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Electricity:

1) Static electricity

2) Electrical charges and its properties,

3) Coulomb’s Law

4) Electroscope, Charging by friction and Induction

5) Lightening

Static Electricity

Introduction

All of us have experienced that our hair on the forearm rise up while removing a full sleeve

sweater. Similar effect is experienced when a comb is brought near the forearm after combing.

We have seen the lightening and heard thundering sound after it. A peacock feather when rubbed

by hand five six times occupies double area. Its individual strands become straight, they spread

away and it looks as if it is live and it has awakened. All these are our common experiences that

occur naturally. We say that this is due to ‘charges appeared due to friction’.

The study of stationary charges and their effects is called as ‘static electricity’ or ‘Electrostatics’.

The fields came in to existence due to static charges, forces experienced in these fields,

potentials etc. comes under the branch of science ‘Electrostatics’. We will learn these things in

this chapter.

ELECTRIC CHARGE

It was known to Greeks that when amber is rubbed with wool or silk attracts dust, pieces of

feathers, and small bits of any light objects. This phenomenon was further systematically studied.

It became a branch of science, known as ‘electricity’. Amber is termed as ‘elektron’ in Greek

language. The word electricity originates from this.

Many such pairs of materials were discovered later, Ebonite or plastic with wool and glass with

silk cloth are commonly used substances in laboratories.

Laboratory activity:

a) Take a plastic (ebonite) rod. Hang it to a stand with silk thread or light but tenacious

cotton thread such that it remains horizontal or almost horizontal. Rub its one end by a

woolen cloth. Take another similar rod of same material. Rub its one end with woolen

cloth and take it near hanged rod. Observe what happens.

physics.tutorvista.com › Electricity and Magnetism › Electrical Theory‎

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Diagram:

http://www.physics.sjsu.edu/becker/physics51/elec_charge.htm

It is observed that as we start bringing second rubbed rod near the first rod, the hanged rod

rotates. It goes away from the rod in the hand. It repels away. This is seen in the first blue

diagram. The rod gets displaced is seen in the second diagram. The hanging arrangement of the

rod is different for second diagram. It looks clearly in the second diagram.

b) Let us do another activity. Take two glass rods. Hang one glass rod to another stand as

we have hanged plastic rod before. Rub its one end with silk cloth. Take another glass rod

and rub it with silk cloth. Bring it near the already rubbed glass rod which is already

hanging.

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We again observe that the hanging glass rod turns away from the glass rod coming near.

It also gets repelled like the first one. The displacement is seen in the second diagram.

c) Now in an experimental set up, there are two stands. A plastic rod is hanged to one stand

and glass rod is hanged to another stand. We have two more rods one plastic rod and one

glass rod.

Rub the glass rod with silk cloth and bring it near the plastic rod which is hanging to first

stand. Again the plastic rod turns towards the glass rod in the hand. It is also attracted.

This can be seen in the second half part of the first blue diagram.

Now rub plastic rod with woolen cloth and bring it near the glass rod. Surprisingly it is

observed that the glass rod turns towards the plastic rod. It is attracted by the plastic rod.

Let us analyze the results. Both plastic rods are rubbed by woolen cloth, so it is logical to

conclude that ‘same types of charges appear on both plastic rods’. Similarly, both glass

rods also are rubbed by the silk cloth, so again it is logical to conclude that ‘same type of

charges appear on the glass rods’. It is observed that both rubbed plastic rods and rubbed

glass rods turn away from each other, when they are brought near to each other. So we

conclude “like charges repel each other”.

Let us assume that the charges developed on plastic rod on rubbing by woolen cloth and

charges developed on glass rod on rubbing by silk cloth are of the same type. Then in one

case when plastic-plastic are brought together, they repel. Similarly, glass-glass rods also

repel each other. In second case, when plastic and glass rods are brought nearer, they

attract. If the charges were of same type they would have repelled. They will not show

attraction in one case and repulsion in other case.

Hence we conclude, the charges developed by rubbing on plastic-plastic are of same type.

Charges developed on glass-glass are again of same type. But both types of charges are of

different from one another.

Hence the conclusion of third activity is “there are two types of charges and unlike

charges attract”.

Careful experimentation further concluded that there are just two types of charges. They can be

transferred from one glass rod to other glass rod or one plastic rod to another plastic rod. In

general, charges can be transferred from one body to other body. The bodies on which there is

net excess charge are said to be electrified or charged. The property which differentiates the two

kinds of charges is called the polarity of charge.

