We know that every matter consists of
protons and electrons. Do you know that there are more than 1028 protons and
almost equal number of electrons in your body? The basic property of these
protons and electrons is charge. The charge of proton is considered as positive
and of electron is negative. The charged objects that apply force on each other
are known as electric force. The electric force is a basic and important force of
nature. In this chapter we will see how a body is charged. We will also learn
detect the presence of charge and measure the force between them. As the
discussed charges will remain static we will call the chapter Static Electricity.
Finally we will discuss the usage of this static charge and some of its danger
and the ways to remain safe from them.
Static Charge
In a winter morning John took his plastic comb to comb his hair.
Before combing his hair he rubbed the comb with his woolen pullover for a while.
While combing his hair he observed surprisingly that he cannot do it because
all the hair was becoming straight and repels away each other. As soon as John
brings the comb close to the tale he observes the comb attracting some pieces
of paper on the table. Some of you may have the same kind of experience like John.
In our daily life we observe that many objects around us behave like the comb
of John.
Do it
yourself: Rub your dry hair with your plastic scale for a while and hold it
near some pieces of paper.
We see that an object attracts another object in a special
condition or becomes charged that is electricity produced in the object. Their
charge remains static in the place of its production. So it is called static
electricity. Now let us see what we mean by being charged.
We know that each matter is consisted of small particles. The atom
of every matter
consists of electrons and which revolves around the nucleus. In the nucleus
there are two type of particles-proton and neutron. The fundamental and special
property of primary particles (electron and proton) of which the matter consist
of is electrical property, which is determined by charge. The charges of
electrons are negative and the charges of protons are considered to be
positive. Neutron is electrically neutral that is it has no charge. The amount
of charge in a proton is equal to the charge of an electron. Naturally an atom
has an equal number of protons and electrons. As a result in the atom whole atom
there is no electrical property to be detected. The number of electrons and
protons in different matters is different.
As long as the number of electron and proton are equal in any
atom, it is electrically neutral. But if their number is not equal in any atom,
then the atom will be charged. If the number of electron decreases, the number
of proton increases. This situation is said to be charged positively. Again, if
these scattered electrons are joined with any atom, the number of its electron
increases, as a result it is charged negatively. The shortage or excess of
electrons in any atom is considered to be charged.
The substances through which electricity or electric charge can
pass easily are called conductors, e.g. metal, soil, human body etc. Generally
the metals are good conductors. Copper, silver, aluminum etc. are good
conductors. On the other hand the substances through which electricity or
electric charge cannot pass easily are called nonconductors or insulator, such
as wood, paper, glass etc.
Electrification by Friction Experiments
Experiment: Suspend a
light pith ball freely with a string from a stand or a hook. Rub one end of a
glass rod with a piece of dry silk cloth very well. It will be more convenient
if the glass rod and the silk are dried to make them warm. Now bring the rubbed
end of the glass rod near the freely suspended pith ball.
In normal condition the number of protons and electrons in
any atom is equal. But every atom has got affinity for getting excess
electrons. This affinity for excess electron is different in different
substances. That is why when two bodies are brought in contact with each other,
the body which has greater affinity for electron collect electrons from other
body and gets charged negatively this happens when a glass rod is rubbed with
silk.
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Figure 1
|
[Fig: 1] Silk has more electron affinity than glass and as such
these two are rubbed together, the electron of glass goes to silk. As a result,
the silk gets charged negatively and the glass rod becomes positively charged.
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| Figure 2 |
This is why glass rod attracts pith ball [Fig: 2].
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| Figure 3 |
Again when a rod of ebonite or polythene is rubbed with
flannel, polythene rod gets charged negatively and the flannel becomes
positively charged. Because polythene has more affinity for electrons than that
of flannel and so when they are rubbed together free electrons of flannel moves
to ebonite or polythene and gets charged negatively [Fig: 3].
Electrical Induction
We know that when two bodies are rubbed together electric
charge is produced. Again when a charged body is brought in contact with
another neutral body later is charged. When the neutral body is placed not in
contact with the charged body but close to the charged body, the former becomes
charged. This happens due to induction. The process of charging a neutral body
by bringing it very near to the charged body is called electrostatic induction.
With a simple experiment the electrostatic induction has been explained below.
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Electric
Induction
|
Experiment: A dry glass rod is rubbed with silk. One end
of the rod is brought very near to the neutral conductor rod AB keeping the
other end of the glass rod in hand [Fig: Electric Induction (a)]. As a result the free electrons of the
conductor are attracted by the positive charges of the glass rod and move the
end A. Therefore, the end B falls in short of electrons and is charged
positively. The end A is charged negatively. With a charge collector [a small
metal sheet attached to a nonconductor handle] if some charges are collected
from end B and their nature is detected with the help of an electroscope then the
above discussion will be proved.
