electricity: 1) static electricity 2) electrical charges and its properties
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1) Static electricity 2) Electrical charges and its properties, 3) Coulombs Law 4) Electroscope, Charging by friction and Induction 5) Lightening
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
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.
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
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
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
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
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
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 others 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
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
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.
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.
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