Static electricity definition, explanation with examples

Static electricity definition, explanation with examples



Static electricity occurs when the intra-atomic or intramolecular equilibrium is disturbed due to the acquisition or loss of an electron. Usually, an atom is in equilibrium due to the same number of positive and negative particles - protons and electrons. It is noted that electrons can move from one atom to another easily. In doing so, they form positive (where there is no electron) or negative (a single electron or an atom with an additional electron) ions. When this imbalance occurs, static electricity is generated.


The electric charge of an electron is (-) 1.6 x 10-19 coulomb. A proton with the same charge has a positive polarity. The static charge in coulombs is directly proportional to the excess or deficit of electrons, i.e. the number of unstable ions.


The pendant is the basic unit of static charge that determines the amount of electricity passing through the cross-section of a conductor in 1 second at 1 ampere.


A positive ion lacks one electron, therefore, it can easily accept an electron from a negatively charged particle. A negative ion can be either a single electron or an atom/molecule with many electrons. In both cases, there is an electron that can neutralize the positive charge.

How static electricity is generated                  

KEY FEATURES;

1-Contact between two materials and their separation from each other (including friction, winding/unwinding, etc.).

2-Rapid temperature drops (for example, when the material is placed in the oven).

3-High energy radiation, ultraviolet radiation, X-rays, strong electric fields (not common in industrial applications).

4-Cutting operations (for example, on cutting machines or paper cutting machines).
Guidance (static electricity generated).

5- Surface contact and material separation are perhaps the most common causes of static electricity in roll film and plastic sheeting industries.

Static charge is generated during the unwinding/rewinding of materials or the movement of various layers of materials relative to each other.


This process is not entirely clear, but the most truthful explanation for the appearance of static electricity, in this case, can be obtained by drawing an analogy with a flat capacitor, in which mechanical energy is converted into electrical energy when the plates are separated:


Resulting stress = initial stress x (final plate spacing / initial plate spacing).

When the synthetic film touches the feed/take-up roll, a slight charge flowing from the material to the roll will cause imbalance.

As the material overcomes the contact area with the shaft, the voltage rises, just like with the capacitor plates when they separate.

Practice shows that the amplitude of the resulting voltage is limited due to electrical breakdown that occurs in the gap between adjacent materials, surface conductivity, and other factors. At the exit of the film from the contact area, you can often hear a faint crackle or observe sparking. This occurs at the moment when the static charge reaches a value sufficient to breakdown the surrounding air.


Prior to contact with the roller, the synthetic film is electrically neutral, but in the process of movement and contact with the feeding surfaces, a stream of electrons is directed to the film and charges it with a negative charge. If the shaft is metal and grounded, its positive charge will drain quickly.


Most equipment has many shafts, so the amount of charge and its polarity can change frequently.

The best way to control static charge is to accurately measure it in the area just in front of the problem area. If the charge is neutralized too early, it can recharge before the film reaches this problem area.

If the object has the ability to store a significant charge, and if there is a high voltage, static electricity will cause serious problems such as arcing, electrostatic repulsion/attraction, or electrical shock to personnel.

Charge polarity


The static charge can be either positive or negative. For DC arresters (AC) and passive arresters (brushes), charge polarity is usually not important.

Static electricity problems 

Static discharge in electronics;


It is necessary to pay attention to this problem since it often occurs in the handling of electronic components and components used in modern control and measuring devices.


In electronics, the main hazard associated with static electricity comes from the person carrying the charge and cannot be neglected.

The discharge current generates heat, which leads to the destruction of connections, interruption of contacts, and rupture of microcircuit tracks. The high voltage also destroys the thin oxide film on the field-effect transistors and other coated elements.


Components often do not completely fail, which can be considered even more dangerous, because the malfunction does not manifest itself immediately, but at an unpredictable moment during the operation of the device.


As a general rule of thumb, when handling static-sensitive parts and devices, you should always take steps to neutralize the accumulated charge on your body.

Electrostatic attraction / repulsion


This is perhaps the most widespread problem in plastics, paper, textiles, and related industries. It manifests itself in the fact that materials independently change their behavior - they stick together or, conversely, repel, adhere to equipment, attract dust, improperly wind on the receiving device, etc.


Attraction/repulsion occurs in accordance with Coulomb's law, which is based on the principle of the opposite of a square. In simple terms, it is expressed as follows:


The force of attraction or repulsion (in Newtons) = Charge (A) x Charge (B) / (Distance between objects 2 (in meters)).


Consequently, the intensity of this effect is directly related to the amplitude of the static charge and the distance between attracting or repulsive objects. 

Attraction and repulsion occur in the direction of the lines of force of the electric field.

If two charges have the same polarity, they repel; when in the opposite, they will attract each other. If one of the objects is charged, it will provoke attraction, creating a mirror copy of the charge on neutral objects.

Risk of fire

The risk of fire is not a common problem for all industries. But the likelihood of fire is very high in printing and other enterprises that use flammable solvents.