Note: I) We have seen that when the glass rod is rubbed with silk cloth, the glass rod acquires

charges of one type of polarity. We note here that the silk cloth is left with other type of charge

or other type of polarity. Similarly, when plastic rod is rubbed with woolen cloth it acquires one

type of charge or one type of polarity. The wool is left with other type of charges or other type of

polarity.

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This is seen in the second diagram. The rubbed plastic rod is hanging with negative charge. The

fur/wool is left with positive charge. Such charged wool is brought near the plastic rod. Plastic

seems to be attracted. Also when charged silk cloth is brought near the charged glass rod they

also get attracted.

II) The second thing we should note, when two bodies with opposite charges are brought in

contact the charges transfer to nullify their effect. If the amount of opposite charges is exactly

equal, then the bodies are perfectly nullified. They become uncharged as all normal bodies.

Positive and Negative Charges:

We have seen that equal and opposite charges cancel each other’s effect. Benjamin Franklin

named the charges; one type as ‘positive’ and other type as ‘negative’. It is perhaps with the

analogy that positive and negative numbers of equal magnitude cancel each other and their sum

is zero.

Conventionally, the charges on glass rod are called as ‘positive’ and that on the silk cloth are

called as negative. The charges on the plastic rod are called as ‘negative’ and charges which

remain with woolen cloth are positive.

If an object possesses excess electric charge, it is said to be electrified or charged. When it has no

excess charge it is said to be neutral.

PROPERTIES OF ELECTRIC CHARGES:

1) Attraction / Repulsion

2) Conservation

3) Additivity

4) Quantisation

q=Ne

At macroscopic level, the quantisation of charge can be ignored.

5) Charge given to a conductor entirely resides on the outer surface of the conductor. There

is no net charge at any point inside the conductor.

6) Distribution of charge is over the surface of a conductor.

Let us learn these properties in little more details.

1) Attraction / Repulsion:

We have already seen that the bodies can be electrified by rubbing. We also have

logically concluded from activity that ‘there are two types of charges. Further we have

seen through the activities that i) like charges repel and ii) unlike charges attract.

2) Conservation:

We have seen that ‘a glass rod is electrified by rubbing it on silk cloth and plastic rod is

electrified by rubbing it on the woolen cloth.’ We will see this in more details.

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There are enormous numbers of materials in nature. We have seen (In chemistry) that

these materials are classified as elements and compounds. The smallest particle of the

element is known as ‘atom’ and smallest particle of the compound is known as

‘molecule’. Molecules are formed when two or more than two atoms come together under

specific conditions. We also know that the ‘atoms are composed of nucleus and

electrons’. The nucleus is at the center and electrons revolve around the nucleus in

specific orbits, bound by the rules. Atom as a whole is electrically neutral but its

constituents are charged. Electron is negatively charged and the nucleus is positive. The

total negative charge on all electrons is equal to the total positive charge on the nucleus.

Therefore the atom is neutral. We also know that nucleus contains protons and neutrons.

The protons are positively charged and neutrons are electrically neutral. The magnitude

of negative charge on one electron is exactly equal to the positive charge on single

proton. Hence the number protons in the nucleus are exactly equal to the number of

electrons revolving around it.

We further note that protons and neutrons are tightly bound in the nucleus by strong

nuclear forces and electrons are bound to nucleus by comparatively week electromagnetic

forces. The strong forces are approximately 104 to 10

5 times stronger than

electromagnetic forces. It means electrons are comparatively loosely bound in the atoms.

When plastic is rubbed by the woolen cloth, some electrons from the atoms of wool are

removed and they ride on the plastic rod. So there are excess electrons with plastic rod

and so it is negatively charged. The woolen cloth has lost these electrons so it is short of

equivalent amount of negative charge. As a result, it becomes positively charged and

amount of positive charge acquired is exactly equal to the amount of negative charge lost.

In case of glass rod and silk cloth similar thing happens. Glass loses some electrons and

remains with net positive charge while silk cloth receives these electrons and becomes

negatively charged by exactly same magnitude. This process is also known as ‘charging

by friction’.

In these charging processes electrons from the glass rod are transferred to silk cloth and

electrons from woolen cloth are transferred to plastic rod. The bodies are charged

negatively and positively due to transfer of electrons. The positive and negative charges

are not created. This remains always true and it is understood as law of conservation of

charges. In general, it is stated as below.

The law of conservation of charges: The charges can neither be created nor they can be

destroyed. The total charge of the universe remains constant.

