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Physics Project Report on Frictional Electricity
4) Positive and Negative Electricity
5) Insulators and Conductors
6) The Gold-Leaf Electroscope
7) Experiments with a Hold Leaf Electroscope
8) Electrostatic Induction
9) The Electron Theory = Atomic Structure
10) Electrification by Friction
11) Electrons in Insulators and Conductors
12) Distribution of Charge over the Surface of a Conductor
13) No Charge on the Inside Surface of a Hollow Charged Conductor
14) Some Uses of Frictional Electricity
This is to certify that the project report entitled "To Study Energy Bands in Solids" submitted by Master Amit Singh is original and has been completed by him under my supervision and is completed in all respects for AISSCE 2000-2001.
Signature of Faculty Head Physics
As a student of Class XII, I did this project as a part of my studies entitled Frictional Electricity.
I owe a deep sense of gratitude to my Physics teacher whose valuable guidance, advice, lovingly nature helped me in doing this project from conception to completion.
I thank the Principal of our school for his ever lasting blessing and motivation.
I am thankful to my computer teacher for providing me computer facilities.
Finally, I am thankful to my parents for helping me economically and friends for giving me a helping at every step of the project.
Signature of the Student
Everyone is familiar with the fact that if a pen made of certain plastic materials is rubbed on the coat-sleeve it will afterwards attract dust and small pieces of paper. The same, effect if noticed when a mirror or window-pane is polished with a dry cloth in a very dry atmosphere. Dust and fluff from the cloth stick to the glass and are difficult to remove. Perspex, cellulose acetate and the vinyl compounds used for gramophone records also show the attraction, but to a more marked degree. The phenomenon is called electric attraction, and the rubbed materials are said to have become charged with frictional electricity. Knowledge of it goes back as far as the sixth century B.C., when the Greek Philosopher, Thales, described the attractive properties of rubbed amber. The word electricity has, in fact, been derived from the Greek word "elektron", meaning amber.
Friction between certain textiles can also produce electrification. Robert Symmer first described this in the early eighteenth century.
Electrification by friction is sometimes associated with a crackling sound. This may be heard when very dry hair is combed with a vulcanite comb, or when an ebonite or alkathene rod is vigorously rubbed with fur. The cracking is caused by small electric sparks, which may be seen if the room is in darkness. Sparks from frictional electricity can be very dangerous when inflammable vapour is present. Nowadays, accidents are prevented by allowing a short length of chain to trail from the metal frame of the trolley. This conducts the electric charge away to earth, where it can do no harm.
Positive and Negative Electricity
Electric repulsion was first described in 1672 by Otto von Guericke, who noticed that some feathers were attracted to a charged sulphur ball and then repelled from it. One hundred and fifty years later in France, Charles Du Fay discovered that charged bodies did not always repel each other, but that sometimes attraction took place. He came to the conclusion that there were two kinds of electricity. Charges of the same kind repel, while charges of opposite kinds attract one another.
To distinguish between the two kinds, Du Fay used the terms vitreous and resinous electricity. Vitreous (from the Latin vitrum = glass) electricity is obtained when glass is rubbed with silk, and resinous electricity is obtained when amber, sealing-wax, sulphur, shellac, and a host of other substances are rubbed with fur or flannel. Later on, these terms were found to be misleading, since, for example, ground glass gives resinous electricity and very highly polished ebonite gives vitreous electricity. Accordingly, Benjamin Franklin introduced the present day terms positive and negative instead of vitreous and resinous respectively.
Insulators and Conductors
When current electricity was discovered, 200 years after the publication of Gilbertís book, it was found that an electric current would flow through a non electric but not through an electric. Accordingly, these terms became obsolete. We now call an electric an insulator and a non-electric a conductor.
Gilbert mentions the importance of dryness an electrical experiments. Impure water is a conductor, and a film of moisture from condensation or moist hands on the surface of an insulator allows electricity to be conducted away to earth. For successful results, all apparatus used in electrical experiments must be thoroughly dry. Glass rods in particular are best warmed before use.
The Gold-Leaf Electroscope
For the detection and testing of small electric charges, a gold-leaf electroscope is used. This instrument was invented towards the end of the eighteenth century by a Yorkshire clergyman named Abraham Bennet. Fig. shows a common type of electroscope. It consists of a brass rod surmounted by a brass disc or cap and having at its lower end a small rectangular brass plate with a leaf of thin gold or aluminium attached. The leaf is protected from draughts by enclosing it in an earthed metal case with glass windows. The brass rod is supported by passing it through a plug of some good insulating material such as alkathene at the top of the case.
The three horizontal parallel lines shown at E in fig. is the conventional symbol for an earth connection.
Experiments with a Gold Leaf Electroscope
(1) To detect the presence of charge on a body
If a rod of some suitable material is charged by friction and then brought near to the cap of a gold leaf electroscope the leaf is seen to diverge from the plate. A charge has been induced on the leaf and plate, and consequently repulsion occurs between them. On removing the charged rod, the leaf collapses, showing that the induced charge on the electroscope is only temporary.
Very small charges may be detected by this method.
(2) To Charge a gold-leaf electroscope by contact
An ebonite rod is given a small charge by rubbing with fur, and is then rolled over the cap of an electroscope. The leaf will be seen to diverge, and then the rod is removed. If the leaf does not stay diverged the process is repeated until it does. We may now assume that the electroscope is charged with negative electricity by conduction from the ebonite rod.
