can a body hold a "static" charge of 1 coulomb ? Please note that it is mantioned "static" charge

Question

i think its "NO"
am i correct?

Answer 1

I think you are wrong.

The body would probably have to be quite big though.

Lightning bolts are typically the movement of 20-30 coulombs.

Answer 2

Well 1 columb is a mighty large amount but Yes this amount can be stored
impossible is nothing
The container has to be highly insulated to make it hold such a charge.

Answer 3

The following notes give an introduction to the background of static and static problems, to enable you to have a
better insight into your own problem.
Background
Static electricity is an undeveloped science because, historically, it was not seen as useful - this contrasts with
current electricity, which had many uses in providing energy and power.
Since the 1940s the greater use of plastics and new technologies have made electrostatics a more researched area,
but the general level of knowledge about static electricity is still quite low.
In industrial applications, it is often still a matter of judgement, not science. There are too many physics and
chemistry complexities involved to allow a perfect analysis. There are also problems with measurement.
What is Static?
Static is an electrical imbalance on the surface of a material which can interact with surrounding materials.
electrons
This imbalance occurs when an atom (or molecule) gains or loses an electron. - -
Normally the atom is in equilibrium with the same number of positive protons
in the middle of the atom and electrons. Electrons move easily from one atom + + protons
to another. They form positive ions (where an electron is missing)
or negative ions (a single electron, or an atom with an extra electron). An atom in equilibrium
When this imbalance occurs it is called static electricity.
A electron has an electric charge of (-) 1.6 x 10-19 Coulombs. A proton has the same charge with a positive
polarity. The static charge in Coulombs is directly proportional to the surplus or deficit of electrons i.e. the
number of imbalanced ions. The Coulomb is the basic unit of electrical charge which represents the surplus or
deficit of electrons. (An ampere of current is the movement of 1 Coulomb of charge per second).
A positive ion has a missing electron:
So it can easily accept an electron -
from a negative static charge: a positive ion with a deficit of electrons
+ +
A negative ion can be a single electron - -
or an atom/molecule with too many
electrons: - an atom with an extra
+ + - electron
single electron ion
In both cases there is an electron available to neutralise a positive charge
2
How is Static Generated?
The main causes of static electricity are:
1) Contact and Separation between two materials (including friction, travelling over rollers etc)
2) Rapid heat change (e.g. material going through an oven)
3) High energy radiation, UV, x-ray, intense electric fields (not very common in industry)
4) Cutting action (e.g a slitter or sheet cutter)
5) Induction (standing in the electric field generated by a static charge)
Contact and separation is probably the most common cause of static in industry where film and sheet are being
processed. It happens when material unwinds or passes over a roller. This process is not fully understood, but
the clearest explanation of how the static is generated here is an analogy with a plate capacitor where mechanical
energy to separate the plates of a capacitor is converted into electrical energy:
Resultant Voltage = Starting Voltage x Resultant distance between plates
Starting distance between plates.
When the material touches the roller a small charge flows from the material to the roller causing an imbalance.
As the material leaves the roller the voltage is magnified like the separating plates of a capacitor. The size of the
resultant voltage is limited in practice by the breakdown strength of the surrounding materials, surface
conduction etc. You often hear small cracks, or static discharges, as the material leaves the roller. This is where
the static has reached the breakdown strength of the surrounding air.
- - - - - - - - - - - - - - - - -
+ + film
+
The plastic film is neutral + As the film separates from the roller electrons flow go onto the
here before the roller film giving it a negative charge. The positive charge on the
roller will disappear quickly if it is an earthed metal roller
On most machines there will be many rollers and the level of charge and polarity of the charge may change
often. The important place to investigate the static charge is immediately before the problem area. If the charge
is neutralised too early it may regenerate before it reaches the problem area.
3
How is Static Generated cont
In theoretical terms, a static charge can be represented by a simple electrical circuit:
I Charging
Current
V R Spark gap
C
C is the capacitor function which stores the charge, like a battery. It is usually the surface of the
material / product.
R is the charge relaxation ability of the material / system - usually a small current flow. If the
material is conductive the charge will escape to earth and so will not become a problem. If the
material is non-conductive, the charge cannot escape and so can become a problem
