Static Electrical Charge
When two insulatiing materials are rubbed together, negatively charged electrons are transferreed from one to another, making both materials electrically charged. Their charges are equal and opposite static electrical charges.
- Like charges repel: both positive or both negative.
- Unlike charges attract: positive + negative.
An electrical insulator is a material that holds static electrical charge (it is not a conductor).
Explain why static electricity normally only builds up on insulators, such as plastic.
Static electricity is caused by a build up in electrons (or lack of electrons) In a conductor the electrons could flow away and not stay together in one place.
Charges exert a force on each other: like charges repel and unlike charges attract.
Insulators (e.g plastics, glass) hold electrical charge.
Electrical conductors (e.g metals, water) allow charge to flow as an electrical current in a closed circuit. They hold charge if insulated or if not part of a closed circuit.
An electroscope shows if an object has an electrical charge. The cap and rod are metal and the leaf is gold; they are on an insulating base.
The electroscope shows whether the charge is positive or negative.
Van de Graaff generator
In a Van de Graaff generator, two materials rub together and build up equal and opposite static electrical charges. The dome gains a large but safe positive charge.
A girl stands on an insulating mat and touches the dome: the charge is equally redistributed over her and the dome. Her charged hairs repel each other, move apart and stand on end. If she touches an earthed conductor, the charge will flow to earth, discharging her hair.
When a conducting object which is earthed approaches the dome of a charge Van de Graaff generater, a spark (electrical charge) jumps across, and charge flows to earth as an electrical current.
Electrostatic Paint Sprayer
Paint droplets gain positive charge as they stream through the spray gun. The car bodywork is earthed, so a negative charge is induced near the spray. It attracts the paint droplets, because opposite charges attract, and the paint droplets are deposisted evenly over the bodywork.
It is important for the paint droplets to have the same charge, so they repel each other and so they are deposisted evenly over the bodywork.
The drum inside a photocopier is negatively charged.
When you press copy, light reflected from white areas of a sheet being photocopied illuminates the drum.
Charge from those areas flows away, leaving positive charge on the drum only where there is colour on the sheet.
These areas of the drum therefore pick up negatively charged black toner, which is transferred to the copy.