module 3.2 electrostatics

8/16/2019 Module 3.2 Electrostatics

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PIA Training Centre (PTC) Module 3 – ELECTRICAL FUNDAMENTALS

Category – Basic A/B1/B2 Sub Module 3.2 – Static Electricity and Conduction

ISO 9001 - 2008 Certified For Training Purpose Only

PTC/CM/B Basic/M3/01 Rev. 003.2 Mar 2014

MODULE 3

Sub Module 3.2

STATIC ELECTRICITY AND CONDUCTOIN


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PTC/B Basic/COMMON/M3/01 Rev. 003.2 - i Mar 2014

STATIC ELECTRICITY AND DISTRIBUTION OF ELECTROSTATICCHARGES ............................................................................................ 3

ELECTROSTATIC LAWS OF ATTRACTION AND REPULSION ....... 3FIRST LAW ......................................................................................... 12

SECOND LAW (COULOMBS LAW) .................................................. 13

UNIT OF CHARGE ............................................................................. 13

STATIC ELECTRICITY AND THE AIRPLANE .................................. 16

LIGHTNING......................................................................................... 17

CONDUCTION OF ELECTRICITY THROUGH SOLIDS .................... 20

CONDUCTION OF ELECTRICITY THROUGH LIQUIDS ................... 21

CONDUCTION OF ELECTRICITY IN GASES .................................. 213

CONDUCTION OF ELECTRICITY IN VACUUM .............................. 214


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PTC/B Basic/COMMON/M3/01 Rev. 003.2 - ii Mar 2014

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PTC/B Basic/COMMON/M3/01 Rev. 003.2 - 3 Mar 2014

INTRODUCTION

Electrostatics (elect ricity at rest) is a subject with which most

persons entering the field of electricity and electronics are

somewhat familiar. For example, the way a person’s hair stands

on end after a vigorous rubbing is an effect of electrostatics.

While pursuing the study of electrostatics, you will gain a better

understanding of this common occurrence. Of even greater

significance, the study of electrostatics will provide you with the

opportunity to gain important background knowledge and to

develop concepts which are essential to the understanding of

electricity and electronics.

Electrostatics is the branch of science that deals with the

phenomena arising from stationary or slow-moving electric

charges. Interest in the subject of static electricity can be traced

back to the Greeks. Thales of Miletus, a Greek philosopher and

mathematician, discovered that when an amber rod is rubbed

with fur, the rod has the amazing characteristic of attracting

some very light objects such as bits of paper and shavings of

wood.

About 1600, William Gilbert, an English scientist, made a studyof other substances which had been found to possess qualities

of attraction similar to amber. Among these were glass, when

rubbed with silk, and ebonite, when rubbed with fur.

Gilbert classified all the substances which possessed properties

similar to those of amber as electrics, a word of Greek origin

meaning amber. Because of Gilbert’s work with electrics, a

substance such as amber or glass when given a vigorous

rubbing was recognized as being electrified or charged with

electricity.

In the year 1733, Charles Dufay, a French scientist, made an

important discovery about electrification. He found that when a

glass was rubbed with fur, both the glass rod and the fur

became electrified. This realization came when he

systematically placed the glass rod and the fur near other

electrified substances and found that certain substances whichwere attracted to the glass rod were repelled by the fur, and

vice versa. From experiments such as this, he concluded that

there must be two exactly opposite kinds of electricity.

Benjamin Franklin, American statesman, inventor, and

philosopher, is credited with first using the terms positive and

negative to describe the two opposite kinds of electricity. The

charge produced on a glass rod when it is rubbed with silk,

Franklin labeled positive. He attached the term negative to the

charge produced on the silk. Those bodies which were notelectrified or charged, he called neutral.

It took about 50 years to find out that charges are polarized.

http://en.wikipedia.org/wiki/Sciencehttp://en.wikipedia.org/wiki/Phenomenahttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Electric_chargehttp://en.wikipedia.org/wiki/Phenomenahttp://en.wikipedia.org/wiki/Science


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STATIC ELECTRICITY

Electricity is often described as being either static or dynamic.

The difference between the two is based simply on whether theelectrons are at rest (static) or in motion (dynamic). Staticelectricity is a buildup of an electrical charge on the surface ofan object. It is considered “static” due to the fact that there is nocurrent flowing as in AC or DC electricity. Static electricity isusually caused when non-conductive materials such as rubber,plastic or glass are rubbed together, causing a transfer ofelectrons, which then results in an imbalance of chargesbetween the two materials. The fact that there is an imbalanceof charges between the two materials means that the objectswill exhibit an attractive or repulsive force.

