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Courses/Electrical Fundamentals/Atoms, Electrons & Electric Charge

lesson · 7min · Lesson 2 of 37

Electric charge and the coulomb

Course syllabusCourse overview
01Atoms, Electrons & Electric Charge
  1. ReadWhat is electricity?: atomic model
  2. ReadElectric charge and the coulomb
  3. ReadConductors, insulators, and semiconductors
  4. ReadConventional current vs electron flow
  5. quizModule quiz
02Voltage, Current & Resistance
  1. ReadPotential difference: the driving force
  2. ReadCurrent: the rate of charge flow
  3. ReadResistance and resistivity
  4. ReadOhm's Law: derivation and examples
  5. exerciseWorked examples: Ohm's Law problems
03Series & Parallel Circuits
  1. ReadSeries circuits: characteristics and rules
  2. ReadParallel circuits: characteristics and rules
  3. ReadCombined series-parallel networks
  4. ReadVoltage dividers and current dividers
  5. exerciseCircuit analysis practice
04Kirchhoff's Laws
  1. ReadKCL: Kirchhoff's Current Law
  2. ReadKVL: Kirchhoff's Voltage Law
  3. ReadMesh and nodal analysis
  4. exerciseKirchhoff's law problems set
05Power & Energy
  1. ReadElectrical power: watts and horsepower
  2. ReadEnergy: kilowatt-hours and joules
  3. ReadEfficiency and power loss in cables
  4. quizPower quiz
06Alternating Current Fundamentals
  1. ReadAC vs DC: why AC won
  2. ReadSinusoidal waveforms: peak, RMS, average
  3. ReadFrequency and period
  4. ReadPhase relationships
  5. ReadAC circuit analysis introduction
07Capacitors & Inductors
  1. ReadCapacitor construction and capacitance
  2. ReadCapacitors in AC circuits: reactance
  3. ReadInductor construction and inductance
  4. ReadInductors in AC circuits: reactance
  5. ReadRC and RL circuits: time constants
08Measurement & Instruments
  1. ReadMultimeters: AC/DC voltage and current
  2. ReadClamp meters and measuring live current
  3. ReadOscilloscopes: reading waveforms
  4. quizFinal assessment
Lesson · 7min
SIMPLIFIED CHARGE MODEL · I = Q / t+electron drift = current1 A = 1 coulomb per second
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Electric charge and the coulomb

Electrical Fundamentals · Lesson 2 · Beginner

BeginnerReview: professional review pending

Purpose

Quantify electric charge in coulombs and connect charge to current and elementary particles.

Before you beginAtomic charge model · Scientific notation

Learning objectives

  • Define the coulomb
  • Use Q = It
  • Use the exact elementary charge
  • Convert between C, mC, and µC

The coulomb measures an amount of electric charge; the ampere measures how quickly that charge passes a reference point.

CHARGE: COULOMBS AND CARRIERSCHARGE: COULOMBS AND CARRIERS+Q = n·esign and direction are explicit

Core theory

In SI, one coulomb equals one ampere-second. Charge is conserved: it may move or separate, but an isolated system does not create net charge.

The elementary charge magnitude is exactly 1.602176634 × 10⁻¹⁹ C. Electron charge is −e and proton charge is +e.

Charge values in circuits are often derived from current over time rather than by counting carriers.

Terms, symbols, and units
TermMeaningSymbolUnit
Electric chargeQuantity describing electrical interactionQcoulomb (C)
Elementary chargeMagnitude of proton or electron chargeeC
Ampere-secondCharge transferred by one ampere in one secondNot applicableA·s = C
Q = I × t

Charge equals current multiplied by elapsed time.

C = A·s
Worked exampleA 250 mA circuit operates for 40 s. How much charge passes?

Assumptions: Current is constant.

  1. Convert: 250 mA = 0.250 A.
  2. Substitute: Q = 0.250 A × 40 s.
  3. Calculate: Q = 10 A·s = 10 C.

10 C of charge passes.

Reasonableness check: A quarter coulomb per second for forty seconds gives ten coulombs.

Common mistakes
  • Treating coulomb as current
  • Skipping milli-to-base conversion
  • Using electron charge without its negative sign when direction matters

Where this appears in practice

Battery capacity, electroplating, sensing, and fault-energy discussions all depend on charge transferred over time.

SafetyEven modest transferred charge can be hazardous when delivered at dangerous voltage or through the body. Charge alone is not a complete measure of electrical risk.
Local code checkThe physical relationships are universal. Installation methods, permitted work, test procedures, and equipment ratings must be checked against current local rules and manufacturer instructions.

Knowledge check

What current transfers 6 C in 3 s?

2 A. Rearrange Q = It to I = Q/t.

Answer: 2 A. Rearrange Q = It to I = Q/t.

Practical exercise

Calculate charge for 20 mA over 5 min, showing both time and current conversions.

Summary

  • One coulomb is one ampere-second
  • Elementary charge has an exact SI value
  • Convert prefixes before calculating

Sources and review

  • The International System of Units (SI Brochure): BIPM; 9th edition, updated 2026; International
  • SI Units: Electric Current: NIST; Current web edition; United States / SI reference
  • University Physics Volume 2: OpenStax; Current web edition; Physics reference

Editorial review date: 2026-08-21. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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