Here no new charge is produced. Due to the presence of the charges
glass rod equal amount of opposite charges are separated only and move towards
the end o the conductor. As long as the glass rod is present near the conductor
AB, the opposite charges are separated and remain at the end of the conductor.
In the above experiment the positive charges in the glass rod
which has created induction in the conductor AB is called inducing charge. The
charges which are accumulated in the conductor A are called induced charge.
Extended activities: Rub a balloon full of air with your
shirt. Then press the balloon with the wall of your house for a while and then
release it. What did you observe? The balloon becomes attached to the wall.
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| Figure: 1 |
Extended activities: Rub a plastic object with your shirt.
Then hold it near a narrow flow of water falling from a tap. The flow of water
will bend towards the plastic object.
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| Figure: 2 |
The negative charge of the balloon produces electrostatic
induction in the wall. The induced positive charge in the wall attracts the
negatively charged balloon [Fig: 1]. Same case occurs with the narrow flow of
tap water [Fig: 2].
Electroscope
The instrument which detects the presence and nature of charge in
any body is called electroscope. A metallic circular disk is fixed on the top
of brass or any metallic rod R [Fig: Charging the electroscope] and at lower
end of the rod two light leaves of gold are attached. Instead of gold aluminum
or any other light metal can also be used. The lower end of the rod with the
leaves is placed in a glass jar passing through a cork of nonconductor matter
C. As the apparatus is kept inside a glass jar, the humidity and the wind cannot
damage the instrument.
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Charging
the electroscope
|
Charging the electroscope
If a glass rod is rubbed with silk, it becomes positively charged.
If that charged rod is attached to the disk or sphere of the electroscope, some
charge is transferred from the rod to the disk. This charge reaches to the gold
leaves through conducting metal rod. As the gold leaves receive similar
charges, they repel each other and move away from each other or explode. In
this situation, if the glass is removed but the gap between the leaves is not
reduced, then it can be decided that the electroscope is charged with positive
ions. To charge the electroscope with negative ions, an ebonite rod should be
rubbed with flannel and this negatively charged ebonite rod be allowed to touch
the disk in the above mentioned process. As a result, the gold leaves of the
electroscope will be charged negatively and they will repel each other and will
remain in that position. The more is the quantity of charge the more will be
the gap between the leaves.
Detection of existence of electric charges
To determine the presence of charge in any body, the body may be
brought to an unchanged electroscope. If the two leaves move away from each
other then it is understood that there is an existence of charge in the body
but if they don’t move away from each other then it is understood that the body
has no charge.
Detection of the nature of charge
To know the nature of charge in any charged body, the electroscope
should be charged either positively or negatively. Suppose the electroscope is
charged positively. In this position, the leaves having positive charge will
stay apart. Now if the experimental body is brought in contact with the disk of
the electroscope and if the gap between the leaves decreases, then it is
understood that the body is charged negatively. On the other hand if the gap
increases due to touching the disk with the experimental body, then it is understood
that the body is positively charged.
Static Electricity Uses
Electrostatic Painting Spray:
Nowadays painting spray is used to color car, cycle, cupboard or
other things with help of static electricity. Spray gun is made in such a way
that it produces very small charged particles of color. The sharp edge of the
painting spray gun is connected to a terminal of the static electrical generator.
The other terminal of the generator is connected to the metal plate to be
colored which must be connected with earth. In case of coloring a car the
charged small particle emitted from the spray gun attracts the outer frame of
the car. As a result a uniform layer of color is formed on the outer surface of
the car. Moreover these small particles move along the electric lines of force
and reach the narrower places of its surface and color it.
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Electrostatic
Painting Spray
|
Ink Jet Printer:
This is the most ordinary printer which remains connected to the
computer. An ink-gun with its narrow mouth projects very small particles of ink.
These small particles are positively charged. These ink particles move through
the space between two plates. These positively charged ink particles are repelled
by the positive plate and attracted by the negative plate.
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Ink Jet
Printer
|
A computer controls the voltage of the plates in such a way that
the plates are sometimes positively charged and sometimes negatively charged.
And the ink particles fall on moving paper scattered in different places to
give the necessary shape of letters and pictures. For color print four kinds of
color are used.
Photocopier:
Nowadays photocopier or photocopy machine is very essential and
thus it has become very popular machine. Not only the educational institutions
and other offices but also general people use this machine to photocopy any
kind of necessary papers and documents. Static energy is also used in this
machine. There is a rotating dram inside the dark part of the photocopier.
Positive charge is sprayed over the dram. A bright light lights up the page to
be photocopied. The white part of the page reflects light but the dark or
printed part does not reflect light. The reflected light centers on the dram. Charges
releases from the part of the dram where the reflected light from white paper falls.