Attractive And Repulsive ForcesOne of the most fundamental laws of static electricity, as well asmagnetism, deals with attraction and repulsion. Like chargesrepel each other and unlike charges attract each other. Allelectrons possess a negative charge and as such will repeleach other. Similarly, all protons possess a positive charge andas such will repel each other. Electrons (negative) and protons(positive) are opposite in their charge and will attract each other.

Figure 3.2.1: Reaction of like and unlike charges

For example, if two pith balls are suspended, as shown in thefigure3.2.1, and each ball is touched with the charged glass rod,some of the charge from the rod is transferred to the balls. Theballs now have similar charges and, consequently, repel eachother as shown in part B of Figure 3.2.1. If a plastic rod isrubbed with fur, it becomes negatively charged and the fur is

positively charged. By touching each ball with these differentlycharged sources, the balls obtain opposite charges and attracteach other as shown in part C of Figure 3.2.1.


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Although most objects become charged with static electricity bymeans of friction, a charged substance can also influenceobjects near it by contact. This is illustrated in Figure 3.3.2.

Figure 3.2.2: Charging by contact

If a positively charged rod touches an uncharged metal bar, itwill draw electrons from the uncharged bar to the point ofcontact. Some electrons will enter the rod, leaving the metal barwith a deficiency of electrons (positively charged) and makingthe rod less positive than it was or, perhaps, even neutralizing

its charge completely.

A method of charging a metal bar by induction is demonstratedin Figure 3.2.3. A positively charged rod is brought near, butdoes not touch, an uncharged metal bar. Electrons in the metalbar are attracted to the end of the bar nearest the positivelycharged rod, leaving a deficiency of electrons at the oppositeend of the bar. If this positively charged end is touched by aneutral object, electrons will flow into the metal bar andneutralize the charge. The metal bar is left with an overallexcess of electrons.

Figure 3.2.3: Charging a bar by induction


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Triboelectrification

Today, the process of rubbing two materials together to transfersome amount of electric charge is known as tribo electriccharging it can also be said as static charging by frictionbetween two or more dissimilar materials. This type of chargingis an important factor in the design and installation of electricand electronic equipment in aircraft. Table 1 below indicates therelative ability of a material to gain or lose charges due torubbing. More plusses (+) next to a material in the chartindicates a greater ability to obtain a net positive charge. Moreminuses (−) next to a material in the chart indicates a greaterability to obtain a net negative charge.

In general when two objects listed in the chart are rubbed

together, the material listed higher in the chart becomespositively charged and the material listed lower in the chartbecomes negatively charged. The greater the separation of thematerials in the chart, the greater the magnitude of the chargetransferred.

We can also characterize how easily charge can flow along orthrough a material. Materials that easily allow charge to flow

through them are known as conductors. Materials throughwhich charge cannot easily flow are known as insulators. Weunderstand this distinction today in terms of the mobility of

charge carriers within the material. For instance, in most metals(which are often good conductors), valence electrons are free tomove anywhere throughout the metal, and thus can easilytransfer charge from one location to another within the metal. Ininsulating materials, on the other hand, there are relatively few

free charge carriers, and so excess charge tends to stay whereit’s put on the surface of an insulator.

When an insulator is charged by rubbing it with a dissimilarmaterial, the charge remains at the points where the frictionoccurs because the electrons cannot move through thematerial; however, when a conductor is charged, it must beinsulated from other conductors or the charge will be lost.Walking is one of the biggest sources of tribo electric charging.Shoe soles contact and then separate from the floor or carpet,effectively leaving both person and floor charged. Conveyorbelts and other moving machinery are also sources of triboelectric charging.

Table 3.2.1Tribolectric charging


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Electrostatic Induction And Polarization

In general terms, polarization means to separate into opposites.In the political world, we often observe that a collection ofpeople becomes polarized over some issue. For instance, wemight say that the United States has become polarized over theissue of the death penalty. That is, the citizens of the UnitedStates have been separated into opposites - those who are forthe death penalty and those who are against the death penalty.In the context of electricity, polarization is the process ofseparating opposite charges within an object. The positivecharge becomes separated from the negative charge. Byinducing the movement of electrons within an object, one side ofthe object is left with an excess of positive charge and the otherside of the object is left with an excess of negative charge.

Charge becomes separated into opposites.

Electrostatic induction is a method to create or generate staticelectricity in a material by bringing an electrically charged objectnear it. This causes the electrical charges to be redistributed inthe material, resulting in one side having an excess of eitherpositive (+) or negative (−) charges.

