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Electrostatics (also known as static electricity) is the branch of physics that deals with apparently stationary electric charges.

Electrostatics involves the build up of charge on the surface of objects due to contact with other surfaces.

Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. This is because the charges that transfer to or from the highly resistive surface are more or less trapped there for a long enough time for their effects to be observed.

These charges then remain on the object until they either bleed off to the ground or are quickly neutralized by a discharge: e.g., the familiar phenomenon of a static 'shock' is caused by the neutralization of charge built up in the body from contact with non-conductive surfaces.

Attraction and repulsion of electric charges is one of three fundamental non-contact forces in nature. The others are magnetism and the force of gravity.

There are only two different sorts of known charge which scientists have labelled as ‘positive’ and ‘negative’. These names were chosen historically to indicate that they were somehow ‘opposites’ of each other, to help emphasise the two different observable forms. Scientists do not know exactly what charge is or how the two sorts of charge differ from one another; however each affects itself and its opposite form.

Positive and negative charged objects attract or pull each other together, while similar charged objects repel or push each other apart. The charged objects do not need to be touching in order for the repulsive or attractive forces to be experienced between them; i.e. they can be observed to affect each other at short distances and without the need for any substance in between, e.g. air.

When an object made of a good electrical insulator like plastic or glass is rubbed vigorously with another flexible electrical insulator made from fur, cotton or wool it is possible for charge of one type to move from the surface of one insulator to the surface of the other. 

Fig.1 - Before rubbing each item is electrically balanced.

Fig.2 - After rubbing the plastic ruler has become negatively charged and the cotton positively charged.

The rubbing does not create charges, but redistributes the charge between the two objects. As a consequence of their different overall charge, the two surfaces when separated will attract each other.

Charged objects can be created by using alternative methods to rubbing or sliding but all need the two surfaces to be in close contact and then separated. Other examples are peeling two plastic sheets apart or removing adhesive tape from a sheet of glass. Charged objects will also influence all other small ‘non-charged’ objects to become partially oppositely charged when they are brought near to them. This causes them to be attracted to each other. For example, a charged comb will attract small objects such as grains of sugar when brought close to them. If the grains of sugar come into contact with the comb then in a little while some grains will gain the same charge as the comb and will be rapidly repelled.

The amount of charge on the surface of any object will slowly reduce with time as the charge is eventually conducted away by water vapour in the surrounding air. For example a charged balloon will eventually fall from the ceiling as its charge diminishes.

Natural displays of charge movement like lightning mostly occur in storm clouds and less frequently above erupting volcanoes or in dust storms. In storm clouds, the charge is redistributed unevenly (parts will be positive and other parts negative). This imbalance can build up to the point where air becomes a conductor, and the imbalance is reduced by a rapid spark within the cloud or to the Earth. This is the lightning flash we see. The thunder we often hear is the result of this powerful spark rapidly heating the air.

Scientists are still trying to find out precisely why the imbalance of charge is created in storm clouds.