Реферат на тему Isu Physics Essay Research Paper ELECTRICITY ISU

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Isu Physics Essay, Research Paper

ELECTRICITY I.S.U. There are two electric states. Benjamin Franklin identified them as positive and negative charges. All objects possess electricity and a neutral object possess a normal amount. For purposes of identification ; Negative = The charge that an ebonite rod acquires when rubbed with fur. Accordingly , any other charged object that is repelled by such a charged object that is attracted to a charged ebonite rod , such as a glass rod rubbed with silk , must be charged positively. Therefore the Law of Electric Charges can be stated as follows. 1) Opposite charges attract each other. 2) Similar charges repel each other. 3) Charged objects attract some neutral objects. All matter is composed of atoms which contain different charges, positive, negative and neutral charges. Each charge is carried in a particle known as either a proton, electron and neutron. Protons are found in the nucleus of the atom. They are small, heavy and have a positive charge of a certain magnitude known as an “elementary charge”, electrons are found moving around in the orbits of the nucleus which contain a negtive charge. Each one is 1/2000th the mass of a proton and has a charge of same magnitude. Neutrons are also found in the nucleus which contain a neutral charge and a re small heavy particles. All matter falls into one of the three categories, which are conductors, semi-conductors and insulators. A conductor is a solid in which electrons are able to move easily from one atom to another. Some of the best conductors are metals such as aluminum, copper, and silver. Some of the electrons in conductors can be classified as “free electrons”. This is because they have the ability to move about within the atomic framework of a solid. A semi-conductor is an intermediate between a conductor and an insulator. This is because it contains very few “free electrons”. These substances become excellent conductors after a small impurity is added in a proccess called “doping”. All semi-conductors are solid at normal temperatures. An insulator is a solid in which the electrons are not free to move about from atom to atom. Some examples of good insulators are cork, glass, plastic, and wood. Solids become conductors when they have a great amount of “free electrons” given the electron the ability to transfer from atom to atom. Certain liquids are also conductors of electricity. This is because the molecules of a liquid are free to move about. The liquid will become a conductor if the particles are charged (positive and negative ions). This occurs, for example, when some chemicals are added to water to dissociate (break apart) into positive and negative ions when dissolved. Gasses also can be classified as conductors and insulators, depending on the electrical nature of the molecules. For example, air is an insulator until exposed to X-rays or radiation which causes the molecules to ionize and become conductors. Plasma is a collection of very hot positive and negative ions and electrons, which make it a good conductor. Some examples of plasma are neon lights to nuclear fusion reactors. 2. Some methods of charge transfer:a) Charge by friction Some substances acquire an electric charge when rubbed with another substance. An example of this is the ebonite rod that became negatively charged when rubbed with fur. The atoms of the ebonite rod capture some of the electrons from the fur, this then exerts stronger forces of attraction on them than do the atoms making up the fur. Therefore, after rubbing the ebonite rod has an excess of electrons and the fur has a deficit. The same explanation can be said for other pairs rubbed together. These can be determined by the Electrostatic Series, which is a list of substances in a specific order which determine which substances are greater in positive and negative charge when rubbed together. b) Induced Charge Separation When an ebonite rd negatively charged is brought near a metallic- coated pith-ball or metal leaf electroscope ( A simple device which is used to detect and compare electric charges. A transferred charge spreads out over the entire conducting system causing the leaves to diverge and indicate an electrical charge.) with metal a neutral charge, some of the free electrons are repelled by the ebonite rod and move to the far side of the ball or leaf. The negative distribution of charge on the rod causes the separation of the charge on each object, this is known as induced charge separation. This can also result from the presence of a positively charged rod. c) Charging by contact This transfer of charge occurs when two or more substances come into contact. For example , when a negatively charged ebonite rod is touched to a neutral pith-ball, they tend to share electrons to give both objects a negative charge. Likewise when a positive charged rod is used, both