This phenomenon is most effective when the object is aconducting material, such as metals. The only drawback is thatonce the electrically charged object is removed, the conductorloses its charge. This can be solved by temporarily grounding

the conductor.

Certain non-conducting materials can also be given a staticelectric charge by electrostatic induction. In these cases, it iscaused by polarization of their molecules.

Induction In A Conducting Material

In its normal, neutral state, an electrically conducting objecttypically has an equal number of positive (+) and negative (−)electrical charges—such as positive ions, negative ions andelectrons—intermingled within the material. When a staticelectrically charged object is brought near this conductor, theelectrical charges on or near the surface of the object attract theopposite charges in the conductor and repel the like charges.Plastic rod near metal plate

As shown in figure3.2.4(a), if a charged plastic rod is broughtnear a metal plate, the negative charges on the rod attract thepositive charges in the plate and repel its negative charges.This creates a redistribution of electrical charges in the plate. As

long as the electrically charged rod is near the metal plate, theelectrical charges in the plate will be redistributed. But once thecharged object is removed, thermal motion of the atoms in themetal will cause the charges to intermingle again.

Bringing Charge Near Electroscope

Another example is the electroscope. If you bring a charged

object such as the plastic rod near an electroscope, opposite

electrical charges will move towards the metal end of the

electroscope.

In this illustration, the rod has negative (−) electrical charges on

its surface, which attract positive (+) charges in the metal shaft

of the electroscope by means of electrostatic induction.


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The electrical charges in the metal shaft are redistributed, with

negative charges collecting on the leaves at the other end of the

shaft. Since like charges repel, the electroscope leaves push

part, due to the electrical force where opposite charges repel.

The electroscope has not gained any electric charges. They

have just been redistributed, with positive charges near the top

and negative charges by the leaves, as seen in figure 3.2.4.

Figure 3.2.4(a): Electrical charges in the conductor are

redistributed

Figure 3.2.4(b): Electroscope leaves separate because of

electrical charges

Removing Charge From Electroscope

When the charged rod is removed, the electrical charges in the

electroscope intermingle again and the leaves fall back to a

neutral position.


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Figure3.2.5: Electroscope leaves fall back after charged rod

removed

Ground To Keep ChargesYou can cause the electroscope to have an excess of one typeof electrical charge by drawing off the other type of charge.This is shown in figure 3.2.6 by touching the negative (−)electrical charged rod to the shaft containing the positive (+)charges or by simply touching the shaft with your finger. Thiswill result in drawing off many of the positive charges but

allowing the negative charges in the leaves to remain. It is oftencalled grounding, although the charges aren't really going intothe ground.You can tell the leaves are still charged, because they remainseparated.

Figure3.2.6: Electroscope remain charged after grounding

Induction In Non-Conducting Materials

Electrostatic induction can also work in non-conducting ordielectric materials. However, movement of electrical charges ismuch more constrained in nonconductors than in conductors.Electrons are allowed to move about in a conductor, and that iswhat allows the flow of electricity in a metal wire. In anonconductor, the electrons are constrained within the atoms,so separation of charges particles does not work.

However, if the nonconductor consists of polar molecules—thatis, molecule that have one side more positive than the otherside—then electrostatic induction will cause those molecules tobe aligned with positive charges on one side and negativecharges on the other side.


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Figure3.2.7:Water molecule can polarize by electrostaticinduction

For example, the water molecule has more positive charges onone side of the molecule and negative charges on the otherside. Thus, water can be slightly attracted to a static electriccharge.

A demonstration of that can be seen in bending a stream ofwater with a charged plastic comb in the figure below.

Figure 3.2.8

Conduction

Conduction is the transfer of charge through direct contact.Conduction occurs when a charged object directly contacts anobject with a different charge. There must be a conductive pathbetween the objects.

Let's assume we have a negatively charged metal object and anuncharged metal sphere(Illustration 3.2.9 a). The uncharged

sphere is on an insulating stand so it will not interact withanything else.


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We bring the two objects close together. We will see aseparation of charge happen in the neutral object as negativeelectrons are repelled to the right hand side (Illustration 3.2.9b).

At this time, they are not touching and no charges have beentransferred. We allow the two objects to touch (Illustration 3.2.9c).

Some of the negative charge will transfer over to the unchargedmetal object. This happen since the negative charges on thefirst object are repelling each other, by moving onto the secondobject they spread away from each other. When the negativeobject is removed, it will not be as negative as it was (Illustration3.2.9d).

Both of the objects have some of the negative charge… howmuch depends on the size of the objects and the materials theyare made of.

