator timing. E96 values (1% tolerance) are specified when precision matters and standard E24 values do not bracket the target closely enough. The LED Resistor and Voltage Divider calculators on this site show both the exact calculated value and the nearest E12 and E24 standard values.

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