Static Electricity and Electric Fields
Static Charge and Electron Transfer
All matter is made of atoms, which contain positively charged protons, negatively charged electrons and neutral neutrons. In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.
When two insulating materials are rubbed together, electrons are transferred from one surface to the other. The material that gains electrons becomes negatively charged, while the material that loses electrons becomes positively charged. This is called static electricity.
Only electrons move when objects are charged by friction — protons are fixed in the nucleus and cannot transfer between materials.
For example, when a polythene rod is rubbed with a cloth, electrons transfer from the cloth to the rod. The rod becomes negatively charged and the cloth becomes positively charged. If a Perspex rod is rubbed with a cloth, electrons transfer from the rod to the cloth, leaving the rod positively charged.
Attraction, Repulsion and Sparking
Charged objects exert forces on each other without touching. Two objects with the same charge (both positive or both negative) repel each other. Two objects with opposite charges attract each other. A charged object can also attract a neutral object by electrostatic induction — the charge on the nearby object causes electrons in the neutral object to move, creating regions of opposite charge on its surface.
Like charges repel; opposite charges attract. This rule governs all electrostatic interactions.
When a charged object is brought close to an earthed conductor, the potential difference between them can become large enough to ionise the air. Electrons jump across the gap, creating a spark. Lightning is a large-scale example of this — charge builds up in clouds until the potential difference between the cloud and the ground is high enough to cause a massive spark discharge.
Earthing allows charge to flow safely to the ground through a conducting path. This prevents dangerous build-up of static charge. Fuel tankers use earthing straps and aircraft are earthed before refuelling to avoid sparks near flammable fuel.
Electric Fields
A charged object creates an electric field around it. Any other charged object placed in this field will experience a force. The closer to the charged object, the stronger the field — this is shown by field lines drawn closer together.
Electric field lines point away from positive charges and towards negative charges. For a single point charge, the field lines are radial (spreading outwards). Between two parallel charged plates, the field is uniform — the field lines are parallel and equally spaced, meaning the force on a charge is the same everywhere between the plates.
The strength of the electric field depends on the size of the charge producing it and the distance from the charge. A stronger charge creates a stronger field. The field gets weaker as you move further from the charge — this is an example of a non-contact force.
The force between two charged objects also depends on both charges and the distance between them. Doubling the distance between charges significantly reduces the force between them. This relationship is important for understanding how charged particles interact in circuits and in everyday static phenomena.
Uses and Dangers of Static Electricity
Static electricity has practical uses. In electrostatic sprayers, paint or crop spray is given an electric charge so that the droplets repel each other, creating a fine, even mist. The object being sprayed is earthed, so it attracts the charged droplets, reducing waste and giving an even coating — even on the back of the object.
In photocopiers and laser printers, static charge is used to attract toner particles to specific areas of a drum, which are then transferred to paper. Electrostatic precipitators in chimneys use charged plates to remove smoke particles from waste gases before they escape into the atmosphere.
Static electricity can be dangerous in certain situations. When refuelling aircraft or filling fuel tankers, friction between flowing fuel and pipes can build up static charge. A spark near flammable fuel vapour could cause an explosion. This is why earthing straps are used and operators touch metal parts before handling fuel nozzles.
In hospitals, static sparks near oxygen-rich environments or flammable anaesthetics pose a fire risk. Anti-static flooring and clothing help prevent charge build-up. Similarly, electronic components can be damaged by electrostatic discharge, so technicians wear anti-static wrist straps when handling circuit boards.