Charging by friction and electron transfer, like and unlike charge interactions, charging by induction, electric field patterns around point charges and parallel plates, and electrostatic applications.
Electrostatics deals with electric charges that are at rest.
Matter contains protons (positive charge) and electrons (negative charge). A neutral object has equal numbers of each. When objects gain or lose electrons, they become charged:
Only electrons transfer during charging of solids, protons remain fixed in the nucleus.
The law of electrostatics states that like charges repel each other and unlike (opposite) charges attract each other.
When two different insulating materials are rubbed together, electrons transfer from one to the other:
The charge is created by electron transfer, not by creating or destroying charge (conservation of charge).
A neutral conductor can be charged without contact using a charged object:
An electric field is a region in which a charged particle experiences a force. Electric field lines show the direction a positive test charge would move:
Isolated point charge:
Two opposite point charges (electric dipole):
Two parallel plates (uniform field):
| Application | Principle |
|---|---|
| Photocopier / laser printer | Charged drum attracts toner particles to form an image |
| Electrostatic precipitator (air filter) | Charged plates attract dust and smoke particles, cleaning industrial exhaust |
| Spray painting | Charged paint droplets are attracted to earthed metal object, reduces waste and gives even coating |
| Lightning conductor | Pointed conductor on a building provides a preferential path for charge to flow safely to earth |
Static charge can build up when flammable fuels are pumped through pipes or when aircraft move through air. If the charge discharges as a spark, it can ignite vapour. Aircraft are earthed when refuelling; fuel tanker trucks use bonding cables.
When explaining charging by friction, always state that electrons (not protons) are transferred. Identify which object gains electrons (negative charge) and which loses them (positive charge).
For charging by induction, always state that the sphere is connected to earth before the rod is removed, this is the step that causes the net charge to appear.