The Spark Gap is the limit to the voltage build-up.
The charging current is the charge generated by the action of the product over a roller etc.
The charging current charges up the capacitor (i.e the product), increasing its voltage V. As the voltage
increases, current flows through the resistor R. An equilibrium will be reached where the charging current is
equal to the current flowing through the resistor. (Ohms Law applies here: voltage = current x resistance)
A static problem arises where the product has the ability to store a sizeable charge and a high voltage is present.
The static problem will show itself in the form of a spark, electrostatic repulsion/attraction or shocks to
operators.
Polarity of the Charge
The static charge may be positive or negative. For AC static eliminators and passive dischargers (brushes) the
polarity of the charge is not usually important.
4
Measurement of Static Electricity
Measuring the static electricity is important. It allows you to see if there is static present, its size and where it is
being generated.
It was seen earlier that static electricity is actually a surplus or deficit of electrons which is measured in
Coulombs. As it is not possible to measure the charge in a material in Coulombs, the electric field strength or
surface voltage related to the static charge is measured. This is the accepted method of measuring static in
industry.
The relationship between the field strength and voltage is that the former is the voltage gradient at any point.
The Fraser 710 Meters measure surface voltage. They use circuitry:
Buffer Using Q (charge) = C (capacitance) x V (voltage)
Sensor Amplifier The capacitance is set at the measuring distance of
100mm. This means that the charge Q varies
directly with the voltage V.
The voltage across this
capacitor varies directly with the charge.
Fraser meters are simple to use and very useful in analysing problems.
It is important to follow operating instructions when measuring static. The electric field behaves in unique ways
and must be understood. One of the most interesting characteristics of the electric field, which is very important
when trying to measure the charge is shown below.
Electric field: - is a region of space in which electrical (Coulomb) forces act.
- every charged object is surrounded by an electric field.
- the field line run perpendicular to the material and show the direction in which the force acts.
- it can be coupled with other bodies with important consequences for measuring and
neutralising the charge.
the electric field lines run perpendicular to the charged
material when it is in open air. When the electric field is like
this it is easy to make accurate and intelligent measurements
Charged Material
Electric Field Lines when the charged material passes over a roller, the
electric charge couples with the roller and
seems to disappear. It is impossible to make
an accurate measurement near the roller.
The electric field “returns” when the material leaves
the roller and so can be measured again.
5
PROBLEMS CAUSED BY STATIC ELECTRICITY
There are 4 main areas:
Electro-Static Discharge in Electronics
This is considered here briefly because it is important when handling electronic assemblies and components on
modern control systems.
The main danger is from the static charge in the human body - which can be considerable. The current in the
discharge generates heat which evaporates junctions, interconnects and the gap between tracks.
The high voltage also breaks down the thin oxide coatings on MOS and other coated devices.
Often the component is not completely destroyed which can be even more problematic as the failure will occur
later when the product is being used.
General rule: make sure that your body does not contain a static charge when handling sensitive
components. See European Standard CECC 00015 for further details.
Electro-Static Attraction or Repulsion
This is probably the most widespread problem in the plastics, paper, textile and similar industries.
It shows itself as product misbehaviour, sticking together, repelling each other, sticking to machinery, dust
attraction on mouldings, bad winding and many other symptoms.
Coulomb`s law governs attraction and repulsion. Basically it is an inverse square law. In simplified form -
Force of attraction or repulsion (in Newtons) = Charge (A) x Charge (B) in Coulombs
(Distance between objects in m) 2
Thus the severity of the problem is directly related to the size of the static charge and the distance between the
objects being attracted or repelled. Attraction or repulsion follows the field lines of the electric field. (Often
called flux lines when they represent force or displacement).
If the two charges are of the same polarity they will repel each other. If they are of different polarities they will
attract each other. If only one of the products is charged it will cause attraction by creating a mirror-image
charge in the non-charged products:
+ - +
+ - +
+ - +
Charged Object Mirror image charge is created on one side of the uncharged object,
although it is still electrically balanced as a whole.
6
Fire Risk
The risk of fire is not the most common static problem in industry. But it is very important in the coating,