Only the dark part of the dram is charged positively. Negatively charged ink of
carbon powder (toner) is sprayed over the dram. Negatively charged ink
particles get stick to the positively charged part of the dram. A piece of
white paper is positively charged and is kept pressed with the dram. This paper
picks up the pattern of carbon powder on its surface from the dram. Positively
charged paper attracts the negatively charged toner. Then the paper is passed
through the heated roller. As a result the ink of the toner gets melted and
mixed with the paper and makes a permanent copy.
Static Electricity Dangers
Sometimes the presence of static electricity is harmful and may
cause danger.
Loading of fuel in aero plane:
When aero plane flies in the air it may electrify due to the
friction with air. If the charge increases continuously the potential
difference between the aero plane and the earth increases simultaneously. Due
to this high potential difference when the fuel is loaded there is possibility of
releasing some charge to the ground which may create spark. This spark may lead
to a huge explosion. This is why the wheels of aero plane are made of conductor
rubber so that the stored charges in the aero plane can be released safely to
the ground when it lands.
The solution of the problem is that a conductor is to be connected
to the aero plane and to the ground as soon as it lands and just before the
loading is started.
Loading of Fuel into Tanker:
Sparking or explosion may occur when a truck or tanker etc.
carries fuel from one place to another. To prevent this type of danger the
tanker should be connected to the ground by a conductor.
Television and Monitors of Computer:
During the operating period television and monitor of computer,
electrostatic charges are produced. These charges attract the uncharged dust
particles thus they become dirty soon.
Change of clothes:
Sometimes our wearing clothes may be charged due to the friction with
us. When we change our clothes, there is a possibility of getting light shock
as the charges pass to the ground through our body.
Operation Theater:
Necessary measures have to be taken to keep the surgeons, concerned
people and treatment equipment’s of the operation theater in a hospital free from
electric charges as they attract dirt and germs. This is why they have to wear conductor
shoes of rubber and use rubber gloves. So that electron can pass through them to
the ground.
Hanging metal chain with petrol transporting truck:
A metal chain has to be hung touching the road with the trucks
that carries petrol, diesel or other liquid fuels. When a truck moves along the
road, the petrol in the tank dashes against the inner surface of the tank and
oscillates to and fro. As a result of this, friction charge is stored in
petrol. If any spark takes place from the edge of the tank it may cause serious
accident and may set fire. So the charges in petrol are not safe. Therefore a
chain is connected at the back of the tank so that the charge can pass through
it to the ground easily as metal is a good conductor.
No direct connection between metal pillar and electric line:
The metal pillars on the road have any direct connection with
electric lines pulled over the pillars. As metals are good conductors, the
electricity of the wire may pass through the pillar to the ground if the pillar
has connection with wires. If anyone touches the pillar he could immediately be
electrified causing serious accident. So the wires are connected to the pillars
by a porcelain cup.
Thunder bolt and lightning conductors:
We know that there is water vapor in atmosphere. This water vapor
being condensed on the charged ions on the atmosphere and form water droplets
and become charged with electricity. When these water droplets gather together cloud
is formed. Cloud can be charged either positively or negatively. When two
oppositely charged clouds come close to each other than electrical discharge
takes place between them and makes huge spark. This is called lightning or
electric flash.
During electric flash the air around the cloud expands suddenly by
getting heated. Due to sudden expansion the pressure of the air lowers much.
Then neighboring air at higher pressure contracts the expanded air. Due to this
sudden expansion and contraction of air, violent sound is produced. This is
known as roaring of thunder. If there is too much charge on a cloud then it
includes opposite charge on the surface of the earth and electric discharge
takes place. This is known as thunder bolt.
Lightning conductor or lightning arrester:
In order to protect buildings from the ravage of lightning the
lightning conductor is used. The metallic rod R (Fig: Lightning conductor or lightning arrester) with several sharp
points at the top is fixed in such a way that its upper end extends several
feet above the roof of the building and lowers end runs down along the outer
surface of the building and buried well inside the moist earth.
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Lightning
conductor or lightning arrester
|
When a charged cloud passes above the building then the charge of opposite
kind is induced in the conductor rod R. The accumulation of charge is at
maximum at the pointed ends of lightning conductor and pointed ends discharges
their charge to the air particles around it. The air particles around the sharp
points are charged by conduction and are attracted by the opposite charges of
cloud and moves towards the cloud is neutralized. As a result the probability
of thunder bolt decreases.
Electricity
always passes following the shortest path through a conductor. The charge produced
in the clouds tends to reach the earth through the high standing objects.
During storm and rain it is rather good to wet in rain than to stand under an
umbrella, any tree, near any conductor, iron bridge or fence of sharp iron
wire.
End
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