Static Electricity and Electric Charge
Electric Charge and Its Origins
Matter is made of atoms, and atoms contain negatively charged electrons and a positively charged nucleus. In most materials, positive and negative charges balance — the object is electrically neutral. Static electricity arises when electrons are transferred from one object to another, leaving one with a surplus of negative charge and the other with a deficit (making it positively charged).
Rubbing two materials together is a common way to transfer electrons. When a plastic rod is rubbed with a cloth, electrons move from the cloth to the rod. The rod gains a negative charge; the cloth gains an equal positive charge. This process requires insulating materials — if the object were a conductor, the charge would flow away immediately rather than building up.
Static electricity is caused by the transfer of electrons between materials when they are rubbed together. Electrons are negatively charged; the object that gains electrons becomes negatively charged, and the object that loses them becomes positively charged. The total charge is conserved — charge is transferred, not created.
Forces Between Charges
Charged objects exert forces on one another even without touching. The fundamental rule is: like charges repel; unlike (opposite) charges attract. Two negatively charged objects pushed near each other will repel. A negatively charged object brought near a positively charged object will experience an attractive force.
Evidence for these forces can be seen by hanging a charged object on a thread: another charged object brought nearby causes the suspended object to swing — towards it if the charges are opposite, away from it if they are the same. The force decreases as the distance between the charges increases.
Electric Fields
Around every charged object there is an electric field. When another charged object enters this field, it experiences a force — attractive or repulsive depending on the signs of the charges involved. The electric field concept is a useful model: it allows physicists to describe and predict the forces that charged objects exert on each other even across empty space.
Electric field lines are drawn to represent the field. The lines show the direction of force that a positive test charge would experience. Field lines point away from positive charges and towards negative charges. Where field lines are closer together, the field is stronger.
When the electric field around a charged object becomes strong enough, it can ionise the surrounding air — electrons are stripped from air molecules, making the air temporarily conducting. Charge then flows rapidly through this conducting path as a spark. This is why you sometimes get a small shock when touching a metal door handle after walking on a carpet in socks: charge builds up on your body as electrons are transferred by friction, then discharges rapidly when you touch the conductor.
A familiar consequence of static build-up is a jumper crackling when you take it off. Different fabrics rub together as the jumper is pulled over your head, transferring electrons and creating regions of opposite charge. These then attract each other and the rapid discharge creates small sparks and sounds.
Lightning is a dramatic large-scale version of the same process. In storm clouds, rapid movement of ice crystals and water droplets causes charge separation: the base of the cloud becomes strongly negatively charged. When the electric field becomes large enough, a huge discharge occurs between the cloud and the ground (or between clouds), releasing a vast amount of energy as light, heat and sound.
Like charges (both positive or both negative) repel each other; unlike (opposite) charges attract. An electric field surrounds every charge and exerts a force on any other charge within it. Static sparks and lightning are both caused by rapid discharge of built-up static charge through air.
A simple demonstration uses two charged polythene rods suspended on threads. Both rods are charged by rubbing with a cloth (giving them both a negative charge). When brought close together, they repel each other, causing the suspended rod to swing away. Replacing one rod with a charged acetate rod (positively charged when rubbed with the same cloth) and bringing it near the suspended polythene rod produces attraction — the suspended rod swings towards it. The effect diminishes as the distance between the rods increases, showing that the force depends on separation.
Uses and Hazards of Static Electricity
Static electricity has a range of practical applications. In an electrostatic paint sprayer, paint droplets are given a charge; they are then attracted towards an oppositely charged object (such as a car body). The electrostatic force ensures even coverage, including on surfaces that do not face the spray nozzle directly. Laser printers and photocopiers use static charge to attract toner particles onto a charged drum, which then transfers them to paper.
However, static build-up can also be hazardous. In fuel tankers, friction between flowing fuel and pipe walls can generate static charge. If this discharges as a spark near flammable vapour, it can ignite the fuel. For this reason, tankers are earthed before fuel is transferred — connecting the tanker to the ground via a conducting cable allows charge to flow safely away rather than building up to dangerous levels. Similarly, workers in electronics manufacturing wear anti-static wristbands that discharge any built-up static safely.