Insulating materials (like plastic) can be charged by friction due to the transfer of electrons. E.g. When Polythene is rubbed with a cloth, electrons transfer from the cloth to the polythene, resulting in a surplus of electrons on the polythene and an overall negative charge. Or When Acetate is rubbed with a cloth, electrons transfer from the acetate to the cloth, resulting in a deficiency of electrons on the acetate and an overall positive charge.
Attraction and Repulsion – If two objects have opposite charges, they will attract. If two objects have like charges, they will repel. If two objects are charged and uncharged, they will attract, for example: If a negatively charged polythene rod is held near an uncharged piece of paper, the electrons in the paper near the rod are repelled away, leaving this area of the paper positively charged, and thus the two bodies attract.
Uses of Static Electricity – Static electricity has several uses: Spray Painting: Charged paint drops are sprayed at the body to direct the paint towards it. Paint particles also repel to provide an even coat. Printing: Charged ink is sprayed at the paper to help it stick. Smokestack Precipitators: Smoke particles are given a charge and are attracted to collecting plates to remove them from waste gases when fossil fuels are burnt. Dangers: Refuelling: In large vehicles, vast amounts of fuel may result in a build-up of charge on the hose and a spark could ignite the fuel. To avoid this, the hose is usually earthed with a conductor, allowing the charge to dissipate. Lightning: A build-up of charge in clouds can occur and this can be released in the form of lightning. Conductors and Insulators – Conductors contain free electrons which allows them to carry a current (i.e. a flow of electrons), insulators do not. Electron flow moves from – to + (like charges repel and opposites attract) but conventional current was assumed to flow from + to -.
Charge – Charge (measured in coulombs) is given as: Charge = Current x Time Or