--- title: Basic Physical Quantities and Laws url: https://doc.liz6.com/en/hardware/01-basic-electricity/01-basic-physical-quantities-and-laws locale: en area: hardware tags: - hardware - basic-electricity date: 2026-06-30 modified: 2026-07-16 description: Basic Physical Quantities and Laws Charge and Electric Field Charge Electric Field --- Three Fundamental Quantities Voltage Current Resistance Conductance --- C… --- # Basic Physical Quantities and Laws ## Charge and Electric Field ### Charge ``` Symbol: Q Unit: Coulomb (C) Definition: A fundamental property of matter, existing in positive and negative forms Charge of 1 electron: e = 1.602 × 10⁻¹⁹ C 1 C = 6.24 × 10¹⁸ electron charges Like charges repel, opposite charges attract ``` ### Electric Field ``` Symbol: E Unit: V/m (Volts per meter) Definition: The force experienced by a unit positive charge Formula: E = F / q F = k × Q₁Q₂ / r² (Coulomb's Law, k = 9×10⁹ N·m²/C²) ``` --- ## Three Fundamental Quantities ### Voltage ``` Symbol: V or U Unit: Volt (V) Definition: The potential difference between two points, the "pressure" driving charge flow Analogy: Water pressure — the greater the water level difference, the stronger the flow 1 V = 1 J/C (Doing 1 Joule of work to move 1 Coulomb of charge) ``` ### Current ``` Symbol: I Unit: Ampere (A) Definition: The amount of charge passing through a conductor's cross-section per unit time Formula: I = ΔQ / Δt Analogy: Water flow — the volume of water flowing through a pipe per unit time Direction: Direction of positive charge flow (actual electron flow is in the opposite direction) ``` ### Resistance ``` Symbol: R Unit: Ohm (Ω) Definition: The property that opposes the flow of current Determining factors: R = ρ × L / A ρ: Resistivity (Ω·m) L: Length (m) A: Cross-sectional area (m²) Analogy: Pipe thickness and length — the thinner and longer the pipe, the greater the resistance ``` ### Conductance ``` Symbol: G Unit: Siemens (S) Formula: G = 1 / R ``` --- ## Core Laws ### Ohm's Law ``` V = I × R I = V / R R = V / I If any two are known, the third can be calculated. Applies to linear resistors; not applicable to semiconductor devices! ``` ### Power ``` P = V × I (General) P = I² × R (Resistive heating — Joule's Law) P = V² / R Unit: Watt (W) Energy: W = P × t (Joule J, electricity bills are in kWh) ``` --- ## Kirchhoff's Laws ### KCL — Current Law (Node) ### KVL — Voltage Law (Loop) ``` In a closed loop, the algebraic sum of voltages across each segment is zero ┌── R1 ──┬── R2 ──┐ │ +V1 -│ +V2 - │ │ │ │ └── Vs ──┴────────┘ Vs - V1 - V2 = 0 i.e., ΣV = 0 (around the entire loop) Essence: Conservation of energy (the electric field is a conservative field) ``` ### Application Example ``` R1=2Ω R2=3Ω ┌────┤├───────┤├────┐ │ │ ─┼─ Vs=10V R3=5Ω ─┼─ │ │ │ └────────────────────┘ KCL (Node A): I1 = I2 + I3 KVL (Loop 1, Left): 10 - 2I1 - 5I3 = 0 KVL (Loop 2, Right): 5I3 - 3I2 = 0 Solving the simultaneous equations → I1=2A, I2=1.25A, I3=0.75A ``` --- ## Basic Circuit Structures ### Series ``` R1=2Ω R2=3Ω R3=5Ω ──┤├──────┤├──────┤├── Equivalent Resistance: Rt = R1 + R2 + R3 = 10Ω Current: Same everywhere It = I1 = I2 = I3 Voltage: Vt = V1 + V2 + V3 (Voltage division) ``` ### Parallel ``` ──┬──┤├──┬── │ R1 │ ├──┤├──┤ │ R2 │ ──┴──┤├──┴── Equivalent Resistance: 1/Rt = 1/R1 + 1/R2 Rt = (R1×R2)/(R1+R2) (for two resistors) Current: It = I1 + I2 (Current division) Voltage: Same everywhere Vt = V1 = V2 ``` ### Series-Parallel (Mixed) ``` Simplify the parallel parts first → then treat as series Simplify step-by-step from inside out ``` --- ## Common Prefixes | Prefix | Symbol | Multiplier | Example | |------|------|------|------| | Tera | T | 10¹² | 1TB | | Giga | G | 10⁹ | 1GHz | | Mega | M | 10⁶ | 1MΩ | | Kilo | k | 10³ | 1kΩ | | - | - | 10⁰ | 1V | | Milli | m | 10⁻³ | 1mA | | Micro | μ | 10⁻⁶ | 1μF | | Nano | n | 10⁻⁹ | 1ns | | Pico | p | 10⁻¹² | 1pF | --- ## Common Signal Waveforms | Waveform | Characteristics | Typical Applications | |------|------|----------| | DC (Direct Current) | Constant | Power supplies, Biasing | | Sine Wave | V(t)=A·sin(2πft+φ) | AC power, RF | | Square Wave | Alternating high/low | Clock signals, PWM | | Triangle Wave | Linear rise/fall | Scanning, Audio synthesis | | Pulse | Brief sudden change | Triggering, Resetting | ### AC Parameters ``` Peak Value Vp: Maximum value Peak-to-Peak Vpp: Distance between positive and negative peaks = 2×Vp RMS Value Vrms: For sine wave = Vp/√2 ≈ 0.707Vp (Equivalent DC value for thermal effects) Frequency f: Number of cycles per second (Hz) Period T: T = 1/f Angular Frequency ω: ω = 2πf ``` --- *Keywords: Voltage, Current, Resistance, Ohm's Law, KVL, KCL, Power, Series, Parallel, AC, RMS*