Electric charge is an intrinsic physical property of matter that causes it to experience a force when placed in an electromagnetic field. It is a scalar quantity and forms the foundation of all electrical phenomena.
Types and Sourcing of Charge
- Positive Charge: Associated with protons, which are located inside the atomic nucleus.
- Negative Charge: Associated with electrons, which reside in the outer shells of an atom.
- Neutral State: An atom is electrically neutral when the number of protons equals the number of electrons.
Fundamental Properties of Electric Charge
- Quantization of Charge: Charge exists only in discrete integral multiples of the basic electronic charge (e). It is expressed mathematically asQ = ne, where n is an integer (± 1, ± 2, ± 3…) and e ≈ 1.6 × 10-19 Coulombs. Charge cannot be subdivided beyond this fundamental unit in a free state.
- Conservation of Charge: The total electric charge of an isolated system remains constant over time. Charge can neither be created nor destroyed; it can only be transferred from one body to another.
- Additivity of Charge: Total charge of a system is the algebraic sum of all individual charges present in the system, taking their respective signs (positive or negative) into account.
- Like Charges Repel, Unlike Charges Attract: Two positive or two negative charges exert a repulsive force on each other, whereas a positive and a negative charge attract each other.
| Property | SI Unit | Value of Elementary Charge (e) | Dimensional Formula |
| Electric Charge | Coulomb (C) | 1.602 × 10-19 C | [A · T] |
UPSC Prelims Pointer: Quarks
While charge quantization holds true for independent particles, protons and neutrons are made up of fractional charges called Quarks (Up quark = +2/3e, Down quark = -1/3e). However, quarks do not exist in a free isolated state.
Coulomb’s Law of Electrostatics
Coulomb’s Law quantifies the electrostatic force of attraction or repulsion between two stationary point charges.
Mathematical Formulation
The magnitude of the electrostatic force (F) between two point charges q1 and q2, separated by a distance r in a vacuum, is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them.
Key Characteristics of Coulomb’s Force
- It is a central force, acting along the line joining the centers of the two charges.
- It obeys the inverse-square law.
- It is highly dependent on the intervening medium. In a material medium, permittivity increases (ε = ε0 · εr), which reduces the net force between the charges.
Methods of Charging an Object
Material bodies can be charged by shifting electrons from one object to another through three primary mechanisms.
Charging by Friction
- Mechanism: When two distinct insulating materials are rubbed together, thermal energy provides the work function required to dislodge electrons from one material and transfer them to the other.
- Result: Both bodies acquire equal and opposite charges.
- Example: Rubbing a glass rod with silk causes the glass rod to lose electrons (becoming positively charged) and the silk to gain electrons (becoming negatively charged).
Charging by Conduction
- Mechanism: Charging an uncharged lifestyle conductor by bringing it into direct physical contact with a pre-charged conductor.
- Result: The uncharged body acquires the same sign of charge as the charging body due to the free migration of electrons.
Charging by Induction
- Mechanism: Charging an uncharged conducting body by bringing a charged object near it, without making any physical contact. The proximity of the charged object causes a redistribution of charges within the neutral conductor.
- Result: The near end of the neutral conductor acquires an opposite charge, while the far end acquires a similar charge. If the far end is momentarily earthed, the body retains a permanent net opposite charge.
Static Electricity: Concepts and Phenonmena
Static electricity refers to the accumulation of electric charges on the surface of non-conducting materials or isolated conductors where the charges remain stationary until they can move away via an electric discharge.
Triboelectric Series
Materials vary in their ability to gain or lose electrons. The Triboelectric Series lists materials in order of their tendency to become positively or negatively charged when rubbed against other materials.
- High Positive Tendency: Human skin, Leather, Glass, Nylon, Wool.
- Neutral Tendency: Cotton, Steel.
- High Negative Tendency: Wood, Amber, Hard Rubber, Polyester, Teflon.
Electrostatic Discharge (ESD)
When the accumulation of static charge becomes high enough, the electric field breaks down the insulating properties of the surrounding air, leading to a sudden, brief flow of electric current.
Real-World Examples and Phenomena
- Lightning: The most violent natural manifestation of static electricity. Water droplets and ice crystals inside storm clouds collide, generating massive static separation. The cloud base becomes negatively charged, inducing a positive charge on the ground, culminating in a massive electrostatic discharge.
- Comb and Paper Bits: Rubbing a plastic comb through dry hair charges it via friction. When brought near neutral bits of paper, it induces an opposite charge on the surface of the paper closest to the comb, creating an attractive force that lifts the paper.
- Shock from Car Doors or Carpets: Walking across a synthetic carpet builds up static charge on the human body due to friction. Touching a metallic door knob provides a path for rapid electrostatic discharge, resulting in a mild shock.
Applications and Hazards of Static Electricity
Static electricity has vital industrial applications but also presents significant safety hazards that require engineered mitigation strategies.
Industrial and Practical Applications
- Electrostatic Precipitators (ESPs): Installed in coal-fired power plants and heavy industries to control particulate pollution. ESPs apply a high voltage to charge smoke particles, which are then pulled out of the gas stream by oppositely charged collector plates.
- Xerography (Photocopying): Photocopiers utilize a light-sensitive, selenium-coated drum. Light creates a charge pattern mimicking the document, attracting negatively charged toner powder to the charged areas of the drum before transferring it to paper.
- Electrostatic Spray Painting: The paint droplets are given a negative charge at the nozzle, and the target object is grounded or positively charged. This leads to an even, wrap-around coating with minimal paint wastage.
- Inkjet Printers: Highly directed electrostatic deflection plates guide charged ink droplets precisely onto the paper surfaces.
Hazards and Mitigation Strategies
- Explosion Risks in Fueling: Fuel flowing through hoses generates static charge due to friction. A spark during refueling can ignite volatile vapors.
- Mitigation: Fuel trucks and aircraft are connected to the ground using bonding wires (earthing) to safely dissipate static build-up before fuel transfer begins.
- Damage to Electronic Components: Microchips and Integrated Circuits (ICs) are highly sensitive to Electrostatic Discharge (ESD), which can melt internal circuitry.
- Mitigation: Personnel in electronics manufacturing wear anti-static wrist straps, and components are stored in conductive ESD-shielding bags.
- Lightning Damage: Direct lightning strikes destroy infrastructure and cause forest fires.
- Mitigation: Lightning Conductors (Rods) invented by Benjamin Franklin are installed atop tall buildings. They provide a low-resistance path for the lightning strike to discharge harmlessly deep into the earth.
Comparison: Static Electricity vs. Current Electricity
| Characteristic | Static Electricity | Current Electricity |
| State of Charges | Charges remain stationary on surfaces. | Charges are in continuous, directed motion. |
| Primary Media | Primarily observed on Insulators. | Observed primarily in Conductors. |
| Duration | Accumulates slowly, discharges instantly. | Flows continuously as long as a potential difference exists. |
| Magnetic Field | Does not produce a magnetic field. | Generates an associated magnetic field. |