Generating Static Electricty
A insulating material can become electrically charged when rubbed against another insulating material. Electrons (which have a negative charge) are transferred from one material to the other, leaving:
- One material with a positive charge.
- One material with a negative charge.
You can generate static electricity by rubbing a balloon onto you hair. The electrically charged object will be able to attract small objects. For example, dusting brushes can be charged so they pick up dust when it passes over the brush.
Synthetic clothing can become charged due to friction between the clothing and the person's body. When the clothing is removed it can sometimes produce static sparks.
Discharging Static Electricity
A charged object can be discharged (i.e. have the excess charged removed) by earthing it. When an object discharges, electrons are transferred from the charged object to earth.
If you become charged and then earthed, you could get an electrostatic shock.
For example, you can become charged by friction between the soles of your feet and the floor if you're walking on an insulator such as carpet. If you then touch a water pipe, e.g. a radiator, the charge is earthed and discharge occurs, giving you a shock.
Problems Of Static Electricty
In some situations, static electricity can be a nuisance.
For example, static can cause:
- Dirt and dust to be attracted to insulating materials, e.g. television screens and computer monitors.
- Some materials to cling on your skin.
In other situations, static electricity can be very dangerous:
- Flour mills and petrochemical factories have atmospheres that can contain extremely flammable gases (or vapors), or high concentration of oxygen. A discharge of static electricity (i.e. a spark) can lead to an explosion.
- Static is dangerous in any situation where large amounts could flow through your body to earth, for example lightening.
Repulsion and Attraction
Two insulating materials with the same charge will repel each other. For example, if a positively charged Perspex rod is held near to a suspended positively charged Perspex rod, the suspended rod will be repelled. The same thing would happen if both rods had a negative charge.
Two insulating materials with different charges will attract each other. For example, if a negatively charged ebonite rod is held near to a suspended positively charged Perspex rod, the suspended rod will be attracted to the ebonite rod. This would also happen if the charges were the other way round.
Charging Up Objects
Electric or static charge builds up wen electrons (negatively charged) are rubbed off one material onto another.
- The material that receives the electrons becomes negatively charged due to an excess of electrons.
- The material giving up the electrons becomes positively charged due to a loss of electrons.
A Perspex rod rubbed with cloth gives up electrons and becomes positively charged. The cloth receives the electrons and becomes negatively charged. An ebonite rod rubbed with fur receives electrons and becomes negatively charged. The fur gives up electrons and becomes positively charged.
Atoms and molecules that have become charged are called ions.
- If they have an excess of electrons they have negative ions.
- If they have lost electrons they are positive ions.
Reducing The Danger
The chance of receiving an electric shock can be reduced by:
- Making sure appliances are correctly earthed.
- Using insulation mats effectively
- Wearing shoes with insulating soles.
Lorries that contain inflammable gases, liquids or powders need to be earthed before unloading, as friction can cause a build-up of charge. This charge could lead to a spark, which could then ignite the flammable substance.
Anti-static sprays, liquids and cloths help to reduce the problems of static electricity by preventing the transfer of charge from one insulator to another. If there is no charge, they can't be discharged.