PHY - 11. electrical charge

conductor

easily allows electricity to flow through

static electricity

charges are not moving through the material, normally occurs with insulators as they don’t allow electric charge to flow

what happens when a polythene rod becomes negative after being rubbed with a cloth

when the cloth rubs against the rod it removes the electrons from it and there is an unequal number of protons and electrons and its overall charge will be positive

describe what happens when a balloon is rubbed against a wall

• electrons in the balloon repel against the electrons in the wall

• this exposes the positive charge

• the negative charge is the attracted to the positive charge

• this creates a dipole

dipole symbol

δ- or δ+

what is selenium

a semiconductor which means that it insulates in dark areas and conducts electricity when light falls

uses of static electricity

paint spraying

inkjet printers

photocopiers

electrostatic smoke precipitators

explain how photocopiers work

• A drum/ plate is positively charged

• Light shines on the original document and the reflected image falls onto the drum/ plate

• where light hits the drum the charge is lost leaving a pattern of the original in electric charge

• black ink toner (powder) is given negative charge and is blown onto the drum sticking to the positive parts

• the drum rolls over paper and the pattern is transferred as toner

• paper is heated to melt and fix the ink

explain a electrostatic smoke precipitator

as smoke rises it passes through a high voltage net that is negatively charged

this means the soot particles in the smoke gain a negative charge and are ionised

The particles are then attracted to the positively charged metal plates where they stick

the smoke escapes through the top of the chimney but will contain less dust and soot

how do they keep the metal plates from being overloaded with soot

the remove/ reverse the charge on the plates so the pollutants will fall off

danger of static electricity: planes

when refuelling planes, prevent explosions

• planes can become charged as dusty air rubs against them in flight

• they are earthed by connection to the ground when refuelling

• This stops sparks from jumping near the fuel nozzle

extra: the fuel nozzle also needs to be earthed to prevent explosions

what can a build up of static electricity lead to

a spark which can cause certain gases to explode

why do your hair stand on end

combing or rubbing something against your hair transfers electrons from the hair to the (comb) by friction, this results in your hair having a negative charge, if all your hairs have the same charge they will repel from each other

How does a Van Der Graff work

• friction from the rubber belt against brushes causes electrons to move from the belt onto the metal dome

• charge builds up on the dome since it is isolated

• the electrons cannot travel to earth and create a current

• so static electricity builds up