objects will share electrons to be given a positive charge. ” An object charged by contact has the same sign as the charging rod. d) Charging by Induction When a charged ebonite rod as brought near an electroscope, free electrons on the electroscope tend to move away from the negative charge. When the electroscope is touched by your finger, it creates an alternatepath for the negative charge to follow. This process is known as electrical grounding . This also leaves the electroscope with a deficit of electrons and a positive charge. When a positive rod is held near the electroscope, it induces electrons to move through your finger onto the electroscope, when finger is removed the electroscope is left with an excess of electrons and a negative charge. “An object that is charged by induction has a charge opposite to that of the charging rod.” ** An electroscope is a simple device that can be used to detect and compare electric charges. A charge transferred to the knob of the electroscope spreads out over the entire conducting system, including the leaves, causing them to diverge and so to indicate the presence of a charge. A greater divergence of the leaves indicates an increase in the charge. This instrument detects positive and negative charges equally as well. 3. It is the French physicist Charles Augustin de Coulomb (1736 – 1806) who established the quantitative nature of the electric force between charged particles. 4. Coulomb used a torsion balance to conduct the experiment. He used electrical forces between small charged spheres to cause the torsion balance to turn. He was then able to determine the relationship between the electric force with the use of the known torsion properties of the suspension wire. He had also changed the distance between the charged spheres. He also experimented by changing the amount of electrical charge on either sphere and by using an identical neutral sphere, he touched it with the charged sphere and was able to divide it’s change in half and noted the resulting effect on the electrical force. 5. It was discovered that the two relationships that governed the strength was that the magnitude of the electric force is inversely proportional to the square of the distance between the centers of the charged spheres and also that the magnitude of the electrical force was directly proportional to the product of the magnitude of the charges on each sphere. 6. Coulomb’s Law states that the electrical force is equal to the product of the magnitude of the two charged spheres (q1q2) divided by the square of the distances between the center of the two charged spheres

(d2) multiplied by the proportionality constant known as “Coulomb’s constant” (k) F = k(q1q2) / d2 7. Relative to Coloumb’s point charges means that the two charged spheres must be a great disance apart compared to their own actual dimensions, that the detailed positions of the charged particles making up the magnitude of charge in spere 1 and 2 are not siginificant. 8. The S.I. unit of charge in Coulomb’s Law are called coulombs or “c”, the quantity symbol is “q” and the magnitude for the unit of electric charge is 1 C passes through a standard 60 W light bulb in 2s. 9. The value for k is determined by using a torsion balance. By measuring the resulting twist of placing charges of known magnitude a given distance apart. This value found is for the electric force causing the twist . Coulomb’s constant has two magnitudes which are K=9.0 x 109 (Nm2) / C2 (used to calculate the electric charges between point charges) and k = 2.306 x 10-28 (Nm2) (elem.chg2 ) (used when measuring q1 and q2 in elementary charges) Where N is Newton ,m is meters ,C is coulombs and elem. Chg is the elementary charge) 11.A field is a region of space , altered in some way by the presence of a mass or charge respectively, in which a force will be felt if we place a mass or charge into the field. 12. It is stated that the field is always present around an object and is displayed when it comes in contact or close range with another object, which also has a field around it. Therefore if two objects are moving close together, their forces will meet before contact is made causing the object to react at a distance. Actions depend on the interacting fields 13. A field is represented diagrammatically by drawing a series of force vectors around the charged object. The force vectors show the direction and magnitude of the electric force on a small, positive test charge placed at each and every point in the field. For simplicity reasons, continuos field lines are drawn to show the direction of the force at all points in the field. 14. Electrical field intensit is a vector quantity and is stated to be the electrical force per unit positive charge. E = FE / q where e represents electric field intensity , q represents unit positive charge and FE represents the electrical force. The units are Newton per coulomb. In comparison it is shown that both use N/ ? , where ? is the effecting unit. In the case of gravitational field . mass is the effecting variable , therefore kg are used. In the electric field , coulombs are the effecting variable. 