If they are the same size, made of the same materials, then thecharge will be the same on both.

Figure 3.2.9

Total deficiency or addition of access electrons in an atom is

called its charge and the element is said to be charged. The

charge on one electron or proton is 1.602×10-19 coulomb.

One coulomb charge is a charge possessed by a total of

1/1.602x10-19 electrons i.e. 6.24x1018 electrons


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Electric Field

The medium around a charge is surrounded by an invisible fieldof force. The region in which the stress exists or in which the

electric force acts is called Electric or Dielectric or electrostaticfield.

A field of force exists around a charged body. This f ield is anelectrostatic field (sometimes called a dielectric field) and isrepresented by lines extending in all directions from the chargedbody and terminating where there is an equal and oppositecharge. To explain the action of an electrostatic field, lines areused to represent the direction and intensity of the electric fieldof force. As illustrated in Figure 3.2.10, the intensity of the field

is indicated by the number of lines per unit area, and thedirection is shown by arrowheads.

. Figure 3.2.10: Direction of electric field around positive

and negative charges

ELECTROSTATIC LAWS OF ATTRACTION ANDREPULSION

FIRST LAW

Like charges of electricity repel each other, whereas unlikecharges attract each other. Charged objects repel or attracteach other because of the way electrostatic fields act together.This force is present with every charged object.

When two objects of opposite charge are brought near oneanother, the electrostatic field is concentrated in the areabetween them, as shown in Figure3.2.11. The direction of thesmall arrows shows the direction of the force as it would actupon an electron if it were released into the electric field. Whentwo objects of like charge are brought near one another, the

lines of force repel each other, as shown in Figure 3.2.12.

Force between 2 charges each 1Q when they are at 1metre

apart in air : Ɛ0 = 8.854 x 10-12 Farad/metre.

Now F = Q1 . Q2 / 4π Ɛ0 d2 by putting values we get

F = 8.9878 x 109 N


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Figure 3.2.11: Electrostatic Field between Two Charges of

Opposite Polarity

Figure 3.2.12: Electrostatic Field between Two Charges of Same

Polarity

SECOND LAW (COULOMB’S LAW)

The strength of the attraction or of the repulsion force dependsupon two factors:

(1) The amount of charge on each object, and

(2) The distance between the objects.

(3) The nature of medium surrounding the charges

The greater the charge on the objects, the greater is the

electrostatic field. The greater the distance between the objects,

the weaker the electrostatic field between them, and vice versa.

This leads us to the law of electrostatic attraction, commonly

referred to as Coulomb’s Law of electrostatic charges, which

states that

The force of electrostatic attraction, or repulsion, is directly

proportional to the product of the two charges and inversely

proportional to the square of the distance between them

Consider two point charges Q1 and Q2 placed d distance apart.

Then the Force exerted between the two charges,

If, k is the constant of proportionality representing the

surrounding medium,


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F = k Q1Q2 / d 2

The value of this constant for free space is given as,k=1/4πЄ0

Where, Є0 Epsilon is the permittivity of free space, which is

equal to 8.854 PF/m for air.

Therefore in free space,

If the charges are placed in any other medium,

Then,

Where, Є is the absolute permittivity of the medium.

The knowledge of relative permittivity is of importance since in

practice the permittivity of materials is defined by this. e.g.,

relative permittivity of air = 1, water = 81, paper = 2 - 3, glass=

5-10, mica = 2.5 - 6.

Permittivity of free space:

The term permeability of free space µ0 = B/H is used in

electromagnetism whereas the term permittivity is used in

electrostatics. It is also a replica for the Dielectric constant,

which is the ability of an insulator to concentrate flux on it.

As we know thatC of a capacitor = Dielectric Constant K x Area of the plates

Distance b/w the plates

i.e. C = k A/d

K = C d/A = C.m/m2 = F/m…………(a)

Similarly in comparison to µ0 = B/H : Ɛ0 = D/E

Where D = Electric flux density = Coulomb / metre sq = Q/A

E = Electric field strength = Volt/metre = V/d

Which is potential drop per unit length or the potential gradient.

The ratio of electric flux density to the electric field strength is

called the permittivity of free space Ɛ0, hence

Ɛ0 = D/E = Q/m2 . m/V

= Q/V x m/m2 since C= Q/V

Ɛ0 = C/m = F/m…………………………….. (b)

Relative permittivity or the Dielectric constant of the material

inserted between the plates, is the ratio of the C of a capacitor

having a certain material as a dielectric to the capacitance of the

same capacitor having free space or vacuum.

