A Van de Graaf generator can make your hair stand on end, it charges up because:
- The belt rubs against a felt pad and becomes charged
- The belt carries the charge onto an insulated metal dome
- Sparks are produced when the dome can no longer hold any more charge
Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth, so the polythene rod becomes negatively charged.
Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod onto the cloth, so the perspex rod becomes positively charged.
Charges On The Move
The rate of flow of electrical charge is called the current.
- Insulators cannot conduct electricity because all the electrons are held in atoms.
- A conductor can only hold charge if it is insulated from the ground
- To charge an insulated conductor, it needs to be brought into contact with a charged object.
When an object is earthed, it is said to be discharged from conducting electricity.
If we supply a conductor with more and more charge, its electric potential energy increases.
Uses and Dangers of Static Electricity
The electrostatic paint sprayer:
- spray nozzle connected to positive terminal of electrostatic generator
- negative terminal is connected to the metal panel
- panel attracts paint droplets from spray
- droplets of paint pick up same charge and repel eachother, forming fine cloud of paint.
The electrostatic precipitator:
- it removes ash and dust from coal-fired power stations
- ash/dust particles pass through a grid of wires in precipitator
- Grid wires are negative so partices become negatively charged
- Charged particles are attracted onto positively charged metal plates
- Plates are shaken so ash and dust falls off and is removed.
Dangers of Static Electricity
- petrol pipes must be earthed, otherwise it can become charged
- A build up of charge can cause a spark, or an explsion as fuel reacts with O2 in air
- Static electricity is also generated when grains of powder are pumped through pipes
- Friction between the grains and the pipe charges them.
- An explosion could happen due to a spark igniting the powder
- Doctors use anaesthetic gases which are explosive during operations in hospitals
- If the gas escapes into air, a tiny spark could make it explode
- To eliminate this static charge, antistatic materials is used as floor surface
- This material is a poor electrical insulator so it conducts charge to Earth