3) Additivity:

The charges can add as if they are numbers. Same types of charges add to increase their

effect and opposite type of charges add to cancel their effect. This can be understood

from the following activities.

i) Let us do the activity that demonstrated the attraction again. Take a stand to which

glass rod is hanged. Rub this glass rod by silk cloth. Take plastic rod and rub it

with woolen cloth. As we bring plastic rod near the glass rod, glass rod turns

towards the plastic rod. This indicates that glass rod is attracted by plastic rod.

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Now, touch the charged plastic rod momentarily with charged glass rod. Take

them away and again bring the plastic rod near the glass rod. The glass rod may

just turn when plastic rod is much closer. The ‘attraction’ is much reduced.

The reduced attraction indicates the effective charges on glass and plastic rod are

reduced due to touching. Some positive charges on glass rod and some negative

charges on plastic rod have added to cancel out their effect.

ii) The atom as a whole is neutral. The constituents of the atom are electrons and

nucleus. Electrons are negatively charged. The additivity property of the charges

allows us to conclude that ‘the charge on the nucleus is equal in magnitude of the

charge on all electrons and it is of opposite polarity.’

iii) We will do one more activity on additivity property when we will learn

‘electroscope.’

4) Quantization of charge:

We have seen that the bodies can be charged by rubbing. Rubbing is a process due to which

comparatively loosely bound electrons get transferred from one body to another body and both

bodies get charged. Wimshurst developed a machine in which large amount of charge is

separated by friction and transferred to a sphere. It again uses the same principle. The brushes

rub the moving belt at one point. The belt gets charged. It moves and transfers the charges to the

metallic sphere above. Thus the charges are again separated by friction.

Actually the number of charges which are transferred is a huge in amount. Let us try to

understand how much huge is this number. Recall the activity in which we rubbed glass rod with

silk cloth. If the glass rod is of one cm in diameter then its perimeter is about 3 cm. If the length

of 4 cm is rubbed then the area of rubbed part of the glass rod approximately 12 sq. cm. The

distance between two glass molecules is assumed to be 4 A. U. [1 A. U. (angstrom unit) = 10-10

m.] Let us assume the square arrangement of molecules. Then the number of molecules which

get rubbed comes out to be = 7.5 X 1015

. Even if one molecule out of one thousand donates one

electron on rubbing then also the order of number of electrons becomes 1012

. It is a huge number.

Addition of one electron or few electrons is not significant to this number. If we try to calculate

the total charge left on glass rod then we need to multiply 1012

by charge on one electron.

Addition of charge due to one electron or due to few electrons does not make significant change

in the total value of the charge. When the electrons are huge in number the charge variation is

considered as smooth. When the number of electrons is small then the charge variation is

considered as ‘in steps.’

We understand that accumulation of charge is either by removal of electrons or by addition of

electrons. Hence any charge Q can be written as

Q = Ne

Where Q is total amount of charge, N is the number and ‘e’ is the charge on the electron. When

N is large, Q is large and when N is small then Q is small.

Quantization of charge: Any charge Q can be written in terms integer multiplied by the charge on

one electron. This property of the charge is called as ‘the quantization of charge’.**

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5) Charge given to a conductor entirely resides on the

outer surface of the conductor. There is no net charge at

any point inside the conductor.

6) Distribution of charge over the surface of a

conductor:

We understand now that any excess charge resides on the

surface of the conductor. It is always clear that it is ‘one’ type of charge. We cannot give two

types of charges to same conductor at a time, because of additivity property of the charges. Since

the excess charges are of same type, they try to repel each other. They form ‘a specific pattern for

a specific conductor’.

It is easy to understand, if the nature of the surface is uniform, then the charges are uniformly

distributed. The most uniform surface is the sphere. Hence the charges are uniformly distributed

over the entire surface of the sphere. The surface density of the charge remains constant

throughout the surface. It is given by the formula

Surface density of charge σ = Q/A = Q/4πr2

It is clear that if the surface is not uniform the charge distribution will not be uniform. Any

metallic body which is given excess charge acquires the potential. The potential at different

positions on any non-uniform body always remains the same. Hence the charges on the non-

uniform body distribute themselves to create ‘equipotential surface’ just above the body.

If there is a complex surface having two or more than two different radii of curvatures, then they

obey the condition for potential to remain the same.

For equal potential, σ1r1 = σ2r2

or σ1 / σ2 = r2/r1

When we understand this, it is now easy to understand the charge distribution on the following

metallic body.