In hazardous areas, the most common sources of ignition are ungrounded equipment and moving conductors. If an operator in a hazardous area is wearing sports shoes or shoes with non-conductive soles, there is a risk that his body will generate a charge that can ignite solvents. Ungrounded conductive machine parts are also dangerous. Everything in the hazardous area must be well-grounded.


The following information provides a brief explanation of the potential of static discharge to provoke fire in flammable environments.

It is important that inexperienced salespeople are aware of the types of equipment in advance in order to avoid mistakes in the selection of devices for use in such conditions.


The ability of discharge to provoke a fire depends on many variables:


1-discharge type;
2-discharge power;
3-source of discharge;
4-discharge energy;
5-the presence of a flammable medium (solvents in the gas phase, dust or flammable liquids);
6-minimum ignition energy (MEW) of a flammable medium.

Discharge types


There are three main types - spark, brush, and sliding brush discharges.
In this case, it does not take the corona discharge into account, because it is not very energetic and occurs rather slowly.

Corona discharge is most often harmless and should be considered only in areas of very high fire and explosion hazard.

Spark discharge


It mainly comes from a moderately conductive, electrically insulated object. It can be a human body, a part of a machine, or a tool. we assumed that all the charge energy is dissipated at the moment of sparking. If the energy is higher than the MEW of the solvent vapor, the ignition may occur.


The spark of energy is calculated as follows: E (in Joules) = ½ C U2.

Hand discharge


Brush discharge occurs when the sharpened parts of the equipment concentrate charge on the surfaces of dielectric materials, the insulating properties of which lead to its accumulation. A brush discharge has lower energy than a spark discharge and, accordingly, presents a lower risk of ignition.

Sliding brush discharge


Sliding brush discharge occurs on a sheet or rolls synthetic materials with high resistivity, having an increased charge density and different charge polarities on each side of the web. This phenomenon can be triggered by rubbing or spraying the powder coating. The effect is comparable to the discharge of a flat capacitor and can be as dangerous as a spark discharge.

Discharge source and energy


The size and geometry of the charge distribution are important factors. The larger the volume of the body, the more energy it contains. Sharp corners increase field strength and support discharges.

Discharge power


If an object with energy does not conduct electricity very well, such as a human body, the object's resistance will weaken the discharge and reduce the hazard. For the human body, there is a rule of thumb: assume that any solvents with internal minimum ignition energy of less than 100 mJ can ignite, although the energy contained in the body can be 2 to 3 times higher.

Minimum ignition energy MEW


The minimum ignition energy of the solvents and their concentration in the hazardous area are very important factors. If the minimum ignition energy is lower than the discharge energy, there is a risk of fire.

Electric shock


More and more attention is paid to the risk of static shock in industrial environments. This is due to a significant increase in occupational health and safety requirements.


Electrocution caused by static electricity, in principle, does not pose a particular danger. It is simply unpleasant and often provokes harsh reactions.

There are two common causes of static shock:

1- Induced charge


If a person is in an electric field and holds onto a charged object, such as a film reel, it is possible for their body to be charged.


The charge remains in the operator's body as if it is in shoes with insulating soles until it touches grounded equipment. The charge flows down to the ground and hits the person. This also happens when the operator touches charged objects or materials - due to the insulating shoes, the charge accumulates in the body. When the operator touches metal parts of the equipment, the charge can drain and provoke an electric shock.

When people move on synthetic carpets, static electricity is generated by contact between the rug and the shoes. Electric shocks that drivers receive when leaving their car are triggered by a charge that has arisen between the seat and their clothes when they get up. The solution to this problem is to touch a metal part of the car, for example, the door frame, before lifting from the seat. 
This allows the charge to safely drain to the ground through the vehicle's body and tires.

2- Equipment-induced electrical shock


Such an electric shock is possible, although it occurs much less frequently than damage provoked by the material.


If the winding reel has a significant charge, it happens that the operator's fingers concentrate the charge to such an extent that it reaches the breakdown point and discharge occurs. In addition, if an ungrounded metal object is in an electric field, it can be charged with an induced charge. Because a metallic object is conductive, the mobile charge will be discharged into a person who touches the object.

When people move on synthetic carpets, static electricity is generated by contact between the rug and the shoes. Electric shocks that drivers receive when leaving their car are triggered by a charge that has arisen between the seat and their clothes when they get up. The solution to this problem is to touch a metal part of the car, for example, the door frame, before lifting from the seat. This allows the charge to safely drain to the ground through the vehicle's body and tires.


Equipment-induced electrical shock


Such an electric shock is possible, although it occurs much less frequently than damage provoked by the material.


If the winding reel has a significant charge, it happens that the operator's fingers concentrate the charge to such an extent that it reaches the breakdown point and discharge occurs. In addition, if an ungrounded metal object is in an electric field, it can be charged with an induced charge. Because a metallic object is conductive, the mobile charge will be discharged into a person who touches the object.

Sources:

WikipediaBrittanica

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