Static Electricity and Electric Fields
Charging by Friction
When two insulating materials are rubbed together, electrons transfer from one material to the other. The material that gains electrons becomes negatively charged; the material that loses electrons becomes positively charged. The total charge is conserved — charge is not created, only redistributed.
A classic example: rubbing a polythene rod with a cloth causes electrons to transfer from the cloth to the polythene. The rod gains a negative charge; the cloth becomes positive. Rubbing an acetate rod with a cloth has the opposite effect — electrons transfer from the acetate to the cloth, leaving the rod positive.
Only electrons transfer when charging by friction — protons are locked inside atomic nuclei and cannot move in ordinary chemical processes. The object that gains electrons becomes negative; the object that loses them becomes positive. Total charge before equals total charge after.
Attraction and Repulsion
Charged objects exert forces on one another without touching — this is an electrostatic (non-contact) force. The rule is:
Like charges repel; unlike charges attract.
Two negatively charged rods brought close together will push each other away. A negative rod held near a positive rod will pull each other towards the other. These forces act at a distance because each charged object creates an electric field around itself.
Charged objects also attract small uncharged objects — for example, a charged comb attracting small pieces of paper. This happens because the charge on the comb induces a redistribution of charge in the paper, attracting the opposite charges on the near side.
Electric Fields
An electric field is the region around a charged object in which a force acts on another charged object. Electric fields are represented using electric field lines. The rules are:
- Field lines run from positive to negative (the direction a positive charge would move).
- Field lines never cross each other.
- The closer together the field lines, the stronger the field.
- For a uniform field (between two parallel plates), the lines are parallel and evenly spaced.
Around an isolated positive charge, field lines radiate outward in all directions (like the spokes of a wheel). Around a negative charge, they point inward toward the charge. Between two oppositely charged parallel plates, the field lines are parallel, equally spaced, and perpendicular to the plates — this is a uniform field. The field has the same strength everywhere between the plates and only weakens near their edges (edge effects).
Earthing and Electrostatic Hazards
Earthing removes charge from an object by providing a conducting path to the ground. Since the Earth is very large, it can accept or provide huge numbers of electrons without becoming noticeably charged itself.
Electrostatic charge is a hazard in several situations. Sparks occur when the electric field between a charged object and earth becomes strong enough to ionise the air — the resulting spark can ignite flammable vapours. This is why aircraft are earthed before refuelling: fuel flowing through pipes builds up static charge, and without earthing, a spark could ignite aviation fuel vapour.
Surgeons in operating theatres and workers handling explosive materials wear special anti-static clothing and footwear to prevent charge build-up. Computer components are stored in anti-static bags because even a small static discharge can damage integrated circuits.
Electrostatic hazards arise wherever charge can accumulate and spark: aircraft refuelling (earth the aircraft before attaching fuel hose), operating theatres (anti-static clothing), grain silos and flour mills (dust clouds are explosive — anti-static earthing). Lightning itself is a large-scale electrostatic discharge.
Useful Applications of Static Electricity
Static electricity also has many practical applications:
Electrostatic paint spraying: The object to be painted (e.g. a car body panel) is given one charge; the paint droplets are given the opposite charge. The paint is attracted to every surface of the object, including awkward recesses, giving an even coat and reducing waste.
Inkjet printers: Tiny ink droplets are given a charge as they leave the nozzle. Deflector plates with a controlled electric field steer each droplet to exactly the right position on the page.
Photocopiers and laser printers: A drum is given a static charge pattern representing the image to be printed. Toner (charged powder) is attracted to the charged regions of the drum and then transferred to paper.
Electrostatic precipitators: Used in power station chimneys, these give particles in smoke a charge so they are attracted to collecting plates, removing them from the exhaust gas and reducing air pollution.
In a thundercloud, turbulent air movements cause ice crystals and water droplets to collide, transferring charge. The base of the cloud typically accumulates a large negative charge, while the ground below becomes induced into a positive charge. When the potential difference between cloud and ground becomes large enough, the air ionises and an enormous discharge — lightning — occurs. Lightning conductors on tall buildings provide a low-resistance earthing path, protecting the building by allowing the charge to discharge safely into the ground rather than through the structure.