Electrostatics

Revision notes

When two insulating materials are rubbed together, electrons can be transferred from one surface to the other. The object that gains electrons becomes negatively charged and the object that loses electrons becomes positively charged. Protons remain bound inside nuclei, so charging ordinary objects does not involve protons moving between them. Like charges repel and unlike charges attract. Conductors allow charge to move through them relatively easily, whereas charge placed on an insulator tends to remain localised.

An electric field is the region in which a charged object experiences a force. Field direction is defined as the direction of the force on a small positive test charge, so field lines point away from positive charge and towards negative charge. A charged object can attract a neutral object because charges inside the neutral material redistribute slightly: opposite charge is effectively closer to the charged object than like charge. This induced separation of charge does not require the neutral object to gain a net charge.

A large potential difference can make air conduct and produce a spark. Earthing gives excess charge a conducting route to or from Earth, reducing unwanted charge build-up. Electrostatic effects are useful in processes such as paint spraying, powder coating and some particle-removal systems, but sparks can be dangerous near flammable vapours.

Charging by electron transfer

Static charge is produced when there is an imbalance between positive and negative charge. In ordinary charging by friction, electrons transfer between materials; protons remain bound in nuclei. An object gaining electrons becomes negatively charged, while an object losing electrons becomes positively charged. Charge is transferred, not created.

Insulators tend to hold charge in one region because charges cannot move freely through the material. Conductors allow mobile charge to redistribute. A charged conductor connected to Earth can gain or lose electrons through the conducting path until its charge imbalance is reduced.

Forces and electric fields

Like charges repel and unlike charges attract. A charged object creates an electric field around it. Field direction is defined as the direction of force on a positive test charge, so field lines point away from positive charge and towards negative charge. Closer field lines represent a stronger field qualitatively.

A neutral object can still be attracted to a charged object because its charges redistribute slightly. The opposite charge may become effectively closer than the like charge, giving a net attractive force. This process does not necessarily give the neutral object an overall charge.

Sparking, earthing and hazards

A large potential difference can make air ionise and become conducting, allowing a spark. Sparks can ignite flammable vapours, so fuel-transfer systems and other industrial processes use bonding or earthing to reduce charge build-up.

Earthing provides a route for electrons to move between an object and the Earth. Explain which way electrons move when possible rather than saying that 'charge goes to ground' without identifying the sign.

Applications

Electrostatic paint spraying and powder coating can charge droplets or particles so they repel each other into a fine spread and are attracted to the target. Similar principles are used in particle-removal devices and some printing processes. The useful effect always comes from forces between charges or between charged particles and electric fields.

Exam technique

Use electron language accurately. A positive charge means fewer electrons than protons; the protons themselves do not move. Keep electrostatics separate from current electricity: a static charge can exist without a continuous current.