Єr = Cd / Co = Є /Єo


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Є = Єr . Єo

Which is the Absolute permittivity of a medium.

Charge

Static electricity arises as the separation of positive andnegative charges at the interface between two dissimilarsurfaces. If one or other of the surfaces prevent easy migrationof charge, or the conductor on which they reside is isolated,then this charge is 'static' on the surface and remains availableto influence the surroundings. 'Static' electricity can also ariseon surfaces as trapped ions from the air.

Static charges may be electrons, or positive, or negative ions -but they are in the basic units of electronic charge 1.602x10 -19

coulomb

Nature Of Charges

When in a natural or neutral state, an atom has an equalnumber of electrons and protons. Because of this balance, thenet negative charge of the electrons in orbit is exactly balancedby the net positive charge of the protons in the nucleus, makingthe atom electrically neutral.

An atom becomes a positive ion whenever it loses an electron,

and has an overall positive charge.

Conversely, whenever an atom acquires an extra electron, itbecomes a negative ion and has a negative charge.

Due to normal molecular activity, there are always ions presentin any material. If the number of positive ions and negative ionsis equal, the material is electrically neutral. When the number ofpositive ions exceeds the number of negative ions, the material

is positively charged. The material is negatively chargedwhenever the negative ions outnumber the positive ions.

Since ions are actually atoms without their normal number ofelectrons, it is the excess or the lack of electrons in a substancethat determines its charge. In most solids, the transfer ofcharges is by movement of electrons rather than ions. Thetransfer of charges by ions will become more significant whenwe consider electrical activity in liquids and gases. At this time,we will discuss electrical behavior in terms of electronmovement.

Movement Of Charge

Although electric current is referred to as the f low of electronsthrough a conductor, it should be noted that more exactly, anymovement of electric charge constitutes an electric current.Thus, passage of electricity may occur through a:

Conductor such as metal, due to the movement of theloosely held outer electrons of the atoms.

Vacuum or gas, due to the movement of electrons.

Gas, due to the movement of the ionised gas molecules.


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Liquid, due to the ionisation of certain molecules,particularly those of acids and salts in solution (e.g.Electrolytes).

The ampere may be defined in terms of the mechanical units offorce and length, a more helpful picture is that of movingelectrons. When a current of one ampere is flowing in aconductor, one coulomb of charge passes any point in theconductor every second.The ampere is thus a measure of the rate of flow of electrons.

UNIT OF CHARGE

Coulomb is the unit of charge.

Coulomb

We have seen that a current of electricity ( ) is a flow ofelectrons but the electron itself is too small to be of use as theunit of electrical quantity and therefore a more practical unitconsisting of many millions of electrons has been chosen. It iscalled the COULOMB (C) and is 6.28 x 1018 electrons.

This is a Quantity of electricity (Q) not a measure of current, butit is used to define the unit of electrical current the AMPERE (A).When a current of one ampere is flowing in a conductor, 1

coulomb of electrons passes any point in the conductor everysecond. In other words the size of an electrical current isdependent upon the rate of flow of electrons not a number ofelectrons.

We can write this in equation form.

= Q/t amperes (A) where t is the time in seconds

Thus 1 ampere of current flowing in a conductor for 1 hour isequivalent to 3600 coulombs and this is called an ampere-hour.

STATIC ELECTRICITY AND THE AIRPLANE

As mentioned earlier, the effects of static electricity are ofconsiderable importance in the design of aircraft and aircraftequipment. An aircraft in flight picks up static charges as it fliesthrough rain, cloud, snow, dust and other particles in theatmosphere. This build-up of statics is referred to asprecipitation static.

The amount of charge that builds up in any particular part of theaircraft depends on the atmospheric conditions to which it issubjected, and the material of which it is made. If two adjacentpieces of material are able to build up charges at different rates,a potential difference will exist between them. Eventually thepotential difference will be sufficient to break down theinsulation and current will jump as a spark between the 2materials. This spark creates numerous problems; it damagesthe materials, it causes corrosion, it radiates radio frequenciesthat interfere with radio and navigation equipment and it couldignite fuel or oil vapor. In order to prevent this happening, it is

essential that all of the aircraft structure and equipment isinterconnected or bonded. Bonding allows small currents tocontinuously flow between materials and equipment, therebypreventing the buildup of large static charges.


