P2 4.1-4.3 Static Electricity


P2 4.1 Electrical Charges

Adding electrons to an uncharged atom will make it negative, whereas removing electrons from an uncharged atom will make it positive. This is because electrons have a negative charge.

Some insulators become charged when rubbed with cloth.

E.g. A polyethene rod rubbed with a dry cloth transfers electrons to the surface atoms of the rod, causing it to become negatively charged.

E.g. A perspex rod rubbed with a dry cloth transfers electrons to the surface atoms of the cloth, causing it to become positively charged.

Two objects with the same charge will repel each other.

Two objects with opposite charges with attract each other.

Like charges repel. Unlike charges attract.

Charged objects can also attract uncharged objects.

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P2 4.2 Charge on the Move

The rate of flow of an electric charge is the current.

Conductors such as metals conduct electricity because they contain delocalised electrons which can move freely throughout the metal's atomic structure.

Insulators cannot conduct electricity because their electrons are held in the atoms.

Conductors 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.

To safely discharge a conductor, you need to 'earth' it by providing a conduction path, e.g. a wire, between the object and the ground so electrons can transfer into the earth.

The more charge a conductor has, the higher its electric potential energy. The potential difference (i.e. voltage) between the conductor and ground increases.

At high enough potential differences a spark may be produced to jump between conductors and nearby earthed objects. Lightning is an example of this.

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P2 4.3 Uses and Dangers of Static Electricity

Electrostatic Paint Sprayer: A spray nozzle is connected to the positive terminal of an electrostatic generator. The negative terminal is connected to a metal panel. The negative panel attracts the positive paint droplets, which also repel each other and create an even finish.

Electrostatic Preciptator: Used in powerplants to remove smoke particulates from flue gases. The carbon particles pass through a grid of negative charged wires, and so pick up the negative charge. They then reach positively charged metal plates which attract the smoke particles and then are shaken to remove the particulates of ash and dust.

Photocopiers: The have a 'photoconducting' drum which is positively charged until light falls on it. Light is reflected off a document onto the drum, the black areas do not reflect light and so the drum keeps its charge. The black toner sticks to the drum where it has charge and is then pressed onto the paper, and the paper is then heated so they stick together.

A spark from a charged object can make powder grains or certain gases explode. This is why doctors wear antistatic clothing- because they use anaesthetic gases that could be explosive.

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