printing and other industries where combustible solvents are used.
The most common sources of ignition in hazardous areas are ungrounded operators and floating conductors. An
operator walking through a hazardous area in trainers or similar non-conductive footwear risks a discharge from
his body which can cause ignition to sensitive solvents. An unearthed and conductive piece of machinery is
similarly dangerous. Good earthing is essential for everything in a hazardous area.
The following information gives a brief introduction into the ability of a static discharge to cause ignition in
combustible environments.
The ability of a discharge to cause ignition depends on many variables:
- Type of Discharge.
- Rate of Discharge
- Source of Discharge
- Energy of Discharge
- Presence of combustible environment - often a solvent gas, but can be dust or liquid.
- MIE Minimum ignition energy of the combustible environment.
Types of Discharge:
Three relevant types: Spark, Brush Discharge and Propagating Brush Discharge.
Spark: usually comes from a reasonably conductive body which is isolated electrically. This could be a human
body, machine part or tool. It is assumed that the entire energy is dissipated in a spark. If the energy
is more than the MIE of the solvent vapour then an ignition could occur.
The energy in a spark is calculated: Energy in Joules = 1/2 C V2
Brush: usually happens when a corner of a machine part concentrates the charge in a larger sheet or web of nonconductive
material. Generally lower in energy than a spark and so less incendive.
Propagating Brush Discharge:
occurs on highly resistive plastic sheets and webs where there is a high charge density of the opposite
polarity on each side of the material. This may be caused by rubbing or powder coating bombardment.
The effect is like discharging a plate capacitor and can be more incendive than a spark.
Source and Energy of Discharge
Size and geometry are important factors. The larger the body, the more energy it can contain. Sharp points
increase field strength and encourage discharge.
Rate of Discharge
If the body holding the energy is not very conductive, e.g. a human body, the resistance will slow the discharge
and reduce its danger. For the human body rule of thumb is that it should be regarded as capable of igniting all
solvents with an MIE of less than 100mJ, even though 2 or 3 times this energy is stored in the body energy.
7
A corona discharge has not been considered here. It is a slower, low energy discharge from a point. It is only
regarded as problematic in the most sensitive areas.
MIE (Minimum Ignition Energy)
The Minimum Ignition Energy of the solvent and its concentration in the hazardous area are important. If the
MIE is less than the discharge energy a fire could result.
Shocks to Operators
Shocks to operators are becoming more important as health and safety issues increase in importance and scope.
Static shocks are unpleasant, but not usually dangerous, unless they cause a recoil reaction. There are 2 common
causes:
Induction charging:
If a person is standing in the electric field of a charged object, such as a winding reel of film, his/her
body may get charged by induction.
- -
electric field around reel of film
- - - -
- - - - - -
- - - - -
- - - - - - - -
- - - - operator standing in electric -
- - - - - - - field becomes charged -
- -
- - - - - -
The charge stays in the operator`s body if
he/she is wearing insulative shoes until
he/she touches an earthed part of the
machinery. Then the charge will zap to earth
giving the operator a shock.
This happens also when the operator is handling charged objects and materials - the charge builds up in the body
because of insulative shoes. When the operator touches a metal part of the machine the charge can escape and
cause a shock.
The shocks that result from people walking over nylon carpets are due to the static generated between the carpet
and the shoes. The shocks which car drivers receive when they get out of a car is due to the charge generated
between the seat and the driver`s clothes as they are separated. The solution to the latter is to touch a metal part
of the car, such as the door frame, as the driver leaves the seat. This allows the charge to go to to earth through
the car and its tyres without giving a shock.
8
Shocks from the Product
It is possible, but less common, for an operator to receive a shock from the material.
If there was a very big charge in the winding reel shown above, the operator`s fingers could concentrate the
charge until it reaches its breakdown point and forms a discharge.
Alternatively, if there is a metal object which is not connected to earth standing in an electric field, it can become
charged by induction. Because the metal object is conductive the charge is mobile and will discharge to a person
who touches it.
9
METHODS OF NEUTRALISING STATIC ELECTRICITY
Neutralising static electricity on a conductive material is simple: allow it to flow to earth in the form of current.
On non-conductive materials this will not happen because the charge will not flow - hence the name “static”.
In industrial situations the best way of neutralising non-conductive materials is to provide electrical particles of