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An aircraft often accumulates very high electric charges, notonly from precipitation but also from the high velocity gasesexiting the engine exhausts. When the charge is sufficientlylarge, it will start to dissipate into the surrounding atmospherefrom any sharp or pointed parts of the aircraft, such as the

trailing edges of aerofoil sections. The point at which thisoccurs is called the corona threshold. The corona dischargeproduces severe radio interference and needs to be controlled.This is achieved using special devices called wicks that allowthe charge to dissipate in a controlled manner from specificpoints on the aircraft so that it causes minimum interference.The subject of static electricity can be considered amusing orannoying when one suffers from its effects. However, it must betaken very seriously by aircraft maintenance engineers. Thefollowing are a few points to consider.

It essential to maintain the integrity of bonding whencarrying out any maintenance work on aircraft.

You can build up a charge on yourself as you move andwork around the aircraft. Much of the equipment in modernaircraft is electronic, and can easily be destroyed by youdischarging static through it.

When an aircraft is refuelled, is the refuel vehicle at thesame potential as the aircraft. If it isn’t, then it could bepossible for a spark to ignite fuel vapour as the fuel nozzlecomes into close proximity with the aircraft. It is essentialthat the two vehicles are interconnected electrically beforeany hoses or fillers are opened.

An aircraft in flight can have a potential several thousandvolts higher than the ground. This charge is dissipatedthrough the tyres or special straps on the undercarriagewhen the aircraft lands.

When an aircraft is inside a hangar for maintenance itshould be correctly grounded.

LIGHTNING

Lightning occurs as a result of a buildup of static charges withina Cumulonimbus cloud, often associated with the verticalmovement and collision of ice particles (Hail), which result in anegative charge at the base of the cloud and a positive chargeat the top of the cloud. Beneath the cloud, a "shadow" positive

charge is created on the ground and, as the charge builds,eventually a circuit is created and discharges takes placebetween the cloud and the ground, or between the cloud andanother cloud. An aircraft passing close to an area of chargecan initiate a discharge and this may occur some distance froma Thunderstorm.

Lightning strikes on aircraft commonly occur within 5,000 feetof the freezing level.

Lightning is accompanied by a brilliant flash of light and often bythe smell of burning, as well as noise. A lightning strike can be

very distressing to passengers (and crew!) but significantphysical damage to an aircraft is rare and the safety of anaircraft in flight is not usually affected. Damage is usually

http://www.skybrary.aero/index.php/Cumulonimbushttp://www.skybrary.aero/index.php/Hailhttp://www.skybrary.aero/index.php/Thunderstormhttp://www.skybrary.aero/index.php/Thunderstormhttp://www.skybrary.aero/index.php/Hailhttp://www.skybrary.aero/index.php/Cumulonimbus


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confined to aerials, compasses, avionics, and the burning ofsmall holes in the fuselage. Of greater concern is the potentialfor the transient airflow disturbance associated with lightning tocause engine shutdown on both FADEC and non-FADECengines with close-spaced engine pairs.

Lightning may also occur in Volcanic Ash clouds formed in theimmediate vicinity of eruptions because the vertical movementand collision between solid particles within the cloud generatesstatic charges.

Effects

Aircraft Damage

Structural damage to aircraft from Lightning strikes israre and even more rarely of a nature that threatens the

safety of the aircraft. Nevertheless, there have been manyincidents of lightning strikes leaving puncture holes in theradomes and tail fins of aircraft (entry and exit holes) anddamage to control mechanisms and surfaces.

Crew Incapacitation

Momentary blindness from the lightning f lash, especiallyat night, is not uncommon.

Interference with Avionics

A lightning strike can affect avionics systems,particularly compasses.

Engine Shutdown

Transient airflow disturbance associated with lightningto cause engine shutdown on both FADEC and non-FADECengines with close-spaced engine pairs.

Defences

Avoidance

Standard advice to pilots is to remain 20 nautical milesdisplaced from any Cumulonimbus cloud. The dangers fromTurbulence, Wind Shear, and Icing associated withCumulonimbus clouds are far greater than the threat ofLightning.

http://www.skybrary.aero/index.php/FADEChttp://www.skybrary.aero/index.php/Volcanic_Ashhttp://www.skybrary.aero/index.php/Cumulonimbushttp://www.skybrary.aero/index.php/Turbulencehttp://www.skybrary.aero/index.php/Wind_Shearhttp://www.skybrary.aero/index.php/Icinghttp://www.skybrary.aero/index.php/Icinghttp://www.skybrary.aero/index.php/Wind_Shearhttp://www.skybrary.aero/index.php/Turbulencehttp://www.skybrary.aero/index.php/Cumulonimbushttp://www.skybrary.aero/index.php/Volcanic_Ashhttp://www.skybrary.aero/index.php/FADEC


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Bonding

It is a mandatory requirement that aircraft structure andequipment are electrically bonded. Specific bonding and

grounding connections are made in an aircraft to accomplish thefollowing:

dissipate energy from a high intensity radiated fields(HIRF) and lightning strikes

dissipate static electricity

limit the potential difference between equipment

provide a low resistance path for earth return systems.

