# Uses of Static Electricity

Electrostatic charges can be useful in everyday life.

### The Photocopier:

 a copying plate is electrically charged; an image of the page you want to copy is projected on to the plate; where light falls on the plate, the electrical charge leaks away; the parts of the plate that are still charged attract bits of black powder; the black powder is transferred from the plate to a sheet of paper; the paper is heated to melt the toner and therefore make the black powder stick permanently to it there is now a copy of the original page.

There are a few webpages on the photocopier that you might find useful to look at: -

### The Inkjet printer:

 tiny droplets of ink are electrically charged as they are forced out of a very fine nozzle; the droplets pass between metal plates ac ross which a voltage can be applied so that one plate is negative and the other plate is positive; the charged droplets of ink are attracted towards the plate with the opposite charge and away from the plate with the same charge. This means that they are deflected as they pass between the plates; the size and direction of the voltage applied across the plates is controlled so that each droplet in turn is deflected to a particular place on the paper; each droplet of ink produces a tiny dot on the paper and many such dots, each in exactly the right place, produce the printed characters.

### Cleaning the air with an electostatic precipitator

 As the waste gases pass the negatively charged wire grid the smoke particles pick up a negative charge. They are repelled by the grid, but attracted to the positive charge on large collecting plates. They stick to the plates, which are banged regularly by a metal striker causing the smoke particles to fall into the dust traps, from which they are removed.

### Earthing (or 'grounding' in the US)

Earth is said to be at zero volts. If a charged object is connected to earth by a conductor, electrons will pass between the charged object and Earth until the object is at zero volts too. If the object lacks electrons (is at positive potential or lacks electrons) then electrons will run from Earth to the object and if it is at negative potential (has a negative charge - too many electrons!) then the opposite happens, electrons run to Earth from the object.

The symbol for for a connection to Earth is:

 The greater the charge on an isolated object, the greater the voltage (potential difference) between the object and earth. If the voltage becomes high enough, a spark may jump across the gap between the object and any earthed conductor which is brought near it. A charged conductor can be discharged safely by connecting it to earth with a conductor. Metal cased appliances are earthed via the plug (see section on the 3-pin plug and earthing of appliances)