the opposite polarity to the static charge. The best way of supplying these oppositely charged particles is with
ionised air. Ionised air consists of free moving positive and/or negative ions which readily combine with the
electrical imbalance in the material to neutralise it.
The ionised air provides ions of the opposite polarity to
the charge. In this case because the static charge is
positive, the negative ions in the ionised air combine
Ionised Air with the positive static charge and so neutralise it.
The unused ions go to earth or combine with other ions.
- + - +
+ + + + + + + + + - + -
Charged Film Neutral Film
Fraser manufactures two types of equipment which generate ionised air:
1) passive static dischargers (anti-static brushes)
2) electrical static eliminators
They operate in different ways:
Passive Static Dischargers or Anti-Static Brushes
“Passive” should not be interpreted as “weak”. This is not the case. In fact passive static dischargers have an
unrivalled ability to neutralise high charges.
Passive dischargers consist of a large number of very fine conductive points. The fine points concentrate the
electric charge in the material until the electric field strength of the air reaches 3MV/m. At this point the
dielectric of the air breaks down and ionised air is created.
Static Discharger
ionised air is created at the fibre tips
electric field of static charge + - + -
Charged Material
+ + + + + + + + + + + Neutral Material
The ionised air allows the exchange of ions needed to neutralise the charge. The static discharger must be
earthed to allow the exchange charges to go to earth.
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Passive Static Dischargers cont.
Passive dischargers do not need to touch the material. On materials with a low / medium static charge level the
tips of the fibre should be about 5mm from the material. Where the static charge is higher, a well designed
passive discharger may be positioned at a greater distance from the material. Use a static meter to see best
position.
Fraser Static Dischargers use three materials to provide the sharp points: carbon fibre (6-7µ in diameter), soft
stainless steel yarn (11-12µ) and conductive acrylic (15µ). The characteristics of each material are explained in
the product data sheet for static dischargers.
Electrical Static Eliminators
Electrical static eliminators generate ionised air by applying a high voltage to an emitter pin. This produces a
cloud, or corona, of ionised air. The corona is typically about 75mm around the emitter pins. It can be
transported by air or repulsion for longer range performance.
Ionised Air - + - + The ionised air is produced by applying a high voltage
+ - + - to a sharp point. So its performance is not dependent
on the presence of a static field
Electrical static eliminators can use AC high voltage or DC high voltage to produce ionisation. AC high voltage
produces both positive and negative ions from the same emitter pins. DC high voltage produces either positive
or negative from the emitter pins.
Fraser makes electrical static eliminators in the form of bars, blowers, guns, nozzles and airknives using this
principle. Air can be used to transport the ions from the electrical static eliminator to where they are needed.
-----------------------------------
Advantages and Disadvantages of each Type of Equipment.
Advantages
Passive Static Dischargers: Powerful - can cope with the highest charges.
Suitable for high speeds.
Low cost, simple to install, little maintenance.
Can be used in hazardous areas.
Electrical Static Eliminators Variety of forms available including long range, dust removal etc
Can be used on 3-dimensional products
Can reduce charges to zero, if required.
Better engineered solutions, with long life.
11
Advantages and Disadvantages of each Type of Equipment cont.
Disadvantages
Passive Static Dischargers Must be installed close to the material, except for high charges.
Usually a small residual charge threshold remains on the material.
Wear parts – need to be replaced periodically
Electrical Static Eliminators More expensive than passive systems.
Combination of passive and electrical:
For very high speeds and high charges a combination of electrical and passive equipment can be used. Contact
Fraser for details on these Combination Bars.
-------------------------------
Humidity
The generation of static electricity is influenced by the humidity in the factory or general weather conditions.
On a damp day less static will be generated than on a very dry day.
Some people believe that damp air becomes conductive and the static charge leaks away through it. This is
wrong. The conductivity of dry air and damp air is nearly the same.
However damp air can deposit a microscopic layer of water onto the product which increases the surface
conductivity and so allows some charge to travel to earth. This applies to a degree to most materials - even
hydrophobic materials which cannot absorb water.
Hydroscopic materials, like uncoated paper, can also absorb water which increases its volume conductivity as
well as its surface conductivity.
In some factories, for example in the printing and textile industry, it is practical to increase the general humidity
level.