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Coulombs Law:

We have seen that like charges repel and unlike charges attract. We will see now the quantitative

relationship of the force given by Coulomb. Here we assume that the charges are point charges i.

e. their physical size is negligible compared to the distance between them. Coulomb’s law is

based on his experimental observations. The statement is as follows.

Coulomb’s Law: The electrostatic force between any two point charges is directly proportional to

the product of magnitudes of these charges and inversely proportional to the square of the

distance between them. The force acts along the line joining the two charges.

Thus, if q1 and q2 are two point charges of same type separated by the distance ‘r’, then the force

of repulsion between them is given by

F α

F = K

where K is the constant of proportionality. In SI system constant K is written as (

. Here

is the permittivity of the free space i.e. vacuum. Using this, the equation becomes

F =

The symbol for the force is made bold to indicate that it is a vector quantity. The other way of

representing force as a vector quantity is, arrow is drawn on the head of F as . The force is

mutual. It means it acts simultaneously on both the charges with same magnitude. When charges

are of same type, the force is repulsive and acts along the same line, away from each other.

When charges are of different type, the force is attractive and acts along the same line, towards

each other. It is shown in the following diagram.

Note: The value of

in SI system is 9 X 10

9 N-m

2 /C

2. It means if two charges of magnitude

1C each, are separated by the distance of 1m, then the force between them is 9 X 109 N.

F = 9 X 109

N = 9 X 10

9 N

If Q and q are two charges separated by distance ‘r’ then the situation is

as shown in the side diagram.

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System of more than two charges:

Consider a system of three charges q1, q2 and q3. The forces

existing between the charges are mutual. It is unaffected by

the presence of the third charge. This is universal

convention. The forces on charge q1 are written as F12, F13

etc.

F12: the force on charge q1 due to charge q2,

F13: the force on charge q1 due to charge q3, and so on.

Hence to obtain the total force on charge q1, first F12 and F13 are found out by applying

Coulomb’s Law. Then F12 and F13 are added vectorially to find out the resultant. It is shown in

the above diagram.

If there is a system of many charges, then same procedure is adapted. First forces on particular

charge are found out and then they are added vectorially to find out the final resultant force.

Electric field:

Consider that a small charge ‘q’ is placed at point say p. It does not experience any force because

there is nothing except that charge. Let us keep another charge Q at distance ‘r’ from it.

Immediately charge ‘q’ will experience a force. Even Q experiences the force of equal

magnitude. If ‘q’ is removed, it does not experience any force and Q too does not experience any

force. Consider Q remains there and ‘q’ is removed from point ‘p’. Does p lose its property

which caused force? If ‘q’ is brought at point p again, then again p experiences the force.

To understand this behavior of charges, scientists assumed that there is ‘something’ existing at

and around ‘p’. Scientists introduced a concept called as ‘field’. Electric field is the property

acquired by (assigned to) the space due to presence of charge. This concept helps us to

understand the force between two charges. It is understood as follows: There exists the field at

point p due to the existence of the charge Q. Charge q is smaller in magnitude compared to Q,

still it will have its own field. When we bring the charge q near point p, the two fields interact

with each other producing electrical force between each other.

Quantitatively, electric field is defined as follows:

Electric field: Electric field at a point is defined as amount of force experienced by the unit

positive charge placed at that point.

E = F /q

Force is a vector quantity, charge is a scalar quantity. Therefore electric field E is a vector

quantity. The SI unit of E is N/C (Newton per coulomb).

When charge q is placed away from Q at distance ‘r’, the force between them is given by

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F =

Therefore the electric field E due to charge Q at distance ‘r’ from it is given by

E =

Where is the unit vector along the line joining Q and point ‘p’ at distance ‘r’ pointing away

from Q. It is clear that if one of the charges is negative then force is attractive, so points

towards Q. If both the charges are of same type then force is repulsive and points away from Q.

Conventionally Q is called as the source charge and q is called as the test charge. The concept of

field is convenient to understand forces in electrostatics. Since the value of E at a point depends

on its distance from the source charge, the electric field changes from point to point.

The concept of ‘field’ more useful and meaningful when one of the charge is moving. It helps us

to understand how the ‘electromagnetic’ signal travels and transports energy. We will not go in

its details at this stage. The concept of Electric field was introduced by Michel Faraday. Today

this concept is extensively used in physics to understand the forces and its effects.

