Electric charge is the intrinsic property of matter that produces electric forces and electric fields. Charge exists in two signs, positive and negative, and is quantised in multiples of the elementary charge e ≈ 1.602 × 10⁻¹⁹ C.
Learning objectives
- Define electric charge and state its SI unit.
- Explain the two signs of charge and the attraction and repulsion rules.
- Calculate charge from current and time and count elementary charges.
- Explain why charge is conserved and quantised.
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
When you rub a balloon on your hair, electrons move from one surface to the other. The balloon becomes negatively charged and your hair becomes positively charged, so the balloon can stick to a wall through the electric force. Charge is measured in coulombs (C), and one coulomb is a fairly large amount of charge at everyday scale.
Intermediate How it behaves in a circuit
Materials are classified by how easily charge moves through them. Conductors such as copper allow charge to move freely; insulators such as glass or dry plastic hold charge in place; semiconductors sit between the two and their behaviour can be controlled, which is the basis of electronics.
Advanced Limits, assumptions, engineering detail
At the microscopic scale, charging is a redistribution of electrons, never the creation of net charge. Triboelectric charging, induction and conduction are different mechanisms for moving or separating existing charge. In precision instrumentation, very small charge packets (picocoulombs) can dominate measurements, which is why guarding, shielding and low-leakage insulators matter.
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Electric charge | Q | coulomb (C) |
| Elementary charge | e | 1.602 × 10⁻¹⁹ C |
| Current | I | ampere (A) |
| Charge density | ρ | coulomb per cubic metre (C/m³) |
Worked examples
Counting electrons in one coulomb
Problem. How many electrons are needed to make a charge of 1.0 C?
Solution. N = Q / e = 1.0 C / 1.602 × 10⁻¹⁹ C ≈ 6.24 × 10¹⁸ electrons.
Charge delivered over time
Problem. A charger delivers 3.0 A for 2.0 minutes. How much charge passes through the circuit?
Solution. Convert time: t = 2.0 min × 60 s/min = 120 s. Then Q = I × t = 3.0 A × 120 s = 360 C.
Practical applications
- Capacitors and energy storage
- Electrostatic painting and dust filtration
- Battery state-of-charge estimation
- Static-control in electronics manufacturing
Automotive applications
Every automotive module is sensitive to charge: a capacitor stores charge for a short time after the ignition is switched off, and static charge on a technician can damage sensitive electronics. When working on airbags or high-voltage systems, the stored charge must be discharged according to the manufacturer procedure before connectors are separated.
Industrial applications
Charge accumulation is a real hazard around conveyor belts, plastic films and fuel transfer. Bonding and earthing give the accumulated charge a controlled path to earth, preventing sparks. In industrial processes, charge also matters positively: electrostatic precipitators use charged plates to capture particles from exhaust gas.
Measurement procedure
- Charge is often calculated rather than measured directly, from current integrated over time.
- A Faraday cup and electrometer can measure small charge quantities in laboratory work.
- A capacitor charge can be estimated from Q = C × V using the measured voltage.
- For very small charges, use guarded, low-leakage fixtures and short measurement times.
Common mistakes
- Confusing charge with current: charge is measured in coulombs, current in coulombs per second.
- Believing that charge can be created by a battery: the battery separates and moves existing charge.
- Forgetting the sign: an electron carries negative charge, so electron flow is opposite to conventional current.
- Mixing up conservation of charge with conservation of energy: they are separate principles.
Safety considerations
- Discharge capacitors with an approved resistor and verify zero voltage before touching terminals.
- Avoid wearing metal jewellery when working near battery terminals or high-current circuits.
- Use ESD-safe practices when handling electronic assemblies sensitive to static charge.