Static electricity

The effects of static electricity are familiar to most people because they can feel, hear, and even see sparks if the excess charge is neutralized when brought close to an electrical conductor (for example, a path to ground), or a region with an excess charge of the opposite polarity (positive or negative).

The familiar phenomenon of a static shock – more specifically, an electrostatic discharge – is caused by the neutralization of a charge.

The triboelectric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together (e.g., fur against an acrylic rod).

[2] Applied mechanical stress generates a electric polarization and in turn this can lead to separation of charge in many types of materials.

Removing or preventing a buildup of static charge can be as simple as opening a window or using a humidifier, to increase the moisture content of the air, making the atmosphere more conductive.

[6] Items that are particularly sensitive to static discharge may be treated with the application of an antistatic agent, which adds a conducting surface layer that ensures any excess charge is evenly distributed.

[8][9] In the industrial settings such as paint or flour plants as well as in hospitals, antistatic safety boots are sometimes used to prevent a buildup of static charge due to contact with the floor.

For modelling the effect of static discharge on sensitive electronic devices, a human being is represented as a capacitor of 100 picofarads, charged to a voltage of 4,000 to 35,000 volts.

Larger objects will store more energy, which may be directly hazardous to human contact or which may give a spark that can ignite flammable gas or dust.

While the details are unclear and remain a subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds.

The static charge in air typically breaks down in this way at around 10,000 volts per centimeter (10 kV/cm) depending on humidity.

The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence.

[14] The flowing movement of finely powdered substances or low conductivity fluids in pipes or through mechanical agitation can build up static electricity.

In the petrochemical industry, 50 pS/m is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.

Re-filling of large apparatus requires precautions against electrostatic charging of the fluid, which may damage sensitive transformer insulation.

The British standard BS PD CLC/TR 50404:2003 (formerly BS-5958-Part 2) Code of Practice for Control of Undesirable Static Electricity prescribes pipe flow velocity limits.

For fluids with electrical conductivity below 10 pS/m, bonding and earthing are not adequate for charge dissipation, and anti-static additives may be required.

[citation needed] The flowing movement of flammable liquids like gasoline inside a pipe can build up static electricity.

Customers who need to fill containers at gas stations are advised to set them on the ground first so that any static buildup will dissipate without risk of fire or explosion.

[25] It is envisaged that a charge generation mechanism only occurs when solid particles or liquid droplets are carried in the gas stream.

Due to the extremely low humidity in extraterrestrial environments, very large static charges can accumulate, causing a major hazard for the complex electronics used in space exploration vehicles.

Fires from cracked fuel lines have been a problem on vehicles, especially in the engine compartments where ozone can be produced by electrical equipment.

[27] One experimenter estimates the capacitance of the human body as high as 400 picofarads, and a voltage of 50,000 volts, discharged e.g. during touching a charged car, creating a spark with energy of 500 millijoules.

[30] As little as 0.2 millijoules may present an ignition hazard; such low spark energy is often below the threshold of human visual and auditory perception.

For the common industrial hydrocarbon gases and solvents, the minimum ignition energy required for ignition of vapor–air mixture is lowest for the vapor concentration roughly in the middle between the lower explosive limit and the upper explosive limit, and rapidly increases as the concentration deviates from this optimum to either side.