If the cap of the electroscope is touched with the finger the charge flows to earth through the experimenterís body and the leaf collapses. This is called "earthing the electroscope".
(3) To test for the sign of the charge on a body
Having charged the electroscope negatively, the ebonite rod should be recharged and brought near to the cap. An increase in the leaf divergence is noted.
A glass road rubbed with silk (positive charge) is now cautiously brought down towards the cap from a height of about 50 cm. This time, a decrease in divergence is noticed.
The electroscope is discharged by touching it with the finger and afterwards charged positively by contact, using a glass rod rubbed with silk. We shall now find that an increased divergence is caused by bringing a charged glass rod near the cap decreased divergence by a charged ebonite rod.
From these experiments we conclude that an increase in divergence occurs when the charge on the electroscope and the test charge are of the same kind.
The results of these experiments are summarized in the table :
(4) To test the insulating properties of various materials
The insulating or, conversely, the conducting property of a given substance may be tested by holding a sample of the substance in the hand and then bringing it into contact with the cap of a charged electroscope. If the substance is a good insulator there will be no leakage of charge through it and the leaf divergence will not alter. If, however, the leaf collapses instantly it shows that the substance is a good conductor.
We saw earlier that a charged rod brought near to the cap of an electroscope causes the leaf to diverge from the plate, showing that a charge has been induced on both of them. The following experiment provides more information about the charges which are induced on an insulated conductor when a charged rod is brought near it.
(a) Two insulated brass spheres A and B are placed together so that they touch one another and thus form, in effect, a single conductor.
(b) A negatively charged rod is now brought near to A. As a result, a positive charge is induced on A and a negative charge on B.
(c) Still keeping the charged rod in position, sphere B is moved a short distance from A.
(d) The charged rod is now removed and A and B are tested for charge.
The test is carried out as follows. Sphere A is brought near to the cap of a positively charged electroscope. An increase in divergence shows that it is positively charged. Similarly, sphere B produces an increase in divergence when it is brought near to the cap of a negatively charged electroscope, thus showing it to be negatively charged.
In the whole experiment is carried out again using a positively charged rod as the inducing charge, the induced charges on A and B are reversed.
The Electron Theory Atomic Structure
Towards the end of the nineteenth century, Sir J.J.Thomson carried out some experiments with an electric discharge through a tube containing air at very low pressure. Following this investigation he came to the conclusion that negative electricity consists of tiny particles which came to be called electrons. During the succeeding years it became apparent that these negative electrons actually formed part of the atoms of which all substances are composed.
Electrification by Friction
When a glass rod is rubbed with a silk cloth some electrons from the glass attach themselves to the silk. Consequently, the glass becomes positively charged and the silk negatively charged. Likewise when ebonite is rubbed with fur electrons are transferred from fur to ebonite, thus making the ebonite negative and the fur positive.
Electrons in Insulators and Conductors
The difference between an insulator and a conductor is that, in an insulator, the electrons are firmly bound to their atoms and will not move of their own accord, whereas in a conductor the electrons are able to move freely from one atom to another.
If an ebonite rod is held in the hand and rubbed with fur a charge of electrons is formed on its surface. These electrons cannot flow to earth through the hand, since they are unable to move through the insulating ebonite. When a brass rod is rubbed with fur it becomes charged with electrons in just the same way as the ebonite. However, the charge cannot be detected, since it is immediately conducted through the brass and the hand to earth. This may be prevented by mounting the brass rod on an insulating handle. The charge cannot now be conducted away, and its presence can be detected by bringing the rod near a gold-leaf electroscope. The charge can be tested and found to be negative by showing that there is no increase in divergence when the brass rod is brought near to the cap of a negatively charged electroscope.
Distribution of Charge Over the Surface of a Conductor
A proof plane and gold-leaf electroscope may be used to investigate the distribution of charge over the surface of a conductor, by pressing the proof plane into contact with the surface at various places in turn and then transferring the charge to the electroscope. The divergence of the leaf will give a rough measure of the amount of charge transferred, and hence some idea of the surface density of the charge.
Surface density is defined as the quantity of charge per unit area of surface of a conductor.
Fig. also shows how charge is distributed over the surface of conductors of different shapes. In these diagrams the distance of the dotted line from the surface is proportional to the surface density at any point. The most important fact shown by this experiment is that charge is mostly concentrated at places where the surface is sharply curved. This is particularly noticeable at the pointed end of the pear-shaped conductor.
No charge on the Inside Surface of a Hollow Charged Conductor
The electric conductors used in the experiments we have described are generally made of hollow brass or else of wood covered with tinfoil. No advantage is to be gained by making them of solid metal, since the charge resides only on the outside surface. The following experiment illustrate this fact.
Charles Coulomb demonstrated that charge always resides on the outside surface of a conductor with the aid of two hemispherical cups which fitted exactly round an insulated metal sphere.
The sphere first charged, and afterwards the hemispheres are fitted over it while being held by insulating handles. On removing the hemispheres they are found to be charged, but no charge at all remains on the sphere. This shows that all the charge on the sphere must have passed to the outside of the hemispheres.
Some Uses of Frictional Electricity
1. Photocopying machines or zerox machines
Based on the attraction of powdered particles to a metal drum that carries a pattern of charge that is same as the pattern of the desired image.
2. Paint Droplets become charged by friction
When they are sprayed. If the object to be painted such as a car body, is given a charge of opposite sign the paint is attracted to the object & covers it without wastage.
1. Modernís abc of physics.
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