Bonding connections are made between componentsand structure using purpose-made straps, see Fig. 3.2.14.

Bonding is categorized as either primary or secondary; this isdetermined by the magnitude of current being conducted.Primary bonding is designed for carrying lightning dischargesand to provide electrical return paths. Secondary bonding isused to dissipate static electricity and keep al l structure at the

same potential. Bonding straps (or leads) are pre-fabricatedfrom braided copper or aluminum terminated with crimps

.Figure 3.2.14: Bonding

Composite MaterialsThere is an increasing use of composite materials being usedin the construction of aircraft because of their good strength-to-

weight ratio (compared with aluminum). Composite material hasa high electrical resistance and is intrinsically unsuitable forbonding, earth return sand lightning strike dissipation. A groundplane has to be integrated into the airframe ;this is normallyachieved by bonding an aluminum wire mesh into the compositestructure during manufacture. This mesh is accessed at keypoints around the aircraft to gain access to the ground plane.Direct bonding (Fig.3.2.15) is achieved by exposing the mesh(ground plane) and mounting the equipment directly onto theconductive path. Indirect bonding(Fig.3.2.16) is achieved byexposing the mesh and installing a bonding wire and connector.

The mesh must always be coated after making a connectionsince the aluminums will oxidize when exposed to air, leading tohigh resistance and unreliable joints. Lightning protection incomposite aircraft is achieved via aluminum wire integrated intothe outer layers of the composite construction.


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Figure 3.2.15: Direct bonding on compositeStructure

Figure: 3.2.16: Indirect bonding on compositeStructure

The lightning strike will enter and leave the aircraft at itsextremities; the integrated wires are installed in anticipation ofthis and the energy dissipated through the aircraft long pre-determined routes to the exit point(s).

CONDUCTION OF ELECTRICITY

Conduction of electricity through solids

The only solids, which conduct electricity, are:Metals- you can find these on the left side of the periodic table.Graphite is one of the forms of the element carbon.

Nearly all of the other solids in the world - non-metal elements,solid ionic and covalent compounds are non-conductors ofelectricity.

Why can metals conduct electricity?

Figure 3.2.17: Movements of electrons

The conductivity of metals is much higher than that ofsemiconductors and insulators because they have many more

free electrons. The free electrons come from the metal atoms.

In metals the charge carriers are the electrons, and becausethey move freely through the lattice, metals are highly


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conductive. The very low mass and inertia of the electronsallows them to conduct high-frequency alternating currents

Conduction of electricity through liquids

The only liquid elements, which conduct, are the liquid metals. At room temperature liquid mercury is a conductor. Othermetals continue to conduct electricity when they are melted.Covalent liquids like water, alcohol, ethanoic acid, propanone,hexane and so on, are all non-conductors of electricity. Evensolid covalent substances remain non-conductors when youmelt them. Ionic substances do conduct when you melt them.

Why do ionic melts conduct?

Ionic substances are made of charged particles - positive and

negative ions. In the solid state they are held very firmly in placein a lattice structure. In the solid state the ions cannot moveabout at all. When the ionic solid is melted, the bonds holdingthe ions in place in the lattice are broken. The ions can thenmove around freely.

When an electric current is applied to an ionic melt theelectricity is carried by the ions that are now able to move. In anionic melt the electric current is a flow of ions.

Aqueous solution which conduct electricity

Remember firstly, that water is considered to be a non-conductor of electricity. It can allow some electricity through it ifa high voltage is applied to it. This is due to the presence of a

minute concentration of H+(aq) and OH-(aq) ions in the water.However, electrons cannot flow through water.

Covalent substances do not conduct at all in solution. Ionic

substances are able to conduct electricity when they aredissolved in water.

Why can ionic substances conduct in solution?

The reason lies again in the fact that ionic substances are madeof charged particles - ions. When the ionic solid is dissolved inwater the ionic lattice breaks up and the ions become free tomove around in the water. When you pass electricity throughthe ionic solution, the ions are able to carry the electric currentbecause of their ability to move freely. A solution conducts bymeans of freely moving ions.