Electric Field lines:

Electric field is a vector quantity. It has magnitude and direction. The magnitude of E at a point

is given by formula

E =

To represent the E pictorially we use arrows. The point at which the source charge is placed is

taken as the origin. E is represented by an arrow whose length is proportional to the magnitude of

the electric field at that point. The direction is along the line joining origin (source charge) and

the point under consideration. The direction depends on the sign of the source and the test

charge. When both of them are of same nature arrows point outwards and when they are of

opposite nature the arrows point inwards. This is shown in the diagrams on page 8. Following

diagrams represent the field due to single positive charge, single negative charge, one positive

and one negative charge and two positive charges. The nature of the field due to two negative

charges is same as that of two positive charges.

The nature of the field shown is two dimensional as it is on paper. You must understand that

charges are kept in the space. Fields are three dimensional. So these diagrams are planer sections

of the three dimensional field.

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The lines which represent electric field are called as ‘lines of force’. The general properties

followed by the electric lines of force are as follows:

i) Field lines start from positive charges and end into negative charges. In case of single

charge, the lines of force end at infinity in case of positive charge. The lines of arrive

from infinity in case of negative charge.

ii) Two lines of force do not intersect. This is because field at a point has a unique value.

It is represented by only one vector. The electric field vector at a given point is

tangential to the line of force at that point. If two lines intersect at a point then it will

mean ‘there are two values of the field at that point.’ In this case, they will add

vectorially and reduce to unique value again. Hence two lines do not intersect.

Electrostatic Potential:

We know the concept of potential energy in case of gravitational field. A mass ‘m’ placed on

the ground is said to be at ‘zero’ level. Still, gravitational force ‘mg’ acts on it and it is in

downward direction. If we have to lift this mass, we have to work against the gravitational

force. The amount of work necessary to be done to lift mass ‘m’ up to height ‘h’ is ‘mgh’.

This work is stored in it as ‘potential energy’ of the body of mass ‘m’ at that height ‘h’.

The nature of electric field and gravitational field are similar. Both are proportional to the

product of masses or charges and both are inversely proportional to the square of the

distance. Hence electrical potential also can be defined on similar lines.

There is difference in electrical and gravitational fields. The masses are of one type only, the

gravitational field is always attractive. Electrical charges are of two types, hence the

electrical field is attractive for different type of charges and repulsive for same type of

charges.

We have seen that the electric field E due to charge Q at distance ‘r’ from it is given by

E =

Electric field is the vector property acquired by the space due to the presence Q at a point.

The Electric Potential: The electric potential at a point at distance ‘r’ from the point charge Q is

defined as the work done in bringing the unit positive charge from infinity to that point against

the electrostatic field due to the point charge Q. It is denoted by V. Hence

V =

We note here that V, the electric potential is the potential energy. Hence it is a scalar quantity.

Electric potential is a scalar property of a vector field. This is again analogues to the gravitational

field and gravitational potential.

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Electroscope:

Electroscope is the simple instrument used to detect electric charge. It possesses gold leaves, so it

is called as gold leaf electroscope.

It consists of a transparent bottle or a wooden

rectangular frame with glass walls. A metal rod having a

circular metallic disc at the top is vertically fixed using

rubber or cork stopper. A very thin pair of gold leaves is

welded at the bottom of metallic rod. This assembly is

closed, to keep itself dust free as the gold leaves are

delicate.

A charged rod is either touched to metal disc or it is

brought near the metal disc. We see that the gold leaves

separate. This is because when a charged rod is touched

to the metallic disc, some charges on the rod get transferred to the metal disc. They get

distributed over the rod and the leaves. We know ‘like charges repel’. Hence there is force of

repulsion between the two leaves. The leaves being very thin; their lower ends go away and take

the shape as seen in the diagram.

If a glass rod is not touched to the disc but it is brought very near to the disc, then negative

(opposite type) charges from the metal disc accumulate on upper side of the disc. They try to

come as near as possible to positive charges. Whereas the positive charges of the disc get

repelled. The gold leaves at the other end become effectively positively charged. Again they

show separation. This is known as charging by ‘induction’. If the charged glass rod is taken

away, then charge separation on the disc and metallic rod again becomes uniform. The gold

leaves fall back by gravity.

If a scale is attached to the gold leaf electroscope, then it can also predict the strength of the

charges crudely. This is the simplest instrument that demonstrates the existence of excess charge

or charge transferred by rubbing.

