Static electricity: uses and dangers

In a nutshell

Static electricity is the result of an imbalance between positive and negative charges in an object. Static electricity can have both uses and dangers.

Static

All matter contains charge - atoms contain positive protons and negative electrons. Most matter contains an equal number of protons and electrons, so their effects cancel each other out. This results in the matter to be neutral and have zero net charge.

However, in some situations charge can build up on objects. This is known as static electricity:

1.	When two materials are rubbed together, the friction causes electrons to gain energy and transfer from one material to the other.
2.	It is always negative charges (electrons) that move.
3.	The direction of electron transfer depends on the materials.
4.	If enough charge is accumulated, it can discharge suddenly, resulting in sparks or shocks.

Note: Static electricity can be generated by rubbing a plastic rod with a cloth. For a polyethylene rod, electrons move from the cloth to the rod. For an acetate rod, electrons move from the rod to the cloth.

The diagram below shows electrons moving from an acetate rod to a cloth after being rubbed together.

Physics; Electricity and circuits; KS4 Year 10; Static electricity: uses and dangers

Charges

Electrically charged objects exert a force on one another. The magnitude of this force weakens the further apart the two objects are. Objects with opposite electric charges attract each other - this is known as electrostatic attraction. Objects with the same electric charge repel - this is known as electrostatic repulsion.

Electrostatic attraction and repulsion are a non-contact force, similar to the force due to gravity. The objects do not need to be touching to exert a force on one another.

One way to observe the electrostatic force is to suspend a rod with a known charge from a piece of string (so that the rod can move freely). Positioning an object with the opposite charge nearby will attract the rod - the rod will move towards from the object. An object charged with the same charge sign will cause the rod to repel and move away from the object. The diagram below illustrates the behaviour of the first case (the rod and object have opposite charges):

Physics; Electricity and circuits; KS4 Year 10; Static electricity: uses and dangers

Sparks

As electric charge builds on an object, the potential difference between the object and the earth increases. If the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth - this is called a spark. Electrons can also jump to any earthed conductor that is nearby. This usually happens when the gap is fairly small.

Uses of static electricity

In certain situations, static electricity can be useful. Here are a couple of examples:

Electrostatic precipitators

A large number of power stations burn fossil fuels such as coal and oil. When these fuels are burnt, smoke is produced.

Smoke is made up of tiny solid particles such as unreacted carbon, which can damage buildings and cause breathing difficulties. To minimise these issues, smoke is removed from waste gases before they pass out of the power station chimneys. The electrostatic precipitator is a device that achieves this. Below are the working principles of an electrostatic precipitator:

1.	The smoke particles pick up a negative charge as they pass through a negatively charged metal grid at the bottom of the chimney.
2.	The smoke particles are attracted to the positively charged collecting plates, which are situated inside the lower chimney.
3.	The collecting plates are cleaned periodically and the carbon is collected.

Photocopiers

A photocopier is a machine that produces photocopies of an original document. Below are the operating principles of a photocopier:

1.	The original document is placed within the photocopier.
2.	The image of the document is projected onto a positively charged copying plate.
3.	Where light falls onto the plate, the electrical charge falls away.
4.	Negatively charged black toner particles are attracted to the remaining positive areas.
5.	Paper is placed over the copying plate. Toner is transferred to the paper and heated to make it stick.
6.	The photocopy of the original document is complete.

Dangers of static electricity

Below are some examples of the dangers associated with static electricity:

When there are flammable gases or a high concentration of oxygen present in an environment, a spark could ignite the gases and cause an explosion.
It is very dangerous to touch something that has a large electrical charge on it. The charge will flow through your body, causing an electric shock. This could result in burns or even stop your heart. Electric shocks can even lead to death in some cases.
Refuelling aircrafts and tankers also poses a risk. If the fuel flowing through the hose to the vehicle was allowed to build up a static charge, a resulting spark might ignite the fuel. To mitigate this risk, the hoses are earthed.