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

    Static Electricity


    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.


    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.

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    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



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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


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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 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

    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.


    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.

    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


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