Nature provides few more awesome displays than lightning. Benjamin Franklin was colonial America’s foremost scientist. Starting in 1740, Franklin studied electricity produced by friction, charges that result from rubbing two surfaces together. Franklin proposed his famous kite experiment in 1750, and two years later showed that “electrical fire” could be drawn from a cloud. Franklin became famous as s scientist throughout Europe. This fame probably helped assure his diplomatic successes in France during the American Revolution. You may have rubbed your shoes on the carpet hard enough to create a lightning-like spark when you touched someone. Franklin’s kite experiment showed that lightning is similar to frictional electricity. Electrostatics is the study of electrical charges that can be collected and held in lone place. Running a comb through your hair transfers electrons to the comb, giving it a negative charge. If the comb, for example, is brought close to bits of paper, a charge separation is induced on the paper bits. The attractive electric force accelerates the paper bits upward against the force of gravity.
Coulomb found how the force between two charged spheres, A and B, depended on the distance. First he carefully measured the amount of force needed to twist the suspending wire through a given angle. He then placed equal charges on spheres A and B and varied the distance, d, between them. The electric force moved A from its rest position, twisting the suspending wire. By measuring the deflection of A, Coulomb could calculate the force of repulsion. He made many measurements with spheres charged with both positively and negatively. He showed that the force, F, varied inversely with the square of the distance between the centers of the spheres.
The electric force, like the gravitational force varies inversely as the square of the distance between two objects. Both forces can act at a great distance. How can a force be exerted across what seems to be empty space? In trying to understand the electric force, Michael Faraday (1791-1867) developed the concept of an electric field. According to Faraday, a charge creates an electric field about it in all directions. If a second charge is placed at some point in the field, the second charge interacts with the field at that point. The force it feels is the result of a local interaction. Interaction between particles separated by some distance is no longer required.
The electric Field
An electric field is the property of space around a charged object that causes forces on other charge objects.The direction of the electric field is the direction of the force on the positive test charge. The magnitude of the electric field intensity is measured in newtons per coulomb, N/C.
Notice that just as the electric field is the force per unit charge, the gravitational field is the force per unit mass, g=F/m.
The collection of all the force vectors on the test charge is called an electric field. Any charge placed in an electric field experiences a force on it due to the electric field at that location. The strength of the force depends on the magnitude of the field, E, and the size of the charge, q. Thus F=Eq. The direction of the force depends on the direction of the field and the sign of the charge.
A picture of an electric field can be made by using arrows to represent the field vectors at various locations. The length of the arrow shows its direction. To find the field from two charges, the fields from the individual charges can be added vertically.
An alternative picture of an electric field is shown below. The lines are called electric field lines. The direction of the field at any point is the tangent drawn to the field line at that point. The strength of the electric field is indicated by the spacing between the lines. The field is strong where the lines are close together. It is weaker where the lines are spaced farther apart. Remember that electric fields exist in three dimensions.
When there are two or more charges, the field is the vector sum of the fields due to the individual charges. Note that field lines always leave a positive charge and enter a negative charge. The Van de Graaff machine is a device that transfers large amounts of charge from one part of the machine to the top metal terminal. Field lines do not really exist. They are just a means of providing a model of an electric field. Electric fields, on the other hand, do exist. An electric field is produced by one or more charges and is independent of the existence of the test charge that is used to measure it. The field provides a method of calculating the force on a charged body. It does not explain, however, why charged bodies exert forces on each other. That question is still unanswered.
How does the potential difference depend on the electric field? In the case of the gravitational field, near the surface of Earth the gravitational force and field are relatively constant. A constant electric force and field can be made by placing two large flat conductors plates parallel to each other. One is charged positively and the other negatively. The electric field between the plates is constant except at the edges of the plate. Its direction is from the positive to the negative plate.
One important application of the uniform electric field between two parallel plates was the measurement of the charge of an electron. This was made by American physicist Robert A. Millikan (1868-1953) in 1909.
The presently accepted theory of matter says that protons are made up of fundamental particles called quarks. The charge on a quark is either +1/3 or -2/3 the charge on an electron. A theory of quarks that agrees with other experiments states that quarks can never be isolated. Many experiments have used an updated Millikan apparatus to look for fractional charges on drops or tiny metal spheres. There have been no reproducible discoveries of fractional charges. Thus, no isolated quark has been discovered, the quark theory remains consistent with experiments.
Merill Physics:(Priniples by Paul W.Zitzewitz & Robert F. Neff.Chapters 20-20
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