Activity: Students can make their own electroscope as follows. Take a thin aluminum metal rod

of about 4 mm in diameter and 15 to 20 cm in length. It has flattened cut ends. Take a large

bottle that can hold this rod and a cork which will fit in the opening of the bottle. Make a hole in

the cork sufficient to hold the aluminum rod snugly. Slide the rod through the hole in the cork

with the cut end on the lower side. Obtain a hollow ball or small aluminum disc and attach (weld

or solder it) it to upper end. Fold a small, thin aluminium foil (about 3 cm in length) in the

middle and attach it to the flattened end of the rod by cellulose tape. This forms the leaves of

your electroscope. Fit the cork in the bottle with about 5 cm of the ball end projecting above the

cork. A paper scale may be put inside the bottle in advance to measure the separation of leaves.

The separation is a rough measure of the amount of charge on the electroscope.

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Let us try to understand why material bodies acquire charge. You know that matter is made up

of atoms and/or molecules. Although normally the materials are electrically neutral, they do

contain charges; but their charges are exactly balanced. Forces that hold the molecules together,

forces that hold atoms together in a solid, the adhesive force of glue, forces associated with

surface tension; all are basically electrical in nature, arising from the forces between charged

particles. Thus the electric force is all pervasive and it encompasses almost each and every field

associated with our life. It is therefore essential that we learn more about such a force. To

electrify a neutral body, we need to add or remove one kind of charge (electrons). When we say

that a body is charged, we always refer to this excess charge or deficit of charge. In solids, some

of the electrons are less tightly bound to the atom. These are transferred from one body to the

other by the process like rubbing. A body can thus be charged positively by losing some of its

electrons. Similarly, a body can be charged negatively by adding some electrons.

Lightening:

Lightning is an atmospheric electrical discharge (spark) accompanied by thunder, usually

associated and produced by cumulonimbus clouds. Lightning striking Atlanta, United States is

shown in the side photograph.

History of lightning research and explanation

of the phenomenon:

Benjamin Franklin (1706–1790) made the

hypothesis that electrical sparks and lightening

are similar. He demonstrated it experimentally.

He flied the kite on cloudy day. A bunch of keys

was attached at the lower end of the string. The

free fibers of the strings stretch out. This indicated that they were ‘charged’. When he brought

his hand near the keys, a spark struck. This demonstrated that charges from clouds had flown to

keys via wet thread. They produced the spark.

www.surfnetkids.com/video/.../benjamin-franklins-kite-exp...

www.youtube.com/watch?v=4jdfFspodpo

http://www.sciencemadesimple.co.uk/activities/lightning

The experiments involving lightning are always risky and

frequently fatal. One of the most well-known deaths was that

of Professor Georg Richmann of Saint Petersburg, Russia.

Nicola Tesla built electrical generators to produce electricity

on large scale. It needed conducting lines and transmission

towers. The transmission towers needed to be safely erected.

He built tesla coil in 1900 and produced high voltages in

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laboratories. He produced artificial lightening in laboratories and studied the phenomenon in

more details.

We know sea water vaporizes due to sunshine. Vapor rises in the atmosphere and moves with air

currents. Water vapor becomes thicker to form the cloud. The temperature at higher altitudes is

fairly low. So there is a possibility of condensation and forming tiny droplets. The clouds move

through the atmosphere. There is friction between atmospheric molecules and water droplets.

Sometimes the water droplets fall under gravity within the cloud. This creates friction between

droplets and vapor molecules. These frictions charge the cloud and atmosphere. The charged

cloud moves, while charged indivisual atoms and molecules of atmosphere move away and they

are not noticed. The charge developed on the cloud may be negative or positive. As the cloud

moves for hundreds of kilometers before raining, the charge on the cloud goes on increasing.

This develops the potential to thousands of megavolts. The clouds become thicker and thicker

due to fall of temperature. They start travelling at lower altitude due to gravity. It induces

opposite charges on the taller objects. It sets up very large electric fields between cloud and tall

buildings or trees. The electric field increases the ionization of air. This increases conductivity of

air. At critical situation the arc strikes and the ‘lightening falls to earth’. The electrical

breakdown of air occurs at a voltage gradient of about 1 megavolts per meter (MV/m).

Lightning can occur with both positive and negative polarity. An average arc of negative

lightning carries an electric current of 30,000 amperes (30 kA), and transfers 500 mega joules of

energy. Large arcs of lightning can carry up to 120 kA. An average arc of positive lightning

carries an electric current of about 300 kA — about 10 times that of negative lightning.