15. In an electric field diagram, the field intensity at any point is indicated by the relative distance between the adjacent field lines. In a region where the electrical field is strong, adjacent field lines are close together and when they are spread widely , it indicates a weaker electric field. The field diagram for a positively and negatively charged sphere a but the vector lines are in opposite direction. 16. The equation for E at a distance of “r” from the center of a small charged sphere is E = K(Q1 / r2) 17. An example of this is parallel plates close together. Each plate would have to be considered as point “n” charges, each point is distributed uniformly side by side on a plane. Therefore, in determining the net electric force at any point in the vicinity of the plates, the contribution from each of the 2 “n” point sources, each a different distance and direction away would have to be considered. 18. The E pattern in the region surrounding 2 positive point charges tends to repel each other. The vector lines tend to curve away from the location of the other point of charge. Except the 90 degree vectord, these tend to go straight towards the other point charge and repel at a 90 degree angle. This also applies for two negative point charges except that the vector lines are pointing in the opposite direction. For a positive and a negative point charge the vector lines tend to curve towards the other point charge. This proves the Law of Electric Charges. 19. In summary to the properties of E between the plates of a charged capacitor, the region outside the two charged plates has an E value of 0 (except for a slight bulge at the ends of the plates). There is a constant value for E everywhere between the parallel plates. All electrical field lines are straight , equally spaced and perpendicular to the parallel plates (except near the edges) depends only on the magnitude of the change of each plate. E is inversely proportional to q, where q is the charge per unit area on each plate. 21. Electric Potential is defined as a unit positive test charge when in the field of any of any other charge. It is a value of potential energy per unit positive charge. It is given the symbol “v”. It represents the amount of work necessary to move a unit positive test charge from rest to infinity to rest at any specific place in a field. 22. The symbol for electric potential in denoted by ” v ” and has the units of volts or joules per coulomb. 23. the equation that is used to determine electric potential at a distance of ” r ” from a spherical point charge “knot” is Ec = (kq1q2) 24. 1 volt is the electrical potential at a point in a electrical field , if 1 joule of work is required to move 1 coulomb of charge from infinity to their point 1V = 1J / C 25.Potential difference or voltage Difference is the work done per unit positive test charge to move one point to another in a field. It is the difference in electric potential between those two points. Electric difference is the work to move from a point in infinity to a point in a field while potential difference is the work to move from one point to another point in the field. 26. The express ion used for the potential difference VAB for a point charge q being moved between two points A and B in an electric field is DEe = q (VB-VA) = qDV 27. As the charge moves in the direction of the electric field it has to fight against the force of the field, making it increasing difficult and making the value of the potential difference higher. On the other hand, if the potential difference is moving in the direction of the field it no longer difference to decrease. Once the velocity of the other ball that is travelling at cetripital velocity in an eliptical denomintaitoin it will ususlly fell inde 28. The expression for the magnitude of the electric field E between parallel plates seperated by a distance ” d ” and connected across a potential difference Vab is E = Vab / d or qVab = qEd 30. In the experiment conducted by R.A. Milikan it was shown that there was a unit of charge which all other units are simple multiples. 31. The apparatus he used was called an electric Microbalance. 32. The accepted value of the magnitude of the charge on an electron is e = 1.602 x 10-19 C 33. The number of electrons required to constitute a coulomb of charge is 6.242 x 1018 e = 1C 34. Three major accomplishments of Robert Andrew Milikan are1. He discovered the Elementary Charge2. He verified Albert Einstein’s Photo Electric effect3. He named and identified the origin and nature of cosmic radiation. 36. A charged particle 9. Moves the electric field of q2 in such a way that the electric potential energy it loses (- DEe ) is equal to the kinetic Energy it gainse (DEk). Practice Problems

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