Download Electricity: 1) Static electricity 2) Electrical charges and its properties

Electricity:
1)
2)
3)
4)
5)
Static electricity
Electrical charges and its properties,
Coulomb’s Law
Electroscope, Charging by friction and Induction
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)
2)
3)
4)
Attraction / Repulsion
Conservation
Additivity
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 105 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 nonuniform 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
Fα
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.
in SI system is 9 X 109 N-m2 /C2. It means if two charges of magnitude
Note: The value of
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 109 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 Graaffgenerator:
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.
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