SITEMAP   School Physics Notes: Electricity 8.6 Nuisance - dangers of static electricity

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Section 8 Static electricity: Part 8.6 More on the nuisance and dangers of static electricity from friction between material surfaces - use of antistatic agents to counteract the effects of 'static'

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8.6 More on the nuisance and dangers of static electricity

As already mentioned in , 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 !!!

Keywords, phrases and learning objectives on static electricity

Know and explain how to use antistatic agents to counteract effects of 'static'.

Be able to describe and explain the nuisance and dangers of static electricity e.g. car builds up static from friction, nylon clothes contain antistatic agents, danger of fire and explosion of refuelling filler pipes of flammable liquids e.g. at petrol stations and fine powders can ignite - there have been serious explosions in flour mills before safety standards were increased.

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