OCR Gateway Science : P4a

There can be (+) or (-) charges - metals (+) charged have missing electrons; metals (-) charged have extra electrons.

      (+)(-)          =   unlike charges attract

(-)            (-)    =   like charges repel

(+)           (+)   =   like charges repel

Electrostatic phenomena are caused by the transfer of electrons.

If insulators are rubbed together, electrons are rubbed off 1 material and are transferred to the other. This results in effects called electrostatic phenomena or static electricity.

We feel a flow of electrons through our bodies as our nerves detect the flow -electric shock.

Electric shocks are usually small and harmless but large can be dangerous to people with heart problems - the flow of charge can stop the heart!

Lightning is a very large electro-static discharge and can jump from cloud to building. It's often fatal.

To discharge objects/ stop them becoming charged we connect them with a thick metal wire to a large metal plate in the ground. This acts as a large reservoir of electrons.

Electrons flow so quick from/to the Earth that earthed objects don't become charged.

If all metal water pipes in a house can be connected to the ground they can be used as an Earth.

If you stand on an insulating mat you can become electrostatically charged more easily. When you step off the mat your charge leaks away through your shoes. If you wear insulating shoes you'll stay charged until you touch something that is earthed.

Metal cases of electrical appliances must be Earthed to prevent electric shocks if there's a fault (battery/mains).

When an object is discharged, electrons jump across the gap just before the 2 objects touch. This causes a spark.

This can cause an explosion if:

a) The atmosphere contains inflammable gases (hydrogen or methane)

b) There are inflammable vapours (petrol or methanol)

c)There is a high concentration of oxygen (powders in the air - flour/custard, which are carbohydrates). They contain lots of oxygen and burn easily. As a dust, they explode!!!

Small particles of dust and dirt are attracted to charged objects such as : Tv monitors or plastic cases.

Clothing clings to our bodies and other clothes when it is charged (attracted to the opposite charge or uncharged objects).

Anti-static sprays, liquids and cloths stop the build up of static charge and help reduce these problems.