Electric current is the flow of charge, like water flowing through a pipe. Higher current = more charge per second. Measured in amperes (A). Direction: positive to negative (conventional), or electron flow (negative to positive).

• 1 ampere = 1 coulomb per second (1 A = 1 C/s)
• Current is flow rate, not amount
• DC current: constant direction (batteries)
• AC current: alternating direction (wall power)

Charge (Q) = amount of electricity (coulombs). Current (I) = flow rate of charge (amperes). Voltage (V) = pressure pushing charge. Power (P) = V × I (watts). All connected but different!

• Charge Q = amount (coulombs)
• Current I = flow rate (amperes = C/s)
• Voltage V = electrical pressure (volts)
• Current flows FROM high to low voltage

Conventional current: positive to negative (historical). Electron flow: negative to positive (actual). Both work! Electrons actually move, but we use conventional direction. Doesn't affect calculations.

• Conventional: + to - (standard in diagrams)
• Electron flow: - to + (physical reality)
• Both give same answers
• Use conventional for circuit analysis

Ampere (A) is SI base unit for current. One of seven fundamental SI units. Defined from elementary charge since 2019. Prefixes from atto to mega cover all ranges.

• 1 A = 1 C/s (exact definition)
• kA for high power (welding, lightning)
• mA, µA for electronics, sensors
• fA, aA for quantum, single-electron devices

C/s and W/V are equivalent to ampere by definition. C/s shows charge flow. W/V shows current from power/voltage. All three identical.

• 1 A = 1 C/s (definition)
• 1 A = 1 W/V (from P = VI)
• All three are identical
• Different perspectives on current

Abampere (EMU) and statampere (ESU) from old CGS system. Biot = abampere. Rare today but appear in old physics texts. 1 abA = 10 A; 1 statA ≈ 3.34×10⁻¹⁰ A.

• 1 abampere = 10 A (EMU)
• 1 biot = 10 A (same as abampere)
• 1 statampere ≈ 3.34×10⁻¹⁰ A (ESU)
• Obsolete; SI ampere is standard

I = V / R (current = voltage ÷ resistance). Know voltage and resistance, find current. Foundation of all circuit analysis. Linear for resistors.

• I = V / R (current from voltage)
• V = I × R (voltage from current)
• R = V / I (resistance from measurements)
• Power dissipation: P = I²R

At any junction, current in = current out. Σ I = 0 (sum of currents = zero). Charge is conserved. Essential for analyzing parallel circuits.

• ΣI = 0 at any node
• Current in = current out
• Charge conservation
• Used to solve complex circuits

Current = drift velocity of charge carriers. In metals: electrons move slowly (~mm/s) but signal propagates at light speed. Number of carriers × velocity = current.

• I = n × q × v × A (microscopic)
• n = carrier density, v = drift velocity
• Electrons move slowly, signal is fast
• In semiconductors: electrons + holes

USB: 0.5-3 A (standard to fast charging). Phone charging: 1-3 A typical. Laptop: 3-5 A. LED: 20 mA typical. Most devices use mA to A range.

• USB 2.0: 0.5 A max
• USB 3.0: 0.9 A max
• USB-C PD: up to 5 A (100W @ 20V)
• Phone fast charging: 2-3 A typical

Household circuits: 15-20 A breakers (US). Light bulb: 0.5-1 A. Microwave: 10-15 A. Air conditioner: 15-30 A. Electric car charging: 30-80 A (Level 2).

• Standard outlet: 15 A circuit
• Major appliances: 20-50 A
• Electric car: 30-80 A (Level 2)
• Whole house: 100-200 A service

Welding: 100-400 A (stick), 1000+ A (spot). Lightning: 20-30 kA average, 200 kA peak. Rail guns: megaamperes. Superconducting magnets: 10+ kA steady.

• Arc welding: 100-400 A
• Spot welding: 1-100 kA pulses
• Lightning: 20-30 kA typical
• Experimental: MA range (rail guns)

Each prefix step = ×1000 or ÷1000. kA → A: ×1000. A → mA: ×1000. mA → µA: ×1000.

• kA → A: multiply by 1,000
• A → mA: multiply by 1,000
• mA → µA: multiply by 1,000
• Reverse: divide by 1,000

I = P / V (current = power ÷ voltage). 60W bulb at 120V = 0.5 A. 1200W microwave at 120V = 10 A.

