As already mentioned in
Part 3, any object connected to
the 'earth' by a conductor (earthing charged objects) then any
static electricity can be safely discharged.
The most dramatic example is a
lightning conductor!
As previously described, lightning is
a very powerful and potentially dangerously destructive discharge of static
electricity. Apart from their obvious danger to human beings, lightning strikes
can seriously damage buildings, especially tall ones, were the highest point is
nearest to the source of static charge.
For example church steeples have a strip
of copper from the peak of the spire running right down to be embedded in the
ground - earthed. When the lightning strikes, the discharged static electricity
heads for the most electrically conductive material, the copper strip, rather
than the insulating stone, and runs safely into the ground. Without the
lightning conductor the build up of energy at the top of the building is so
great it cause physical damage to stonework and set fire to roof timbers.
As a car,
or any other road vehicle, is moving fast through air, static charge can build
up on the body of the car through friction.
To avoid any irritating or dangerous
consequences, you can have a metal contact e.g. a copper strip in a plastic
sheath (brown strip on the diagram above) that electrically connects the metal
body of the car to the 'earth'.
This allows any static charge formed to drain
away.
If the car is positive the charge is 'neutralised' by electrons flowing
from the road (the 'earth') or if the car is negative, then the negative static
charge of electrons can be safely discharged to the road through the copper
strip.
Static charge is easily formed by a plastic
surface rubbing against another surface e.g. plastic vinyl floor tiles, nylon
comb through your hair, synthetic fibres in clothing, etc.
To minimise these
effects plastic additives called antistatic agents have been developed to
minimise the build up of static electricity.
To these plastic products special
molecules called anti-static agents are added to the polymer mixture from the
object/material is made.
These antistatic agents make the surface of the polymer
slightly conductive and enough to allow any static charge formed to be
discharged and so dispersed to give no noticeable effect.
You can uses
anti-static sprays to coat surfaces to increase the surface conductivity to reduce the problems of static
electricity - you can treat car seats in this way too.
Refuelling and filler pipes:
When road
vehicle fuel tanks at the petrol station, fuel tankers themselves, aircraft fuel
tanks etc. are being filled the friction of the flowing fuel against the pipe
hosing can create static charge.
Therefore, fuel delivery systems must be
(most importantly) earthed and anti-static liquid agents may be added to the
fuel to increase its electrical conductivity to drain away any potentially
static electricity.
The hose piping itself can be treated
with an anti-static agent to avoid the build up of static charge, that,
if discharged, may create a spark potentially causing a fire or explosion in an
air - petrol vapour mixture.
Static electricity can build up on the body
of an aircraft as it flies through the air at great speed, so a great friction
effect cannot be avoided.
Therefore the plane does become charged and this
static charge can interfere with communication systems.
Modern aircraft are fitted with
static dischargers, which moderate the amount of static charge that
builds up on the aircraft.
In a factory, machinery operators using high
voltage machines, stand on insulating mats or wear shoes with insulating soles
to stop any charge flowing through them to the Earth.
Protection against a static electricity
discharge must be in place where equipment is used in atmospheres where explosions could
occur eg inflammable gases or vapours or with high concentrations of oxygen
Most of the situations I've described will be
familiar to most people, but how many of you realise the dangers of very fine
combustible powders moving in the air!
In the past there have been coal dust
(coal mine) and flour (flour mill) explosions due to the friction between moving
fine dust particles and the surrounding air.
The fine powder particles have such
a large surface area for friction to take place that sufficient static charge can build up to create a
spark.
The 'surface area rule' in chemistry kicks in (rates
of reaction factor) and rapid combustion ensues from the heat
generated, causing the powder and oxygen in the air to explode !!!