Lightning rapidly heats the air in its immediate vicinity to about 20,000 °C (to 30,0000C). The

sudden heating effect and the expansion of heated air give rise to a supersonic shock wave in the

surrounding clear air. This supersonic wave travels through the air. It experiences friction and

decays in to acoustic wave. This sound wave is heard as a ‘thunder’.

The arc of lightning is very hot. Air surrounding it also becomes hot. It enhances the reaction of

forming nitric oxide. Nitric oxide produces nitric acid when it is in contact with rain water. The

water soluble nitrogen enriches soil to become more fertile.

The study or science of lightning is called fulminology.

Lightning Protector:

Diagram of a simple lightning protection system

A lightening protector is a system engineered to protect the

building in the event of lightning strike. It was discovered by

Benjamin Franklin in 1749. It is made up of thick metal (copper)

rod, mounted on the top of a tall building. A pit of appropriate size

is dug near the foundation of the building. An iron rod is buried in

this pit. The pit is filled with a mixture of soil, charcoal powder and

common salt. The upper rod on the building and iron rod in the pit

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are connected with thick copper or aluminum rod (or thick strip). Now a days, they are connected

by thick aluminum strip, as copper is costly.

Lightening is a threat to the tall buildings. A large current passes through when lightning strikes.

This may melt metallic part and cause fire to wooden parts of the building. Either the building or

a part of the building is at threat. We have seen that a charged cloud has a potential of billions of

volts. When such cloud comes near the rod on top of the building, it induces charges of opposite

nature on the tip of the rod. The tip being spherical, it generates radial electric field towards the

cloud. The charges on the cloud have larger probability that they will discharge through this

conductor than through building. Thus it protects the building from disaster, at least partially. We

note here that each lightening conductor protects roughly a circular area of 15m. Hence for large

buildings, appropriate numbers of protector rods are installed.

The electric transmission towers and conductors are also protected from lightening. There are

two-three wires mounted to the top of the towers. They function as the protectors of transmission

line. Generally, lines above 50 kV are protected from such lightening arresters.

The lightening arrester systems are essential to aircrafts and also to ships. The lightening arrester

requires special system to avoid electromagnetic interferences in aircrafts.***

Producing Static Electricity

www.youtube.com/watch?v=I2G0IdTWGQU

Van de Graaff generator:

A Van de Graaff generator is an electrostatic generator which uses a moving

belt to accumulate very high amounts of electrical charge on a hollow metal

globe on the top of the stand. It was invented by American physicist Robert J.

Van de Graaff in 1929. The potential difference achieved

in modern Van de Graaff generators can reach 5

megavolts. It can produce a visible electrical discharge to

a nearby grounding surface which can potentially cause a

"spark" depending on the voltage.

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Schematic view of a classical Van de Graaff-

generator:

1) hollow metal sphere

2) upper electrode

3) upper roller (for example an acrylic or glass)

4) side of the belt with positive charges

5) opposite side of the belt with negative charges

6) lower roller (metal)

7) lower electrode (ground)

8) spherical device with negative charges, used to

discharge the main sphere

9) spark produced by the difference of potentials

A simple Van de Graaff generator consists of a belt

of silk or similar flexible bad conducting material. 3 and

6 are two metal rollers or pulleys. The belt runs over

these pulleys. One roller is surrounded by a hollow metal

sphere. (2) and (7) are two electrodes. The nature of

electrode is comb-shaped. One electrode is placed near

the bottom of the lower pulley and other electrode is

placed inside the sphere over the upper pulley. The upper

comb (2) is connected to the sphere, and lower comb (7)

is connected to the ground.

The lower roller or pulley is moved by a motor. The belt

receives charge due to friction between belt and comb. As

the belt moves over upper roller (6), it transfers some

charge to upper comb. The upper comb is connected to

the sphere. The charges received by the comb get

transferred to the outer surface of the sphere. As the belt continues to move, the upper sphere

constantly receives the charge. The potential of sphere goes on increasing. If the sphere is larger

and farther from ground, its final potential will be higher.

The Van de Graaff generator was developed, starting in 1929, by physicist Robert J. Van de

Graaff at Princeton University with help from colleague Nicholas Burke.

The highest potential sustained by a Van de Graaff accelerator is 25.5 MV, achieved by the

tandem at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory.

Van de Graaff generator stores a large amount of charge. It can supply small

current at constant electrical potential of the sphere. It behaves as nearly ideal

current source.

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Van de Graaff generator of the first

Hungarian linear particle accelerator. It achieved 700 kV

in 1951 and 1000 kV in 1952. (Constructor: Simonyi

Károly; Sopron, 1951.)