Figure3.2.18: Ionic compounds which dissolve in water to form

aqueous solutions will conduct electricity


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ELECTROLYSIS AND ELECTROLYTES

Electrolytes are liquids that conduct electricity. Most need to bedissolved into water or another solvent. Batteries have anelectrolyte in them, either as a liquid or as a paste. Liquid

electrolytes are used in electrolysis, electroplating, and otherchemical processes. When electrolytes dissolve they releasepositive and negative ions. The released ions carry electriccharges between electrodes, in the solution. Cat ions (apositively charged ion that migrates to the cathode, a negativeelectrode) carry positive electric charges toward the cathode.

Anions carry negative electric charges toward the anode,positive electrode. Strong electrolytes release many ions andconduct electricity well.

Weak electrolytes, like acetic acid, don’t release many ions and

conduct poorly. Non electrolytes, like sugar, release no ions andform non conducting solutions.

A couple electrolytes conduct electricity as solids. These solidelectrolytes have ions that can move and carry charges withoutsolvents.

Example The electrolysis of copper (II) chloride solution

The products of this electrolysis are:Brown copper metal at the negative electrode.

Chlorine gas (Cl2) at the positive electrode.

Happening Hint

At the negative electrode:

Copper (II) ions (Cu2+) are attracted.The blue Cu2+ ions are forced to accept two electrons.The ion-electron half equations for this reaction is:

Cu2+ (aq) + 2e- Cu(s)

The copper forms as a brown solid on the negative electrode.

At the positive electrode:

Colorless chloride ions (Cl-) are attracted to the positiveelectrode.

The chloride ions are forced to give away their extra electron toform chlorine atoms.

The chlorine atoms join up in pairs to form diatomic chlorine gas(Cl2).

The ion-electron half equation for this reaction is:

Cl-(aq) Cl2(g) + 2e-

The chlorine appears as a gas with a characteristic smell at thepositive electrode.


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Conduction of electricity in a gas

A gas under ordinary pressure is a perfect insulator and cannotconduct current. However, if the gas pressure is low, it ispossible to produce a large number of free electrons in the gasby the process of ionisation and thus cause the gas to becomea conductor. This is precisely what happens in gas filled

tubes. The current conduction in a gas at low pressure can bebeautifully illustrated by referring to the hot-cathode gas diodeshown in Fig. 3.2.19. The space between cathode and anode of

the tube contains gas molecules. When cathode is heated, itemits a large number of electrons. These electrons form a cloud

of electrons near the cathode, called space charge. If anode ismade positive w.r.t. cathode, the electrons (magenta dots) from

the space charge speed towards the anode and collide with gas

molecules (cyan circles) in the tube.

3.2.14 Conduction through a gas at low pressure

If the anode-cathode voltage is low, the electrons do notpossess the necessary energy to cause ionization of thegas. Therefore, the plate current flow in the tube is onlydue to the electrons emitted by the cathode. As the anode-cathode voltage is increased, the electrons acquire morespeed and energy and a point–called ionisation voltage is reached, where ionisation of the gas starts. Theionisation of gas produces free electrons and positive gas

ions (cyan circles with +ve signs). The additional freeelectrons flow to the anode together with the originalelectrons, thus increasing plate current. However, the

increase in plate current due to these added electrons ispractically negligible. But the major effect is that thepositive gas ions slowly drift towards the cathode andneutralise the space charge. Consequently, the resistanceof the tube decreases, resulting in large plate current.


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Hence, it is due to the neutralisation of space charge bythe positive gas ions that plate current in a gas tube is toomuch increased.

Conduction of electricity in Vacuum

Since a "perfect vacuum" contains no charged particles,vacuums normally behave as very good insulators. However,metal electrode surfaces can cause a region of the vacuum tobecome conductive by injecting free electrons or ions througheither field emission or thermionic emission. Thermionicemission occurs when the thermal energy exceeds the metal'swork function, while field emission occurs when the electric fieldat the surface of the metal is high enough to cause tunneling,which results in the ejection of free electrons from the metal into

the vacuum. Externally heated electrodes are often used togenerate an electron cloud as in the filament or indirectly heatedcathode of vacuum tubes. Cold electrodes can alsospontaneously produce electron clouds via thermionic emissionwhen small incandescent regions (called cathode spots oranode spots) are formed. These are incandescent regions of

the electrode surface that are created by a localized highcurrent flow. These regions may be initiated by field emission,but are then sustained by localized thermionic emission once avacuum arc forms. These small electron-emitting regions canform quite rapidly, even explosively, on a metal surfacesubjected to a high electrical field. Vacuum tubes and sprytronsare some of the electronic switching and amplifying devicesbased on vacuum conductivity.

module 3.2 electrostatics

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