• I = P / V (Amps = Watts ÷ Volts)
• 60W ÷ 120V = 0.5 A
• P = V × I (power from current)
• V = P / I (voltage from power)

I = V / R. Know voltage and resistance, find current. 12V across 4Ω = 3 A. 5V across 1kΩ = 5 mA.

• I = V / R (Amps = Volts ÷ Ohms)
• 12V ÷ 4Ω = 3 A
• 5V ÷ 1000Ω = 5 mA (= 0.005 A)
• Remember: divide for current

USB port delivers 5V. Device draws 500 mA. What's the power?

P = V × I = 5V × 0.5A = 2.5W (standard USB 2.0)

5V supply, LED needs 20 mA and 2V. What resistor?

Voltage drop = 5V - 2V = 3V. R = V/I = 3V ÷ 0.02A = 150Ω. Use 150Ω or 180Ω.

Three devices: 5A, 8A, 3A on same circuit. What breaker?

Total = 5 + 8 + 3 = 16A. Use 20A breaker (next standard size up for safety margin).

Standing on ground, your body constantly has ~100 µA leakage current to earth. From EM fields, static charges, radio waves. Completely safe and normal. We're electrical beings!

Average lightning bolt: 20-30 kA (20,000 A). Peak can reach 200 kA. But duration is <1 millisecond. Total charge: only ~15 coulombs. High current, short time = surviv able (sometimes).

1 mA 60 Hz AC: tingling sensation. 10 mA: muscle control loss. 100 mA: ventricular fibrillation (lethal). 1 A: severe burns, cardiac arrest. Current path matters—across heart is worst.

Zero resistance = infinite current? Not quite. Superconductors have 'critical current'—exceed it, superconductivity breaks. ITER fusion reactor: 68 kA in superconducting coils. No heat, no loss!

LEDs are current-driven, not voltage. Same voltage, different current = different brightness. Too much current? LED dies instantly. Always use current-limiting resistor or constant-current driver.

Electromagnetic rail guns: 1-3 MA (million amps) for microseconds. Lorentz force accelerates projectile to Mach 7+. Requires massive capacitor banks. Future naval weapon.

Volta invents battery. First source of continuous electric current. Enables early electrical experiments.

Oersted discovers current creates magnetic field. Links electricity and magnetism. Foundation of electromagnetism.

Ohm publishes V = IR. Ohm's law describes relationship between voltage, current, resistance. Initially rejected, now fundamental.

Faraday discovers electromagnetic induction. Changing magnetic field creates current. Enables generators and transformers.

First international electrical congress defines ampere as 'practical unit' of current.

Tesla's AC system wins 'War of Currents' at World's Fair. AC current can be transformed, DC cannot (then).

CGPM defines ampere: 'constant current which produces 2×10⁻⁷ N/m force between parallel conductors.'

SI redefinition: ampere now defined from elementary charge (e). 1 A = (e/1.602×10⁻¹⁹) electrons per second. Exact by definition.

Voltage is electrical pressure (like water pressure). Current is flow rate (like water flow). High voltage doesn't mean high current. You can have 10,000V with 1 mA (static shock), or 12V with 100 A (car starter). Voltage pushes, current flows.

Current kills, not voltage. 100 mA through your heart can be lethal. But high voltage can force current through your body (V = IR). That's why high voltage is dangerous—it overcomes your body's resistance. Current is the killer, voltage is the enabler.

60 Hz AC causes muscle contractions at frequency of power grid. Can't let go (muscle lock). DC causes single jolt. AC is 3-5× more dangerous at same current level. Also: AC RMS value = effective DC equivalent (120V AC RMS ≈ 170V peak).

Whole house: 100-200 A service panel. Single outlet: 15 A circuit. Light bulb: 0.5 A. Microwave: 10-15 A. Air conditioner: 15-30 A. Electric car charger: 30-80 A. Total varies, but panel limits maximum.

In superconductors, yes! Zero resistance means current flows with zero voltage (V = IR = 0). Persistent current can flow forever. In normal conductors, no—you need voltage to push current. Voltage drop = current × resistance.

USB cable is thin (high resistance). Too much current = excessive heating. USB 2.0: 0.5 A (2.5W). USB 3.0: 0.9 A. USB-C PD: up to 5 A (100W). Thicker wires, better cooling, active negotiation allow higher current safely.