A Van de Graaff generator integrated with

a particle accelerator. The generator produces the high

fields (in the megavolt range) that accelerate the particles.

Conductors and Insulators:

The materials are usually classified with respect to certain property i.e. how given material

behaves in context of certain particular property. In context of electricity materials are divided in

two main classes. They are described as follows.

Conductors: The materials which can carry electricity from one place to other are called as the

conductors or good conductors of electricity.

All metals are good conductors of electricity. The reason lies in their structure. The atoms of

metals are bound together by a bond called as a ‘metallic bond’. Generally the metals are in first,

second or third group of the periodic table. They possess one, two or three electrons in the

outermost orbit. We know, there can be maximum eight electrons in the outermost orbit. The

outermost electrons are shared by neighboring atoms and form the bonds with the neighbors. In

simplest cubic structure, there are six nearest neighbors. If we concentrate on central Na/

Sodium (blue balls in the diagram) atom, we can easily see that there are ‘six’ chlorine (green)

atoms as nearest neighbors. Other atoms are diagonal hence they are slightly away.

Hence the outermost electrons are shared by six (truly seven) atoms. Once the electron changes

the atoms, some neighboring atoms change. As a result, if we focus our attention on one electron,

it may be found around any atom in the given metal piece. It means it may be found anywhere in

given piece of material. It is described as ‘the electrons in outermost orbit (valency electrons) are

free to move inside the given piece of metal’. Electrons carry charge with them. This gives the

metals an ability to conduct electricity. Therefore they are called as the good conductors.

These valency electrons are in random motion. Hence

there is no net current in any direction in a sample of

metal .

1. lrrpublic.cli.det.nsw.edu.au/lrrSecure/Sites/Web/.../superc_03_02.htm‎

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When external potential difference is applied between the

two ends of the metal piece (or a metal wire), it sets up

electric field in the metal. Each electron experiences

electric force in the direction of positive electrode.

Electron moves towards the positive electrode under the

action of this force. So an avalanche of electrons achieves a ‘drift velocity’. They are neutralized

at positive electrode. Metal sample has to remain electrically neutral. Hence equal number of

electrons are poured from the battery or power source. Therefore effectively negative charge

flows from negative electrode to positive electrode. The direction of conventional current is from

positive electrode to negative electrode.

The ability of carrying electric charge is measured in terms of ‘conductivity’. We will see

definition and other details of this property when we will study ‘current electricity’.

Insulators: The elements from group VI and VII possess six and seven valency electrons

respectively. They need two or one electron to complete outermost subshell. As a result these

elements have high affinity towards electrons. They do not lose their own electrons while

forming their structure. Hence the samples of these elements do not possess ‘free or partially

free’ electrons.

When external potential difference is applied to such sample, electric field is set up in the

sample. The electrons revolving in the parent atoms do experience the electric force. This force

is comparatively week and is unable to pull the electrons out. Hence there are no free charge

carriers available in such materials. Therefore there is no net flow of charge and there is no so

called conventional current. Such materials are called as the ‘insulators’ or ‘bad conductors’ of

electricity. These materials are the non-metals.

Rubber, wood, glass, plastic are materials having complex molecular structure. They do not

show any conductivity under normal potential differences. They show very feeble conductivity

under very high voltages. Currents are of the order of microamperes and nano-amperes when

applied voltages are of the order of mega-volts. Such materials are used as the ‘insulators’.

Semiconductors: There is one more class of materials. The conductivity of these materials is

intermediate between good conductors (metals) and insulators/bad conductors. They are called as

the semiconductors. They are the forth group elements from the periodic table. They are also

called as the metalloids.

Semiconductors made revolution in miniaturization of electronics. We will see this in much more

details later.

Occurrence:

Lightening: We have seen how clouds get charged. It is due to friction of the cloud and air. Many a

times charged clouds result in to lightening. Most of the lightening events are in air itself. It strikes

earth on very few situations like cloud is heavily charged and also it is at very low altitude.

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Sometimes it is observed that there goes a spark when key is put in a car to open the door. The

metal body of the car gets charged due to friction between car and air. Car rests on tires which are

bad conductors. The charges do not leak to earth due to tires. It is generally at higher positive

potential. The key is normally at zero potential. As the distance between key and car whole reduces,

sparking becomes possible. A person may also get shock if he does not were rubber or polythene

shoes.

Airplanes also get charged due its friction with air. There is possibility of sparking between the

airplane and clouds in rainy seasons.